I'd Like To Thank The Acadamy...

It's hard to believe it, but it's been over a year since I started throwing these (for the most part) weekly tidbits out for general consumption.  I made a commitment to myself to see if was disciplined enough to do it for a year.  Except for those Saturday mornings when I got in at 4:30 from a gig, it's been a pleasure to do this. 

Please know that this exercise has been educational for me as well.  It's forced me to stop and think "how do I actually do (fill in the blank)" as opposed to just going through the motions.  As a result, I've gotten more efficient in the way I do certain things.

And what has amazed me is hearing from people that have absolutely no connection to performing or audio that say they enjoy reading the articles.  For that, I'm extremely grateful.  It's funny, because I wasn't a big fan of writing when I was in school.  But now, I find it quite therapeutic.

So...for now I think I've said everything I've needed or wanted to.  But before I close this out, I did want to share a few things with you.

And The Winner Is...
People ask "What has been your most read article?".  By far, it was Pedals To The Metal from February 23rd, with Part 2 right behind.  After that, the recent rant on power amp companies, Watts The Meaning Of All This would be in third place.

In general, the popular articles tended to be the ones about monitors, personal gear, and rehearsals.

Far & Wide
The most mind blowing part of this year long exercise for me is that this blog has attracted readers from every state, as well as South America, Mexico, Russia, Italy, Denmark, Norway, England, and a host of other places around the globe.  That tells me that crummy clubs, broken gear, and neurotic bandmates are universal!

And Now...
I want to thank everyone for the feedback, advice, and ideas for articles.  I've gotta give props to my wife Barb for understanding why I disappeared into the studio on Saturday mornings after breakfast.  A big thanks to Michael Ingalls for coming up with the title.  And of course, to everyone who has kept up with these morsels of information along the way.

I'm still going to write, albeit on not as much of a schedule now.  If something interesting comes along or a cool piece of gear comes across the bench, I'll share it with you guys.  If I run into a club owner that's figured out a new way to screw a band over, I'll let you know.  Conversely, if I play a venue that really knows how to treat musicians I'll share that as well.

If I had to summarize the last year, and boil it down into a bite-sized piece of advice, I would just say this...

 - Take care of your gear
 - Take care of your bandmates
 - Take care of business

Do that, and everything else will take care of itself.

Thanks for a great year!  And even though I'm not around here weekly now, you can always email questions to me.  If I don't have an answer, I probably know someone who does!

As always, keep the meters out of the red.


Ken

Ken Carver has been a musician and performer since the early 70's, and involved with live music production since the mid 70's. He worked for 15 years as a broadcast engineer, building numerous studios and transmitter sites around Texas. He's also worked in Critical Care Communications for the medical industry, R&D for an automated lighting manufacturer, and owned Project Lighting & Sound in the 80's. He currently heads up an R&D Hardware Technician Team at National Instruments in Austin, and still performs on the weekends in the Central Texas area. You can reach Ken at itsjustlogistics@gmail.com

And The Answer Is....

So...how did everyone do on last week's test? 

I know you're waiting with baited breath (or is that coffee breath).  Anyway, here are the answers.

---

1. When does the show start?
   1. On the downbeat of the first song
   2. At the last rehearsal before the gig
   3. At load-in
   4. After everyone has had a shot of Patron or Jack Daniels

Like it or not, you're in the spot light as soon as you start rolling into the venue.


2. What should you do before hooking up to power at a new venue?
   1. Check that the outlets are wired correctly and have a functioning ground
   2. Determine what outlets are on the same circuit
   3. Both a. and b.
   4. Get everyone a shot of Patron or Jack Daniels

Use that handy dandy outlet tester that you ran out and bought last year.  It's cheap insurance against frying a piece of gear...or worse yet, you!


3. How should you "build" your stage setup?
   1. Back to front, top to bottom
   2. Stage Left to Stage Right
   3. From the middle out to the edges
   4. The same way you play Jinga

Don't make any more obstacles than you have to.  Hang your banner and lights from the back wall, run your upstage cross connections, and then work to the front of the stage.


4. You've determined that two normally well behaved pieces of equipment start humming as soon as they are connected together. The cause is most likely:
   1. An power supply failure
   2. A faulty cable
   3. A ground loop
   4. A fruit loop

A power supply failure can cause hum, as can a faulty cable.  But the hum would be there before you connect the gear. 


5. When performing at private parties, the key to a successful gig is:
   1. Remaining patient and flexible
   2. Getting half the money up front
   3. Loading in and out through the kitchen
   4. Doubling up on your Prozac before leaving the house

You will likely be dealing with people that have no idea what's involved in putting a live band's production together.  I will also accept answer D, as it helps with A.


6. The function of a compressor to:
   1. Limit the dynamic range of a signal
   2. Re-equalize a signal
   3. Correct the pitch of a singer
   4. Make up for a crappy pack in the trailer

By reducing the dynamic range of an audio source, it's easier to get it to sit in the mix.


7. When should you use sub-groups?
   1. When you need to control the level of several sources without changing the balance between them
   2. When you need to apply processing or effects to several sources
   3. Both a. and b.
   4. When the main group is late to the gig and you need a replacement

Drum kits, big keyboard rigs, and horn sections are all good candidates for sub-grouping.  Vocals work really well in a sub-group with compression across the bus.


8. What is "Power Alley"?
   1. 20Hz to 50Hz
   2. 200Hz to 500Hz
   3. 2000Hz to 5000Hz
   4. Where most of the crack dealers in Austin hang out

Power Alley is the portion of the audio spectrum where just about every source (except cymbals) has fundamental frequencies.  This is where your mix can get away from you.


9. When your band picks a song to cover, make sure everyone learns:
   1. The same version and key of the song
   2. All of the lyrics so anyone can sing it
   3. English and Spanish versions
   4. The entire Phil Collins catalog

Nothing makes you want to throw things at rehearsal more than to find out you've learned the radio edit version, while the rest of the band picked up on the extended dance remix.


10. Generally speaking, when assembling a pedal board which effect should go last in the signal chain?
    1. Compression
    2. Distortion
    3. Delay
    4. The on/off switch for the fog machine

While there are no hard and fast rules, your time-based effects will have the greatest effect at the end of the pedal signal chain or in the effects loop of your amp.


11. When discussing "American" watts and "British" watts, it's important to remember:
    1. American watts are louder
    2. British watts are louder
    3. It has to do with the differences in line voltage
    4. The person talking about the two terms is a idiot

Power equals volts multiplied by amps. There's no variable for "country of origin" in any of the formulae.


12. When EQ'ing monitors stop when:
    1. All of the sliders are at the top or bottom of their travel
    2. You've moved about one-third of them
    3. The peak detect light comes on
    4. There's blood coming from the lead vocalist's ears

Once you've reached that point, you're really not going to get anymore out of the system.


