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

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

Watts The Meaning Of All This

I'm going to let you in on a little secret about most consumer grade electronics manufacturers. They're counting on most people having absolutely no idea what all of those numbers on a specification sheet mean. They're banking on the fact that most people believe a bigger number is a better number.

Now before you beat yourself up over this, just keep in mind that unless you've got an electronics background and understand how specifications are derived then terms like S/N ratio and Total Harmonic Distortion are just buzzwords.

Furthermore, consumer grade audio manufacturers are really bending the rules nowadays...all in the quest of that all mighty big number. The pro grade guys...not so much. Because the guys that are ready to spend a few thousand dollars on a power amp can spot a B.S. spec in a split second.

So for the next couple of weeks, I'd like to impart a little bit of this inside information to you. We'll talk about what certain specifications are, whether a bigger number is actually better (not always), and how a spec affects how a piece of gear sounds. We'll also talk about the ways that manufacturers "fudge" the numbers...especially wattage.

I had originally planned to cover all of the major specs of a power amp in one article, but after I started writing this I realized that it would wind up being a very long post.  So we're going to split it up.  This week, we'll talk about wattage and power ratings.  Next week, the other specs.

Which Watt?
The output power of an amp is the most manipulated, massaged, and maladjusted spec on a power amp.  After all, that's the first thing people look at.   "Ooooo....2000 watts for just $100...I'll take it".  Yeah, you'll take it all right...but I'm not going to say where you're going to take it.

First, let me clear the air here. A watt is a watt...period. There are no "tube" watts, "solid state" watts, "British" watts, or "American" watts. Anyone who uses these terms falls under the category of "repeats whatever they read on bathroom walls". The formulae for calculating power does not contain a variable for any of these terms (but maybe there should be an "moron" variable). You hear these terms used with guitar amps mostly, but be aware they're out there and they're bunk!

There are several variations of the power formulae, depending on the variables that you know, but at the end of the day it boils down to this:

Power (watts) = Volts * Amps

 

 

Like I said, there are variations of that if you know the load that the amp is operating into.  The most frequently used is:

 

 

Power (watts) = Voltage Squared / Resistance (ohms)

 

 

You'll see that one written as P= E^2/R, and we'll be using that one later.

So with that said, lets look at a good set of wattage numbers, and then some not so good.

In With The Good
I consider Crown and QSC to be amoung the best amp manufacturers out there.  You'll see their amps on a lot of pro touring rigs, and the care they put into those high end products are reflected in their consumer grade stuff.  Since we've been talking about the Crown XLS 802 for the last couple of weeks, we'll give 'em a little more free advertising.

These are the average power numbers and are very conservative.  Most amp manufacturers use RMS (root-mean-square) power (which would result in a slightly higher number). Either measurement is a good indicator of what the amp can pump out all day long without breaking too much of a sweat.  Crown specs the frequency, the distortion level, and the loads they test into.  If it looks simple, it's because it is.  These are honest numbers without any trickery involved.

Taking A Peek At Peak To Peak
Prior to the mid 70's, amp manufacturers had no guidelines on how to spec power.  So how did they do it?  Easy, by using methods that would make for the biggest numbers! So you saw wattage ratings expressed in either Peak Power, or the notorious Peak To Peak Power.

So what are these numbers?  This is definitely one of those cases where a picture is worth a thousand algebra equations, so let's look at a sine wave:

The point at which the waveform crests and reaches it's maximum (or minimum) value is the peak.  Peak to Peak is 2 times that.

So lets say that I'm testing an amp into a 8 ohm load (typical for a loudspeaker), and I measure the peak value at 28.28 volts (there's a reason for that weird number).  OK, so how much power is that?  If you plug in the numbers you'll see:

 

(28.28 * 28.28) / 8

 

800 / 8

 

100 watts

 

 

So, the peak power of this amp is 100 watts.  Any guesses as to the Peak to Peak power?  Anyone?  Yep, 200 watts.  Sound pretty impressive, doesn't it.  But there's a slight problem with this number.

 

 

Not So Peak Performance

The problem with the Peak power numbers is that for most amps they're not sustainable.  What I mean by that is they can produce that power for a very short period of time (a few hundredths of a second).  If you tried to produce that continuously, you would burn the amp out pretty quickly. 

 

Looking at the sine wave above...you can see that the amp actually spends very little time (relative to the rest of the waveform) at the peak value. 

 

At some point in the mid 70's, the Federal Trade Commission finally stepped in and put some rules in place about how manufacturers can specify the power of their amps.  And with that came two new values...RMS and Average. 