13. From a reliability and noise stand point, the best way to power a pedal board is:
    1. Batteries
    2. A switching power supply (like a 1-Spot)
    3. A linear power supply (like a Voodoo Labs Power Plus)
    4. A tank of electric eels being aggitated by a trained monkey

Batteries will give you the quietest performance from a pedal. However, they are not the most reliable (and it's easy to forget to change them before a show). The switching supplies are the most efficient, but can be a noise source. The linear supply is the best compromise (but I'd pay money to see that eel power supply).


14. You've just fired up your PA. The drum and instrument mics are working, but none of the vocal mics are coming through. The problem is likely:
    1. The main power amps did not get turned on
    2. The monitors are not connected
    3. The vocal sub-group on the board is muted
    4. President Obama's fault

If the main amps weren't on, you wouldn't hear anything. Not having the monitors up shouldn't affect the mains. Check for a button on the mixer that shouldn't be pushed.


15. The #1 enemy of electronics is:
    1. Humidity
    2. Temperature
    3. Vibration
    4. A 1st semester electronics student with a Radio Shack soldering iron.

For every 18 degrees F you can drop the temperature of a component, you essentially double it's life. Cold can affect some electronic components (mainly electrolytic capacitors), but you've got to get down below -100 F before it's a problem.  Unless you're doing an outdoor gig in Siberia, don't worry about it.


16. When should you learn your parts to a song?
    1. On your own time
    2. During rehearsal so the rest of the band can help you
    3. During sound check
    4. On the way to the gig

Showing up to a rehearsal unprepared is unfair to your band mates. Don't be that guy!


17. When working with a house sound man or hired PA, the most helpful thing you can give them is:
    1. Verbal instructions on how your band is set up
    2. Nothing...they're pros and will figure it out
    3. An up to date stage plot
    4. Weed

A stage plot is a prime example of a picture being worth a whole lot of words.


18. PA speakers should be aimed:
    1. Straight ahead to minimize bleed onto the stage
    2. Toward the main audience area
    3. At the bar
    4. Right at the guy yelling for "Freebird"

You're there to entertain the audience, not the bar staff. Remember, if they can't hear the drink orders, your night is going to go down hill really fast.


19. When booking an extended tour, the first thing you should look at is:
    1. Routing
    2. Accomodations
    3. Getting plenth of merchandise to sell after the shows
    4. Having a "connection" in each city you're playing in

You have to know where you're going before any other plans can be made. The routing will make or break you on a tour.


20. Keep the meters out of:
    1. The green
    2. The red
    3. The black
    4. Your corn hole

Please tell me you got that one right!


---

All right folks...the winner of the iTunes gift card is Murray Jones from right here in Austin!  Congrats Murray.


Next week, random thoughts. 

Until then....see question #20.




Ken







Ken Carver has been a musician and performer since the early 70's, and involved with live music production since the mid 70's. He worked for 15 years as a broadcast engineer, building numerous studios and transmitter sites around Texas. He's also worked in Critical Care Communications for the medical industry, R&D for an automated lighting manufacturer, and owned Project Lighting & Sound in the 80's. He currently heads up an R&D Hardware Technician Team at National Instruments in Austin, and still performs on the weekends in the Central Texas area. You can reach Ken at itsjustlogistics@gmail.com

Open Your Test Booklets

All right gang...you were warned last week.  And here it is, the official "It's Just Logistics" Quiz.  The first person to email the results of the test back and get 'em all right will get an iTunes gift card...a small one, but a gift card none the less.

In the event that nobody gets a perfect score, the gift card will go to the highest score with the earliest time stamp.

Good luck.  And....begin!

---

 

1. When does the show start?
   1. On the downbeat of the first song
   2. At the last rehearsal before the gig
   3. At load-in
   4. After everyone has had a shot of Patron or Jack Daniels


2. What should you do before hooking up to power at a new venue?
   1. Check that the outlets are wired correctly and have a functioning ground
   2. Determine what outlets are on the same circuit
   3. Both a. and b.
   4. Get everyone a shot of Patron or Jack Daniels


3. How should you "build" your stage setup?
   1. Back to front, top to bottom
   2. Stage Left to Stage Right
   3. From the middle out to the edges
   4. The same way you play Jinga


4. You've determined that two normally well behaved pieces of equipment start humming as soon as they are connected together. The cause is most likely:
   1. A power supply failure
   2. A faulty cable
   3. A ground loop
   4. A fruit loop


5. When performing at private parties, the key to a successful gig is:
   1. Remaining patient and flexible
   2. Getting half the money up front
   3. Loading in and out through the kitchen
   4. Doubling up on your Prozac before leaving the house


6. The function of a compressor to:
   1. Limit the dynamic range of a signal
   2. Re-equalize a signal
   3. Correct the pitch of a singer
   4. Make up for a crappy pack in the trailer


7. When should you use sub-groups?
   1. When you need to control the level of several sources without changing the balance between them
   2. When you need to apply processing or effects to several sources
   3. Both a. and b.
   4. When the main group is late to the gig and you need a replacement


8. What is "Power Alley"?
   1. 20Hz to 50Hz
   2. 200Hz to 500Hz
   3. 2000Hz to 5000Hz
   4. Where most of the crack dealers in Austin hang out


9. When your band picks a song to cover, make sure everyone learns:
   1. The same version and key of the song
   2. All of the lyrics so anyone can sing it
   3. English and Spanish versions
   4. The entire Phil Collins catalog


10. Generally speaking, when assembling a pedal board which effect should go last in the signal chain?
    1. Compression
    2. Distortion
    3. Delay
    4. The on/off switch for the fog machine


11. When discussing "American" watts and "British" watts, it's important to remember:
    1. American watts are louder
    2. British watts are louder
    3. It has to do with the differences in line voltage
    4. The person talking about the two terms is a idiot


12. When EQ'ing monitors, stop when:
    1. All of the sliders are at the top or bottom of their travel
    2. You've moved about one-third of the sliders
    3. The peak detect light comes on
    4. There's blood coming from the lead vocalist's ears 


13. From a reliability and noise stand point, the best way to power a pedal board is:
    1. Batteries
    2. A switching power supply (like a 1-Spot)
    3. A linear power supply (like a Voodoo Labs Power Plus)
    4. A tank of electric eels being aggitated by a trained monkey


14. You've just fired up your PA. The drum and instrument mics are working, but none of the vocal mics are coming through. The problem is likely:
    1. The main power amps did not get turned on
    2. The monitors are not connected
    3. The vocal sub-group on the board is muted
    4. President Obama's fault


15. The #1 enemy of electronics is:
    1. Humidity
    2. Temperature
    3. Vibration
    4. A 1st semester electronics student with a Radio Shack soldering iron.


16. When should you learn your parts to a song?
    1. On your own time
    2. During rehearsal so the rest of the band can help you
    3. During sound check
    4. On the way to the gig


17. When working with a house sound man or hired PA, the most helpful thing you can give them is:
    1. Verbal instructions on how your band is set up
    2. Nothing...they're pros and will figure it out
    3. An up to date stage plot
    4. Weed


18. PA speakers should be aimed:
    1. Straight ahead to minimize bleed onto the stage
    2. Toward the main audience area
    3. At the bar
    4. Right at the guy yelling for "Freebird"


19. When booking an extended tour, the first thing you should look at is:
    1. Routing
    2. Accomodations
    3. Getting plenth of merchandise to sell after the shows
    4. Having a "connection" in each city you're playing in


20. Keep the meters out of:
    1. The green
    2. The red
    3. The black
    4. Your corn hole

---

OK...put your pencils down...or however you're doing this.  Don't forget...the first person to get 'em all right and email their answers to itsjustlogistics@gmail.com  gets the glory and the iTunes card.

The answers will be posted next week.

Until then....you know.

Ken

 



Ken Carver has been a musician and performer since the early 70's, and involved with live music production since the mid 70's. He worked for 15 years as a broadcast engineer, building numerous studios and transmitter sites around Texas. He's also worked in Critical Care Communications for the medical industry, R&D for an automated lighting manufacturer, and owned Project Lighting & Sound in the 80's. He currently heads up an R&D Hardware Technician Team at National Instruments in Austin, and still performs on the weekends in the Central Texas area. You can reach Ken at itsjustlogistics@gmail.com

Phantom Of The Microphone

I've never really liked the term "Phantom" power.  It makes it sound like it's not really there, or that it's just a figment of someone's imagination.  But since I don't have a better suggestion (and apparently neither has anyone else), we're stuck with it.

If you're using condenser microphones in your set up, you're going to need phantom power to them.  Condenser mics will not work without some kind of power source.  Period.

Condenser mics aren't the only thing that can take advantage of phantom power.  Some active DI boxes can use it as their power source, as can a few preamps and effects pedals (the Sansamp Tech 21 series being a good example).  So lets start the discussion today with exactly what phantom power is.

Special Delivery
Phantom power is a way to deliver DC power down a balanced audio connection without causing problems for the audio coming back the other way.  The original specification was written to allow 1mA (.001 amps) per connection, but as with everything else electronic, power demands have increased over the years, and now that power budget is 10mA.

Thinking back last week to our discussion of balanced and unbalanced connections, keep in mind that phantom power can only be done through a balanced system.  The positive voltage is applied to pins 2 and 3, and the shield carries the ground return current.  At the mixer (or voltage source end), the DC is blocked using capacitors as a high pass filter (DC is essentially a 0Hz signal, so we can filter that out long before we get to audio frequencies).

Electrically, here's what it looks like:

 

At the microphone end, the transformer that's built in keeps the DC from messing with the output of the preamp (DC can't cross a transformer, only AC / audio can).  At the mixer input side, you see the blocking capacitors I talked about.  Capacitors don't pass DC after they've charged (which only takes a few microseconds in a circuit like this).

 

So now you can see "how" we get the phantom power down to the mic and get the audio signal back.  I suppose you could think of it like a double-decker highway.  The audio is riding down the upper deck, and the DC the lower deck. 

 

Did You Want Medium, Large, or Extra Large?

I was brought up thinking that phantom power was always supposed to be 48 volts DC.  When you're using phantom for condenser mics, that's where they operate best.

 

However, it's a well know fact that most consumer grade electronics don't put out that much...it's typically 15 volts or so.  Why?  Easy...it saves them the cost of putting another power supply in their product (as most audio electronics run off of +/- 15 volts or less).  And in all fairness, you're not going to hear the difference with a mid-grade mic in a live situation.

 

It's interesting to note that the first mic to use phantom power (the Schoeps CMT-20) was built in 1964, and was designed to use 9-12 volts.  It was built for French radio, and that value was picked because that's what was easily available in the studio.

 

It was in 1966 that Neumann Audio of Berlin built a mic with a transistorized preamp for the Norwegian Broadcasting Network.  That mic was designed for the now accepted 48 volts.  Again, why?  Because the emergency lighting in their studios ran off of 48 volts.  Again..it's what they had available.  And you thought there was a bunch of math and egg heads involved in picking these values.

 

There was a 24 volt standard proposed in the 1970's, but it never really caught on.

 

But I Don't Want Any...

So, what happens when you hook a plain old dynamic mic (that doesn't require phantom) up to a cable that is providing it?  Nothing.  Since the same voltage is being applied to both sides of the mic's transformer, there's a net zero potential across it.  In other words, it doesn't need it so it ignores it.

 

Big Bang Theory

Now, it's never a good idea to connect or disconnect an audio cable unless the channel is muted.  But with phantom power on a cable, it's worse.  Even if you're working with something that doesn't use phantom power, connecting or disconnecting the cable with the channel unmuted is going to sound like a .22 caliber weapon going off.  You won't damage the device you're hooking up to, or the mixer.  But, there's a good chance you'll blow a high frequency horn out.  This is why (when the equipment allows it) I only turn phantom power on the channels that need it.  Most digital and higher end consoles allow this.  But for less expensive gear, it's typically all or nothing.

 

You've been warned.

 

No, You Can't Do That

I mentioned earlier that some active DI boxes and preamps can use phantom power instead of their internal batteries.  This is cool (as anytime you can take a battery out of the equation is a good thing in my book).  But, there's one major drawback.  That is, when using phantom power you can not use the ground lift if you need to.  Phantom counts on the ground being there as the return path for the current.  If you have to use the ground lift, you'll have to run the device on batteries.

 

We've been at this for almost a year now, and I've wondered how much of this stuff I've been writing about has stuck with the readers.  So next week...we're having a quiz!  Yeah, you read that right.  It will be multiple choice, and all of the questions come from what we've covered here. 

 

And just to give you a little more motivation, the first person to email me with all of the correct answers....heck, I'll send you an iTunes gift card.  In the event no one aces the test, the highest score with the earliest time stamp wins. 

 

Study up and get some rest.

 

Ken

 

 

 

Ken Carver has been a musician and performer since the early 70's, and involved with live music production since the mid 70's. He worked for 15 years as a broadcast engineer, building numerous studios and transmitter sites around Texas. He's also worked in Critical Care Communications for the medical industry, R&D for an automated lighting manufacturer, and owned Project Lighting & Sound in the 80's. He currently heads up an R&D Hardware Technician Team at National Instruments in Austin, and still performs on the weekends in the Central Texas area. You can reach Ken at itsjustlogistics@gmail.com



 

 



Lost My Balance...