 

Sustainable Power

RMS and Average values are what an amp can produce over an extended period of time without destroying itself.  So...what are they and how do they compare to the Peak numbers?  Glad you asked:

 

 OK...so that graph looks a little intimidating, but here's the bottom line.  The RMS (root mean squared) power is about 71% of the peak, and that is by far what most amp manufacturers use.  The average power is even more conservative, and is about 64% of the peak.  Crown uses the average power, but then again they've always been very conservative with their specs.

Hey Man, What Happened
So lets use what we just learned on the example I gave earlier.  Prior to about 1975, a manufacturer could have advertised the amp in my example as a 200 watt amp, and not told you how they came up with that number other than to say it was Peak to Peak.  But what can this amp really do?

First, anytime you see Peak to Peak, divide it in half.  Again...it's a BS number.  Now, take the 100 watts you have left and multiply it by .7, which leaves you with 70 watts.  That's a much more realistic number.  But what's more impressive...70 watts or 200 watts (sounds like one of those phone commercials with the kids, doesn't it)?  So now you know how the game is played.

Full Disclosure
A wattage rating alone is useless without knowing at a bare minimum the load you're operating into and the distortion.  The example from Crown, that's a complete spec.  But that's not so much the case with our next contestant.  And I don't mind calling them out, because Pyle is one of the worst offenders as far as manipulating their numbers and playing off the average consumer's lack of electronics knowledge.

What A Pyle Of...
OK, we saw a good wattage spec.  How's about something from the other end of the scale:

Two things to point out.  First, they are not doing anything illegal.  Second, keep in mind that they are advertising this amp as a 1,400 watt product.

Under the section for "Continuous Output Power", they list the amp as being able to produce 70 watts per channel into an 8 ohm load, and 110 watts into a 4 ohm.  That alone tells me that the power supply for this amp is under designed.  On a decent amp, it would produce about twice the wattage into 4 ohms as 8.  On this amp, the power supply runs out of gas before then.  That's a red flag.

There's another flag on this as well.  In the 8 ohm mode, they spec that the amp can deliver 70 watts over the full audio bandwidth (20Hz to 20kHz).  But, on the 4 ohm ratings, the 110 watt figure is only spec'd at a single frequency.  So that does that mean?  It means that the distortion figure likely goes to hell at frequencies above and/or below this.  That's a bad sign.

Anyway, we have power and load.  The Total Harmonic Distortion (THD) is listed as 0.1% at "rated output power".  So, that infers that it's at the 70 watt / 110 watt figures.  But then they throw out "Maximum Power" for the two loads.  And guess what, there's no definition for "Maximum Power" other than what they define.  Most likely, that wattage is at a higher distortion value...probably 1% (which still isn't terrible).

And then there's that Peak Power....which I'm at a loss on how they came up with.  If you take the 100 watt figure as an RMS value and work the math backwards, the Peak value I come up with is 142 watts.  So how the hell did they come up with 700 watts?

Hey Rocky, Watch Me Pull A Spec Out Of My A$$
Here's how they did it.  If you measure the power supply rail of the amp at idle, it's likely coming in at around 75 volts (a typical value for a 200 watt amp).  Now, if you plug that number into the power formula we used earlier, then you get:

(75 * 75) / 8

5625 / 8

703 watts

 

 

So in effect what they are saying is that the amp has the "potential" to produce a peak output of 700 watts.  However, there's a whole lot wrong with this:

• As soon a you put a signal in the amp, the supply rail is going to drop a bit
• Even if the supply rail didn't drop initially, it will in a few thousaths of a second
• Assuming they've got a really good power supply that maintains it's voltage under load(which they don't), you're going to have switching losses through the output transistors.

So the 700 watt spec is valid...until you actually try to amplify something.  That's real useful...not!

 

You should be on the lookout for other obscure power ratings.  There's no official industry definition for "Music Power".  Some manufacturers will make that 6 or 7 times their RMS and then come up with some tech-speak about non-recurring non-simultaneous power excursions in typical program material.  Screw you guys...you're trying to sell a 200 watt amp (on a good day) as a 1400 watt amp.

Levelling the Playing Field, And Then Not
As I said earlier, the Federal Trade Commission standardized how wattage could be advertised around 1975.  Amp makers either played by the rules or got fined for false advertising.