There's a long list of buzzwords in audio that people toss around, but really don't understand what they mean.  Two of those terms are "balanced" and "unbalanced".  Now, if you're talking about your band mates...then everyone knows what you mean.  But if you're talking about an audio connection or interface...that's where a lot of people's understanding gets a little murky.  So, this week we'll try to bring some understanding to these terms.  I'll attempt to keep it light on the math and heavy on the practical applications.

First off, lets start with an unbalanced audio connection.  Every time you hook up a guitar to an amp, an iPod to a computer or mixer, or anything else that just uses a 2 conductor cable, you're using an unbalanced connection.  This is what that type of cable looks like:

A balanced connection requires three conductors.  If you're hooking up a professional mic to a mixer (especially if it's going down a long cable), then you're likely using a balanced connection. Here's what that type of wire looks like:

OK, we've identified the physical difference between the two types of cable.  But what's going on here electrically, and why is one better than the other.

Let's look at the unbalanced (also called single-ended) connection first, since it's a little easier to understand.  With unbalanced connections, the shield is performing two functions.  It's serving as the return path for the signal (the "negative" side of the audio if you will), and it's shielding the center conductor (the "positive" side) from external noise.  It does a pretty good job, but if the interference is strong enough, it's going to start bouncing the ground around...and you're going to hear that in the audio.

Here's a very basic premise...the shield is only going to do so much to protect the signal from interference.  Unless you're using wire with a solid outer shield (which would make the cable extremely stiff), then some amount of noise is going to get through.  And the longer the cable, the more chance for garbage to get in.  That's one of the reasons why you rarely see unbalanced connections longer than 30 feet.

OK, so why is a balanced audio connection so much better?  It has to do with the fact that there's a second line of defense against noise that's built into the architecture.  It's a concept called Common Mode Rejection.   No, I'm not talking about when you got turned down for dates back in high school.  This is far more useful and much less painful.

Before we talk about common mode rejection, we're going to have to have to touch on a mathematical concept here (sorry...I've put it off as long as I can).  That concept is Vector Addition.  OK...let that sit for a moment...the sting will go away.

The classic (and simplified) definition of a vector is a ray with magnitude and direction.  Putting that concept into a real world example...a car heading north at 50 MPH could be considered a vector.  You have both magnitude (50 MPH) and direction (north). 

So far so good, but how does vector addition work?  For that example, let's use a tug of war game.  Let's say that there are 4 people on each side of the rope pulling.  Pulling from the left side, it's 4 Marines.  On the right side, it's 4 members of the Austin Botanical Society.  When we say "go" there's force being applied from both directions, but guess which way the rope is going to go.  Yeah, I'm betting the that vector force coming from the Marines side is going to be a little greater.

Now, let's even this up.  We'll send the Botanical Society home and bring in 4 Army Rangers to stand in for them.  And now let's assume it's a dead even match and both teams are pulling with the same force.  Which way will the rope go?  Right, it doesn't move because the 2 forces are cancelling each out.  (Now, in reality I know that one team will probably prevail...but I have friends who are ex-Marine Corp and ex-Army...so I'll just let them talk trash to each other).

Back to the world of electrons.  A balanced audio connection works by running the same signal down 2 wires, but they're running opposite of each other.  If you've heard the term "out of phase"...this is it.

By the way, here's another tech term you can throw around and impress people.  Whenever you have two signals like this that are compliments (opposite) of each other, it's called a Differential Pair.  That's going to turn some heads at your next beer bust.

I know...you're thinking "if those signal cancel out, then how do we hear anything?".  Well, you're right, but this is just to show you what happens to two signals that are out of phase with each other when they're added together.  I'll show you how we recover the signal in a bit.

Now, we've got our two out of phase signals running down the cable.  All of a sudden, noise decides to show up and join the party.  Now, our otherwise pristine signal looks something like this:

I want you to notice something very important here, because it's key to understanding how we're going to get rid of the noise.  While our audio signals are out of phase with each other, the noise is the same (or in phase) on the 2 lines. 

So what do you suppose would happen if we could flip one of those audio signals over so that is was in phase with the other one.  What would happen to the noise? 

Now our two audio signals are in phase with each other, but the noise is now out of phase!  When we add those two signals together...the noise is cancelled out.  And just to throw another tech term out there for you, the specification that describes how well a piece of equipment does this job is called the Common Mode Rejection Ratio, or CMRR.  That's one of those specs where the bigger the number the better.

The "phase flip" and signal addition was done with a transformer in the olden days.  Now, it's typically done with a ....wait for it...differential input amplifier.  Electrically, what's going on looks like this:

Given this significantly improved method of getting rid of noise on audio lines (along with a couple of other electrical characteristics), we're able to run much longer cables without degrading our signal.  How much longer?  If you've got a good pro mic and you're plugging it into a decent mixer, you can typically run up to 2000' feet of cable before you start messing the signal up.  Yeah, you read that right...about 4/10ths of a mile!  This is why you have to used balanced audio connections when you have a console out front to mix the band.  In that application, it's not uncommon to have 120 feet of wire between the mic and mixer.

So, can unbalanced and balanced connections ever mix?  The short answer is yes.  You can unbalance a balanced connection through wiring:

You would use a cable like this if you wanted to plug a balanced microphone into a guitar amp.  You loose the noise cancellation and some signal, but it works.  When you do this, though, the rules for unbalanced wiring apply...especially the length of cable limits.

Going the other way from unbalanced to balanced can't be done through wiring alone.  You're going to have to have some electronics or a transformer.  If you're a bass player or keyboardist, you've like already used a device that performs this function.  You just probably know it by it's common name...a Direct Injection (DI) box.

The transformer does the job of converting the signal from single-ended (unbalanced) to differential (balanced).  It also electrically isolates whatever you're hooking up to prevent ground loops.  And again, this can be done with electronics to...which would make this an active DI box (as opposed to a passive one shown above). 

I want to leave you with one final thought here.  If you happen to have a balanced cable that's had one of the signal conductors fail, you will continue to get a signal through it, because that that point you've got an unbalanced connection.  It's going to be quite a bit lower in level, and likely have more noise on it (since you've lost the ability to cancel it out).  So, if you're setting up your PA and happen to notice that a mic is quieter than you expected, it's probably a good idea to bust out the handy dandy cable tester that I'm sure you have now and check that cable.

This is has been a pretty heavy installment, and I've only covered a small part of theory behind all of this.  If you have additional questions, you're always welcome to email me.

Next week, as long as were talking about these connection schemes we might as well tackle another "mystery subject" to a lot of people, and that's phantom power.

Until then, keep the meters out of the red.

Ken

Ken Carver has been a musician and performer since the early 70's, and involved with live music production since the mid 70's. He worked for 15 years as a broadcast engineer, building numerous studios and transmitter sites around Texas. He's also worked in Critical Care Communications for the medical industry, R&D for an automated lighting manufacturer, and owned Project Lighting & Sound in the 80's. He currently heads up an R&D Hardware Technician Team at National Instruments in Austin, and still performs on the weekends in the Central Texas area. You can reach Ken at itsjustlogistics@gmail.com

Heading Off Trouble

The last few weeks have been pretty technical, so I promise...no math this week!

We've spent a lot of time here talking about how to troubleshoot your systems when they fail.  And being able to do that is a skill that will carry you far.

But as most of you know, one of my big mantras is that you shouldn't be fixing things at a gig...you should set up, play, and load out.  Or, as one of my past co-workers said, "Show, blow, get the dough, and go!"

To that end, there's a lot to be said for preventive maintainence.  In a couple of my previous careers, preventive maintainence (or "PM" as we called it) was crucial.  As the chief engineer for a number of commercial broadcast stations, my job was simple...keep the stations on the air and legal.  To that end, I would schedule time monthly (usually on a Monday morning at midnight) to take a station off the air and go through the transmitter.  I would look for loose components or connections, anything that looked like it was getting hotter than it should, and give the cooling paths a good cleaning.  As a result, I had an excellent record for minimal down time. 

You can adopt the same mentality for your PA systems and instrument rigs.  There's nothing really hard about it...it's just talking yourself into doing it.  A few hours a couple of times a year can really save you a ton of aggrevation and frustration at a gig.  If you get the entire band involved, it's something that can be knocked out very quickly.  And unlike the radio stations, you don't have to wait until the wee hours of the morning (unless that's when you function the best).

So...what would preventative maintainence look like on a PA system?  Well, in check list form, it would probably look something like this:

• Test all audio interconnect connect cables (with your trusty cable tester).  Identify any that fail and quarantine them for repair.
• Inspect all cables for knicks or cuts.  Repair with either heatshrink tubing or electrical tape.
• Clean all cables, including power. (Just pull them through a damp cloth as you coil them up).
• Inspect amp racks for loose connections and hardware.
• Clean any fans or filters on force-air cooled gear.
• Exercise all audio connections (disconnect and reconnect them a few times).  Cleaning the connectors with DeOxit is highly recommended.
• Hook your PA up and run some music through it at a low level.  Now, listen to each individual speaker component and make sure everything is still working (I do this every gig).  It's possible to blow a woofer or midrange speaker and not realize it when everything is going full blast.
• Speakers vibrate...vibration causes things to work loose.  Check the mounting hardware for your speakers!
• Any case hardware (handles, latches, or casters) that's busted should be repaired / replaced at this time.  You curse that broken caster every time you load in...get rid of now!
• Replace any missing labels on any equipment.  Remember the 4x4 test!
• Check all of the pots on your main console for quiet operation.  Slide pots are easy to clean, so take care of that now. 

Your personal rigs should be checked in a similar manner.  And don't think that because you don't gig that much that you don't need to do this as often.  I find more problems with systems that sit idle for an extended period of time than those that go out on a regular basis, especially with the connectors in amp and effects racks.

Yeah, I know...this isn't the most "rock star" thing you can do with your band time.  But the payoff is that you're going to lower the stress level at your gigs by not having to troubleshoot/swap out/fix something 10 minutes before show time.  And that (at least to me) has a great deal of value to it.

Next week, I'm going tech on you again!  We're going to talk about one of those oft-quoted but rarely understood terms in electronics, and that's "balanced" wiring.  We've all heard of it, but who really understands it?  We'll break it down next week.

Until then, keep the meters out of the red!

Ken

Ken Carver has been a musician and performer since the early 70's, and involved with live music production since the mid 70's. He worked for 15 years as a broadcast engineer, building numerous studios and transmitter sites around Texas. He's also worked in Critical Care Communications for the medical industry, R&D for an automated lighting manufacturer, and owned Project Lighting & Sound in the 80's. He currently heads up an R&D Hardware Technician Team at National Instruments in Austin, and still performs on the weekends in the Central Texas area. You can reach Ken at itsjustlogistics@gmail.com

I Feel So...Used.

Nice, shiney new equipment is cool.  It's fun to open up the box and get that whiff of "new electronics". Pulling it out of the box, setting it up, and then carefully peeling the protective plastic off the front panel (although some friends of mine frown on that...probably because their grandparents had heavy vinyl covers on the couch).

There's a lot of reasons to buy new gear.  But sometimes, used ain't a bad way to go.  Certain items, I wouldn't think twice about looking at used gear.  Other things...it just doesn't make sense.  So this week I thought I would share my thoughts on used equipment...what makes sense to consider, and what makes absolutely no sense.

First off, I'm not going to talk about instruments and instrument amplifiers.  Once you start getting into that "vintage" thing, I think people loose a lot of their common sense.  Yeah, some older amps sound pretty good.  But others....they sounded like a broken speaker at Sonic Drive-In.  But, because it was from the 60's some folks will convince themselves that they're in tone nirvana.  So, I'm not going there.  We'll limit our talk to PA equipment only.

Speaking Of...
Speakers are something that as long as they don't look like they've been dragged behind a truck are safe to consider.  But, you definitely want to look things over carefully and actually fire them up before you hand over your money.  Take a flash light to shine through the grill (or better yet, ask if you can take it off) to inspect the speaker.  You're obviously looking for tears in the cone, but also look for water/beer stains...especially if it's a monitor slant.  Speaker cones are (for the most part) paper, and they don't like liquid.  A lot of liquid spilled on a speaker will shorten it's life and affect the sound.

If you get the grill off, gently push the speaker in using your finger tips on both hands.  Push about halfway between the center and edge.  What you're looking for is if you feel like anything is rubbing or catching.  The speaker should move freely.  If it doesn't, that indicates that the voice coil is damaged or not aligned properly with the magnet.  That's caused by either excessive power, over excursion (from too much bass), or the thing was dropped from a significant height.  Fixing that calls for a re-cone.  Figure $50 to $150 dollars depending on what brand, model, and size of the speaker.  For me, that's usually a deal breaker. 

Listen to each speaker in a system individually.  You don't want to find out after you get home that one of the woofers in a dual 15 cabinet isn't working.  Unless you run them at a low level and put your ear right up to the cabinet, it's hard to tell.

Getting Amp'd Up
I used to have no trouble buying used power amps as long as they were less than 8 years old.  Why that age?  Well, after 10 or so years most gear needs to have the electrolytics capacitors replaced.  That's something I do all the time, but unless it's just a killer deal I don't want to have to do it immediately.

Again, check out the physical condition.  If it's scratched up, no big deal.  But, if it looks like a corner or front panel is caved in, the amp could be hiding circuit board or component damage inside. 