However, in the last couple of years the FTC appears to be bowing to pressure from electronics manufacturers.  They are now considering no longer requiring the distortion figure in print or radio advertising.  Their "rationale" behind this is a crappy as Pyle's power numbers:

• Most equipment produced today has a low distortion value.
• Most consumers don't understand that number
• Consumers that want to know the figures can find them on the Internet.

As to the first point, it should be noted that most equipment made today is capable of low distortion.  But if you're going to advertise it at a power level that 10x of reality...that goes out the window.  The other two points, I unfortunately can't argue with.

A Dead Give Away...
I've gotta give Behringer the "Most Exaggerated Wattage" award for their new iNuke series amps.  And here's why.

There's a basic law of energy conservation that says you can't get more power out of something than you put into it.  Keeping that in mind, and the fact that a watt is a watt regardless of how it's being used take a look at this spec sheet for what they claim is a 3000 watt amp:

There are a lot of numbers here, but I've highlighted the two of interest.  First is the claim that the amp can produce 1500 watts per channel into 2 ohms (and don't get me started on 2 ohm load measurements).  Then look down at the Power Consumption at 2 ohms...they're stating that it's 350 watts total.

If there's one number you can usually trust on a spec sheet, it's the power consumption number.  You see, if the manufacturer wants to get a UL (Underwriters Laboratories) sticker or equivalent, they have to state the max power the amp can draw under worst case conditions. 

So, what Behringer is saying is that they can put 350 watts into the amp, and get 3000 watts out.  Really?  Where did the other 2,650 watts come from?  OK...here's how they do it.

They're using the same trick as Pyle, and basing that 1500 watt per channel number on the power supply rail voltage (probably about 110 volts) The power supplies used in light weight amps (switching power supplies) can deliver a lot of power instantaneously, but can only do it for maybe a 1/10,000th of a second.  And then that's it...it's out of juice until it recharges on the next power cycle.  So it's a completely useless number other than for marketing.  If someone could make a device that could continuously output more power than it took in, they wouldn't be jacking around with amps.  They would be making trillions of dollars selling cheap energy to the world.

The bottom line here is that you can't have more continuous power out than you take in.  If you read anything to the contrary, it's more marketing BS.  In fact, the true power output of an amp should always be less than the power it draws from the AC outlet.  No amp is 100% efficient.  Some really good (and expensive) ones approach 90%...but most are in the range of 50% to 70%.  So, expect a true 500 watt amp to draw between 700 to 1000 watts.  The difference between the two figures is what's lost as heat.

Oh, and what do our friends from Pyle list as the power consumption for their "1400 watt" amp:

Wow...it's blank! Go figure.  Losers....

I realize we've covered a lot of stuff today, but to me this is important stuff.  I'm really tired of seeing decent working musicians spend their hard earned money with these companies thinking they're getting 3000 watts of power only to find out it's really less than 1/10th of that. 

In fact, it bugs me so much I'll make this offer.  If you're considering buying a power amp and you're not sure of what you're getting, then email me at the address below.  Either tell me the make and model or the amp or send me a PDF of the spec sheet.  I'll read it and tell you what you're really getting, not what the magazine ad wants you to believe.

Next week, we'll wrap up our discussion on specifications and how they impact the way an amp sounds.

Until then, keep the meters out of the red.  Especially if you don't have as much power as you think!


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

Nice Rack...Mount Equipment

When we left off last week, I was troubleshooting a mid 2000's Crown power amp that had decided to not work for a customer at a gig.  It turns out that the pots were a little dirty and the XLR input connections had fatigued.  A shot of DeOxit, some solder, and deionized air to blow the dust out and this bad boy is good to go.

While I had it apart, it really admired the construction techniques used.  Even though this amp was built in China, the attention to detail and a few little tricks to make the amp road worthy really stood out.

People talk about why one piece of gear is better than the other.  Sure, electrical specs are a big part of it, and we'll talk about how to actually read those in another installment.  But the bottom line is that if you're a working band and moving your gear from gig to gig, it's got to be able to stand up to that.  I've seen some equipment (designed for home use) that sounded fabulous, but wouldn't survive 10 miles in a U-Haul...even in a road case.

So this week I thought I would point out just what makes a piece of equipment "nice".  And even though we're just looking at a power amp, a lot of these points apply to any piece of electronic gear you're going to haul around.  And this is not an exclusive endorsement for Crown...there are a lot of other companies today building some very good gear at reasonable prices for the average working musician.  But on the other hand...there are a lot of 'em that are building some really bad stuff.