In addition to listening to the amp you really want to look at it.  If it's filthy to the point that the air intakes / exhaust are clogged, I'l pass right there.  That tells me that the owner gave it no maintainence whatsoever, and that the components have been running at an elevated temperature.  And as I previously mentioned, every 10 degrees C (18 degrees farenheit) the temperture is elevated, you cut the life of some components in half!  People that treat their gear like that typically don't change the oil in their vehicles at the appropriate intervals either.  Here's a photo taken right off of Craigslist:



 

That filthy air filter on the bottom would kill the deal for me.  That, and the Power Base series wasn't exactly one of Crown's best amps.  They were built for a price point first and foremost, and performance / reliability second.

 

That said, it doesn't have to be operating room sterile.  Here's that fan intake on an XLS 402 (one of Crown's best amps for the money) that I consider normal use:

 

Yeah, I'm good with that.  And the fact that I know this amp inside and out.

 

He Ain't Heavy...Wait, Yeah He Is

Now, my biggest arguement against buying used power amps now...weight!  If I'm going to install them in a fixed installation, no big deal.  But if I'm moving this stuff in and out of clubs several times a month, it quickly becomes a consideration.  The pounds-per-watt has been coming down rapidly in the last few years thanks to improvements in switching power supply technology (and that lets the transformers get small and lighter).  It's at the point now where I can get into something in the range of 1000 watts (real ones) that weighs under 15 pounds.  In the late 80s I was using Peavey CS-800s.  Almost the same power, but almost 60 pounds.  A rack of 4 of those was absolute death when you loaded out at 3:00 in the morning!

 

And The Rest Of The Stuff...

Consoles, EQ's, Compressors, Crossovers...the low level signal stuff.  That stuff very rarely fails due to electrical problems.  It's almost always physical damage or dirty potentiometers.  Equalizers are especially susceptible to dirt, but they're also the easiest to clean.  What you do want to look for is evidence of something getting spilled into them.  Beer is tough to clean out.  Sodas...forget it.

 

Missing knobs (if there's just a couple)...no biggie.  I can usually get replacements from the manufacturers.  But ones where the shafts are sheared off...again that's showing some potential physical abuse. 

 

And then, it comes down to the brand.  Quality brands Rane, White, Ashley, DBX, BSS...hard to go wrong with that stuff.  It's built well and can be repaired if needed.  One brand of rack gear I will never buy again is Behringer.  Period.  The first thing that fails on their gear is the pushbutton switches.  And here's the kicker...you can't even disassemble the stuff to clean them.  Or I should say you can't reassemble it when you're finished.  Why?  Because they put that stuff together with double-sided tape!  Yeah, you read that right.  To fix their stuff, you literally have to duct tape it back together.  I've thrown away a crossover and two 4-channel compressors for this very reason.  Naw...screw those guys if they can't even use screws to put their stuff together.  That stuff was replaced with DBX and Presonus gear, and I've never looked back.

 

Check The Spare When You're Kicking The Tires

Whatever you're looking at buying, make sure you check every single function on the device unless you like being surprised later on.  That means every button, knob, and switch.  If the seller is rushing you along or gives you the "don't worry about that...it's fine", then ask them what kind of warranty they're giving you.  If they offer a return period, as them to give it to you in writing (or better yet, ask them to send you an email to that effect with their contact into).  If they don't want to do that, then they're either hiding something or just douche bags.  Either way, you don't need to be dealing with 'em.  Your time and money are better than that.

 

The Bottom Line

I've reached the point that I only buy used gear if:

• I'm saving at least 50% off of the new street price
• It's a killer deal with a problem that I'm reasonably confident I can fix
• It's some oddball piece of gear that's no longer made that I want to make up for selling the same thing when I was younger.

 

And there you go...Ken's guide to used gear.  Take it for what you will.  And just remember that only you can decide if a deal is a good deal.

 

Until next week, keep the meters out of the red (even if the red is a little faded)

 

 

Ken

 

 

 

Ken Carver has been a musician and performer since the early 70's, and involved with live music production since the mid 70's. He worked for 15 years as a broadcast engineer, building numerous studios and transmitter sites around Texas. He's also worked in Critical Care Communications for the medical industry, R&D for an automated lighting manufacturer, and owned Project Lighting & Sound in the 80's. He currently heads up an R&D Hardware Technician Team at National Instruments in Austin, and still performs on the weekends in the Central Texas area. You can reach Ken at itsjustlogistics@gmail.com

 





We Are The Road Crew...

Most of our discussions here have revolved around working on your own sound system and how to get the most out of it.

But sometimes that's not the case.  In downtown Austin it's fairly common for a venue to have a house sound (and sometimes lighting) system.  And if you're lucky enough to be playing on a festival, it's almost guaranteed that the production is going to be hired in. 

Of course, like everything else in the music biz there are companies and people that are really good at what they do and are equipped to deliver a professional product...and then there are those "sound companies" that are little more than "a friend with a lot of stereo equipment".  When you've been doing this as long as I have, you can walk into a venue and in 30 seconds and have a pretty good idea what you have to work with.

So this week, let's talk about things you can do as a musician to make interfacing with a house or hired production team easier for all of you.

Plotting To Take Over The World
First and foremost, if you don't have an up to date stage plot...make one!  It doesn't have to be anything fancy...just something that lets the sound guys know how many inputs they're dealing with and where things need to go.  Here's one I did for The Michael Ingalls Band:

There's actually a lot of information in this drawing.  From one sheet of paper, the sound man can tell:

• How many mics/channels we're requesting
• The brand of mics we're partial to
• Starting points for what we like in our monitors
• Where we need AC power drops
• The frequencies our wireless gear operates on, just in case they use any wireless stuff.

If you had tried to verbally explain all of that, how long would it have taken?  Yeah, a while.  And don't worry that your drawing isn't all pretty and to scale.  A sharpie and the back of the flyer from the band that played there last week works just as well.

If the sound man is worth a flip he will appreciate this because it makes his job faster and easier.  But I'll warn you now that there are more than a few guys that have been mixing crappy bands in crappier clubs a lot longer than they should have, and they'll blow this off.  Instead, they cop a "you'll get what I give you" attitude.  They're burnt out and fed up with the endless parade of Prima Donna bands that come through thinking they're the next big thing.  I feel bad for those guys, but at that point their putting themselves through it.  All you can do is not be another one of those Prima Donnas, work with them the best you can, and play the best show you can.  Then you decide if you want to go back to that venue again...or not.

Checkin' In
Since these guys are (hopefully) a lot more familiar with their sound systems than you are, let them run the sound check.  All the pros pretty much do it the same way, build the mix from the bottom up starting with the drums.  Standard sound check etiquette applies here...keep your instrument quiet while someone else is sound checking.  You will get your turn.

The first step is usually a line check and EQ.  Next, I like to see the monitors done.  Finally, you play and let 'em build a rough house mix.  I've run across one sound man that had us play before the monitors were up.  I tell you, that's painful and difficult in a big room (and it was a big room).  Sound guys...if any of you are reading this please don't do that to the bands you're mixing.  You won't get a good sound check because we can't hear ourselves singing and will hold back.  And speaking of monitors....

Monitoring The Situation
Sometimes this is the hardest part of the sound check...getting the monitors dialed in.  Keep in mind that when you're setting up your monitor you'll likely have things too quiet (because the whole band is not playing).  Remember that in 5 or 10 minutes, sonic chaos is going to be competing with that monitor slant in front of you...plan accordingly.

And again...don't everyone shout at the same time for something changed in your monitor.  There's an old joke that applies here:
     Q:  What's the difference between a monitor engineer and a toilet?
     A:  The toilet only has to deal with one a$$hole at a time.

During the show, if you need something changed then use the universal monitor adjustment sign language.  Get the sound man's attention, point the the source or person you need adjusted, point to your monitor, and then point up or down.  If you're pointing to a singing guitarist, then mouth the word "vocal" or "guitar" while pointing at them.  Otherwise, you don't know what you're going to get.

OK...but what if the sound man is reading a book or has walked away from the board to get a beer?  See "burn out" above.  You just have to deal with it.

A good sound man will have his eye on you during the show, and won't walk away from the console unless something needs to be fixed on stage.  A good sound man will also not try to make you significantly alter how you set up and perform.  Which brings me to...

One Bad Apple...
I played a show several years ago in Waco with The Rhythm Dawgs where there was house PA and lights.  We got there early to set up, and geez...what a pile of crap.  There were probably 8 different manufacturers speakers piled up on top of each other across the back of the stage.  I thought it was just for looks, but the sound guy (who showed up about 15 minutes before we were scheduled to start) was going to use those as monitors.   Really....monitors behind the band???  Nope...don't think so.

He also wanted us to break our amps down and move them to the side of the stage pointing in...and then tried to BS us with some mumble jumble about cross-fire phase cancellation resulting in a cleaner sound out front.  (By the way, have I mentioned that a good soundman will not try to make you change the way your perform).  At that point, I told him that if he had been there when he was supposed to be (over an hour ago), we would have tried to accommodate him.  But now, our amps were up and were staying where they're at.

Then, no sound check...no line check...nothing.  His excuse..."I know this room, I'll get you dialed in by the second song."  He may know the room, but not the band.  He didn't have it dialed in by the second set!

I really couldn't believe how bad this guy was...until I saw him across the street on break toking up.  Later on he was bragging about how he was a "prescription drug and medical marijuana user".  Sorry dude...the only thing you are is a stoner.  Needless to say, we never went back.

The Friend With The Stereo
Sometimes you walk into a situation where you've got good well meaning people that have no idea what they're doing.  Even worse, the equipment they bring is better suited for an iPod in a dorm room than mixing a band.  I walked into one of those situations back in the 80's.  The guy had a DJ mixer.  Seriously...a Numark DJ mixer that he was going to do a 5 piece rock band with.  His first question..."How many of you sing?".  "Four" I told him.  "Uh...I can only do two mics...can you work with that?". 

No..we can't.  So being the nice guy I am I pulled our 16 channel board and our mics from the truck.  I then asked where the mains and amps were.  He pointed to a Kenwood AM/FM receiver and two Sansui stereo speakers.  "It kicks, man...just try it".  We did, and the amp went up in smoke during the first verse.

At that point, I told the event planner that we could do one of two things...we could bring in our PA system (for an upcharge), or we could keep our deposit, leave, and promise never to come back.  They took the latter, which was fine by us.

The point I'm trying to make here is that when you get into these situations, concentrate on what you have (not what you don't) and how to make it work.  But sometimes...it's beyond saving.  Yelling at people don't get it done.  Either try to fix it, deal with it as is, or shake hands and walk away.

It Will Not Be Perfect
Even with the best gear and the best crew operating it, when you're playing on a hired or house system it's not going to be perfect.  Period.

The big tours...there are usually a couple of weeks of pre-production somewhere to work out all the kinks.  Two weeks...at least.  For folks like us...we've got maybe a half hour.  Maybe.

Do the best you can, be cool, and thank everyone at the end of the night.  Be the band that the makes the crew smile when they see you walk in...not the band where the crew all says "geez...not those guys again".

It don't cost nothing to be nice.


Ken

 

Ken Carver has been a musician and performer since the early 70's, and involved with live music production since the mid 70's. He worked for 15 years as a broadcast engineer, building numerous studios and transmitter sites around Texas. He's also worked in Critical Care Communications for the medical industry, R&D for an automated lighting manufacturer, and owned Project Lighting & Sound in the 80's. He currently heads up an R&D Hardware Technician Team at National Instruments in Austin, and still performs on the weekends in the Central Texas area. You can reach Ken at itsjustlogistics@gmail.com

Seeing Specs Before My Eyes

Last week, we talked about all the ways that the wattage on an amplifier can be grossly overstated and that finding out the true power capabilities can be a game of cat and mouse.  This is especially true of the manufacturers of lower end gear.

This week, I wanted to at least touch on some of the other specifications you will typically see on a power amp.  You don't see these exaggerated near as much...mainly because most folks never get past the wattage spec.  But, they are there, and I thought you might like to know how them impact the way an amp sounds.

First off, let's take a look at the specs our trusty Crown XLS 802.

So, for a live sound application which specs are we really concerned with?  While I can make a case to consider all of 'em, the first 4 are really the important ones.  Let's talk about what they are.

Sensitivity Training
The spec for sensitivity of an amp is actually one of the easiest to understand.  The spec for this amp is 1.25 volts.  What that means is that if you turn the gain controls on the amp all the way up, it will take 1.25 volts at the inputs to drive it to full power.  This is a reasonable spec, as most modern mixers and crossovers can output several times this. 

The trick is that console outputs are almost always spec'd as dBu.  To convert that to voltage requires....ah hell, I'll just give you the answer today.  My Presonus StudioLive can do +24dBu.  That works out to a little over 12 volts...plenty to drive the amp.  (We'll do an article on decibels for non-math majors one of these days).

But decibels are funny critters, being logarithmic.  That means they're kind of like the Richter Scale they use for earthquakes...where a 7 is a whole lot worse than a 6.  If your console can output +18dBu, then that's about 6 volts.  And if it can only do +4dBu...that's right at 1.25 volts...which means you'll have to have it floored to max your amps out.  It also means you're completely out of headroom on your console and you're probably clipping the signal.
And once the signal clips at the console there's nothing you can do to clean it up later. ...you're S.O.L. at that point. 