OK students...what is the #1 enemy of electronics?   What?  Who said heat?  That's right...proper system cooling goes a long way toward extending the life of the gear.  So that's usually the first thing I look at.  On that note, starting at the front panel notice the large vent for air flow:

 

And those aluminum fins that are sticking up...heat sinking for the output transistors.  The cooling fans are located directly behind them.  What all that adds up to is a straight path for the air to flow and take the heat out of the amp.  This is called back-to-front cooling, and just about every amp over 100 watts / channel uses that now.  But that wasn't always the case.  When I built my first really big PA (2,000 watts) in the late 80's, I used Peavey CS-800s.  They had a fan in the back, but exhausted the hot air out the sides of the amp.  Put 'em in a rack, and guess what...you're recirculating the hot air!  I had to hack up my road racks to get them to exhaust correctly.  It's something to watch out for.

Moving around to the back panel, check out the speaker connections:

This amp can output up to 800 watts per channel...far more than a 1/4" connector can safely carry.  Crown was one of the first companies to move to Speakon connections which are far more secure and can carry a whole lot of power.  Binding posts also give you the option of using banana plugs or wire lugs.  This is all standard on high power amps now, but this was cutting edge in 2006.

Moving across the back panel, Crown used two cooling fans for this amp:

Two fans give coverage to the entire heat sink field in the amp.  Electrically, they're two speed and thermostatically controlled.  They run at half speed (which makes 'em pretty quiet) until things get hot and heavy, and then they cut over to full speed.  And in the event that the amp still overheats due to a fan failure or extremely high ambient temperatures, the control system will shut everything down before any major damage can occur.  You're not going to find that on a cheap Pyle amp!

Inputs are pretty straight forward, a pair of balanced XLRs.  This is one place I've got to pick on this amp...it would have been really nice to have a switch that parallels the inputs.  In other words, let me plug into one channel, but feed both of them (useful if you're using the amp for subs).  I'm not crazy about the plastic housings on the XLRs, as those 4 screws you see (that go into the plastic) are what are holding that panel in place.  A couple of them were stripping out, so I used a filler on the holes to give the screws some new meat to bite into.

But you know what makes up for my gripes with the XLRs?  Look under the AC power connection.  That's not a fuse, it's a circuit breaker!!!!  That's right, if something happens and you pull too much power, this trips.  No fumbling around to find a replacement fuse...just clear the fault condition, reset the breaker, and you're back up and running.  In bulk, these really don't cost that much more than a fuse and fuse holder...and I really wish more manufacturers would go this route.

Now, there's a few things I want to show you on the inside of the amp.  If you remember from article I did a few weeks ago about rebuilding a bass amp, I talked about how the power supply is critical to the proper operation of the amp.  Check this out:

There are two things to point out here.  First, on the left, are the bulk storage filter capacitors.  There's about twice an many as theoretically required.  And that white stuff between them...industrial adhesive.  Components that are tall have a tendency to physically "rock" (not in a good way), and that can result in lead breakage and failed solder joints.  Gluing them all together helps prevent that. 

To the right, that round thing...it's a toroidal transformer.  This is what steps the line voltage down from 120 to the various voltages the amp needs.  Toroidals, because of their construction, have a very small magnetic field around them when compared to a standard rectangular transformer.  So why is that a big deal?  Easy, magnetic fields can induce hum into nearby audio circuits.  A transformer like this makes for an amp with very low background noise. 

There's one other construction note on this photo.  Right below the capacitors, you'll see a row of white rectangular objects.  Those are power resistors (we'll look at some different ones next).  These essentially balance the loads going to the output transistors, and they can get hot.  By selecting this style of resistor (which is relatively expensive), the heat is not going back into the circuit board and deteriorating it.  And notice a bead of silicone across the top of them...again to stabilize them and keep them from moving more than they should.

Finally, there's some construction points that the designers really got right on the circuit board.

There are three things I want you to notice here.  In the upper left, there are two power resistors (the grey cylinders with the stripes).  These are electrically the same kind of thing as the white rectangular ones above, but in what we call an axial package.  They're going to get hot as well, but if you look closely they're not sitting down on the circuit board (which would eventually scorch it).  They're set up off the board where they won't do damage and will radiate heat better.  That's a small thing that a lot of manufacturers miss.

In the top center is a ribbon cable that connects the front panel (pots and LED indicators) to the main board.  Remember last week when we had trouble with a ribbon cable because of the connector?  You won't see that here...the ribbon is soldered in!  It's more expensive because it increases your manufacturing build time, but you won't have a connector problem.