Freq Out
The frequency response of an amp is the range of frequencies that it can pass.  Almost every amp will have that spec, but 80% of them are incomplete.  A frequency response without a tolerance on it is really useless.  The spec for this amp is 22Hz to 20kHz, but notice the numbers after that...+0 dB, -1 dB.  That means the amp is flat to within 1dB which is incredibly good. 

Most amps are spec'd +/- 3dB which is still perfectly acceptable for live work.  But when you start seeing numbers like +/- 10dB...it's a BS spec.  That means you're going to have to put a lot of EQ in front of that amp to get it to flatten out.  And if there's no tolerance...well...that just means they're hiding something.

Now, how much frequency response do you need for live work?  Unless you're using concert grade subwoofers and doing electronic dance music, you're not going to need to reproduce much of anything below 40Hz.  And going to the other way, most PA systems start rolling off at 12 kHz.

So why do amp manufacturers spec the frequency response outside of this range.  While you may not be able to hear (or a speaker pass) anything about 15kHz, an extended high frequency response can be an indication of how "fast" an amp is.  There's a spec not mentioned about called slew rate, and you'll usually see it on consoles and preamps.  Slew rate can be a scary looking spec, but what it amounts to is "how fast can this amp move on a waveform".  It's usually expressed in volts per microsecond.  An extended high frequency response can mean that the amp has a decent slew rate.  What that that translates into sonically is amp that sounds clear and punchy.  The slew rate along with the damping factor (we'll talk about in a bit) has a large impact on the "sound" of a power amp.

Bring Da' Noise
The signal to noise ratio (frequently abbreviated S/N) gives you an idea how much background noise the amp produces on it's own.  Here again, a bigger number is better.  For this amp, they're saying that the noise is 100dB below the signal.  Putting that into more familiar terms, it means that the noise is 1/100,000th of the signal...pretty darned quiet.  The difference between weighted and unweighted is that the weighted number is filtered and limited to what we can actually hear.  The weighted number is more meaningful in the real world here.  But even the unweighted is still pretty impressive. 

Again, how much do you need?  Keep in mind that your system S/N can not be any better than the worst component in the chain.  So, if you amp is 100dB, but your Behringer console is only 55dB...then that's as good as your system can get.  Personally, I look for 80dB or better unless the client just has no budget and is only going to be playing $#!+ hole clubs.

Distorting The Truth
When most musicians here the term "distortion" they think of the pedal a guitar player steps on before the intro to "Satisfaction".  (OK, bad example...that's a fuzz box). But where I'm going with this is that while distortion is usually desirable to a guitar (and some bass) players, it's not in a PA system.  So this is where a smaller number is better.

Now, how many of you actually know what "distortion" is?  The technical definition for the intellectually curious is any non-linearties introduced into a signal.  For the mere mortals, it's probably easier to explain how it's measured.

To measure distortion in an amp, you input a single tone (1kHZ is the standard, but some folks use 400Hz) and measure the output signal level.  Then, you filter out the original tone at the output and measure the signal level again.  The crap that's left over...that's your distortion.

Again, how much (or little in this case)?  It depends on the application.  If this amp were going into a studio or other critical listening environment, I would want 0.1% or less.  But for live work, I'll generally accept anything at 1% or below.  This amp has 0.5%, so it's fine.

But remember, for the distortion numbers to be meaningful it has to be a rated power.  And this is where you can get tripped up.  The Crown spec above is written to say that distortion will never be worse than half a percent.  But other manufacturers will bound the distortion measurement by doing things like spec'ing it at a lower power or not over the entire frequency bandwidth of the amp.

If you don't see this number on a spec sheet...it's likely because the manufacturer doesn't want you to know that it sucks.

Et All
The other numbers (IMD, Damping, Cross Talk, and Input Impedance) are still important, but unless you're putting a very large system together (I'm talking 10 power amps or more), don't concern yourself too much.  Most folks getting into a system that big are going to hire a geek like me to do it for 'em anyway.  But just for giggles, here's what they are and how the affect what comes out of the speakers.

• IMD - Intermodulation Distortion.  Dang...that hurt.  IMD is one of one of those distortions that is really uncool.  To explain it, you have to have a quick primer on the concept of Intermodulation.  Whenever you mix 2 different frequencies together, you wind up with 4!  Yep...it's magic.  You get the original 2, plus the sum and difference of them.  So..if you mixed 800Hz and 1200Hz, you would also get 400 Hz (1200-800) and 2000Hz (1200+800).  To bad we can't do that with currency.  Anyway, it's an annoying artifact, so the more negative this number is the better.
• Damping Factor - I would tell you how this number is derived, but you would just want to throw something at me.  Let's just go with this...the damping factor is an indicator of how well an amp can physically control a speaker.  Think about what's happening with a speaker...it's travelling one direction and then all of a sudden has to go the opposite way.  If the amp doesn't have enough control to do this, then the first thing you notice is that the low end is muddy and undefined.  The Damping Factor (sometimes written as DF) is one of those "bigger is better" numbers.  There are a lot of other things that affect the system Damping Factor (when you add in the speaker and the wire that hooks it up).  If you see an amp with 200 or better, you're in good shape.  Don't look for this spec on a budget amp...it would reek if they printed it.
• Crosstalk - This one is actually pretty easy to understand.  On a stereo (or multi-channel amp), the crosstalk is a measure of how much of the signal on one channel gets into another one.  Again, the more negative the number the better.  I only pay attention to this spec if the amp is going into a critical listening environment (like a studio).  But live...all the noise coming off the stage is going to mask the minor amount of crosstalk in the PA system.  As long as it's at least -40dB I'm not gonna sweat it for a PA.
• Input Impedance - This is almost useless to spec now unless you plan on hooking up 10 amps in parallel.  Back in the 60's when stuff had real transformers in it for the audio, this mattered.  Now...no so much.  It's just another big number to confuse the average user. 

I realize the last couple of weeks have been really tech-heavy.  Those of you who got through this...kudos.  Regardless of how much of this sank in, I hope this helps those of you buying equipment in the future.  Again, I hate seeing people get ripped off or wind up with less than they thought they were getting.

Next week...I promise no (OK...very, very little) math.  We'll talk about working with sound and lighting companies.  I recently played a show where we hired in the production (and man oh man was it nice to not have to pack a PA at the end of the night).  A lot of what we'll discuss also applies to playing on house systems at a club. 

Until then, keep the meters out of the red.

Ken

Ken Carver has been a musician and performer since the early 70's, and involved with live music production since the mid 70's. He worked for 15 years as a broadcast engineer, building numerous studios and transmitter sites around Texas. He's also worked in Critical Care Communications for the medical industry, R&D for an automated lighting manufacturer, and owned Project Lighting & Sound in the 80's. He currently heads up an R&D Hardware Technician Team at National Instruments in Austin, and still performs on the weekends in the Central Texas area. You can reach Ken at itsjustlogistics@gmail.com