And finally, this is something I've only seen super high end touring amps.  See the red and black wires that are going into the board?  Those connections are stiffened with adhesive.  Otherwise, those wires would eventually break at those points (I've repaired plenty of them).  This cost a fraction of cent to do, but improves the reliability immensely.

This amp has a lot of other things going for it electrically that you can't see.  Things like a soft start turn-on (it reduces the inrush current when you power up), and a 3 second delay after turn-on before the speakers are connected...so you don't get that loud "THUMP" when you're bringing your system up.  And sonically it's just plain good,  but that's all in the specs.

So how about next week we talk about amp specs...what is THD, sensitivity, and damping factor?  How do these specs impact what the amp sounds like?  And probably most importantly, I'm going to show you how to spot a bogus wattage rating from a mile away.  You won't need a calculator, but if you're planning on drinking heavily next Friday night you might want to wait to read this one until Saturday afternoon.

Until then, keep the meters (or LEDs) 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

Change The Oil...Rotate The Tires

In my last article, I had mentioned that I was going to write on lighting for a band.  Well...two things happened.  First, it's been pointed out to me that very few bands on the circuit I play now actually carry any lighting (their loss).

Two...you have to keep in mind that I built automated lighting for 8 years, so once I started into this it became an endless string of acronyms, buzz words, and technical jargon that at times required trigonometry.  And seeing that this comes out on Saturday morning (which is prime hangover time), I didn't want to risk exploding any heads.  So, maybe another time.

In the last couple of months, I've gotten back into servicing other peoples gear (albeit on a limited and referral only basis).  It starts off with "hey...I heard you know how to fix amps...can you look at mine?"  The next thing you know, I got gear waiting to be looked at. 

As many of you know, I'm an Engineering Electronics Technician by trade.  At my day gig, I'm dealing with very low power, and units of measure that are ridiculously small like nanovolts, picoamps, and femtoseconds.  That's all well and good, but when I get the chance to get my hands on something in the range of hundreds of watts...well, this is where you insert that Tim Allen grunt.  I'll admit it...there's a certain rush to working on a piece of equipment that has the potential to kill you!

Lots of gear snobs go on and on about how good the "vintage" amps from the 60's sound.  And some of them are really great.  If I had a few thousand dollars laying around I would love to score a mid 60's Marshall JTM-45.  But I would never take it out...so much of that stuff from that era was so fragile. 

It was in the late 70's that manufacturers figured out how to build good sounding amps that could stand up to being thrown in the back of a van and driving down 200 miles of washboard road by a ticked off roadie who has been up for 3 days on bathtub crank.

Recently, I've had a couple of bass amps from the 80's come across my bench...an SWR and a Gallien Kruger that I'm finishing up today.  Both of these amps represented some really outstanding engineering and manufacturing processes.  I mean, we're talking 30+ year old equipment that's been working...not sitting in a closet some where.

Both of them just needed some TLC to their power supplies.  The GK had a blown output transistor, but it was one of the originals from with a date code on it from 1984!  The owner really loves this amp, so we're completely rebuilding the output section in addition to the power supply.  This amp will hopefully run another 30 years.

Which brings me to the heart of the article today...what can actually wear out in a piece of gear? 

First, let's get the vintage thing out of the way.  If you have a 30 plus year old amp that gives you a tone that defines "your sound", then by all means keep it running.  It pains me to know that thousands of pieces of vintage instruments and gear have been bought up by foreign investors and have left this country.  So anything we can do to keep part of our history here is OK by me.

Stuff Wears Out
In general people think that tubes in amps are the only things that wear out and require replacement.  Solid state amps (like most bass amps) just keep running forever.

WRONG!

Even the solid state stuff will wear out after time.  You want to know the number one enemy of electronics?  It's heat.  Yep...it wears things out more than you realize.  This is a real generalization, but for every 10 degrees Celsius (18 Fahrenheit) that you can reduce the temp on electronics, you double the life!  Yeah, you read that right.  This is why tube amps typically need to be serviced more...they just run hotter.  And this leads to...

The Power Supply Is Everything
Without adequate and clean power, the rest of the amp can't perform.  I've seen some brilliant designs that were compromised by cheaping out the power section (although one amp, the Vox AC-30, actually capitalized on this as part of their sound).

Here's the power supply from the GK amp I'm working on now:

All of those blue cans...those are the electrolytic filter capacitors, and that's the primary component that wears out (or should it say drys out) in amps.  Yeah, there's actually a liquid in those things that makes them work.  Their job, when they're working right, is to take all of the ripple and junk out of power that's going to amp.  When they're not working, you'll notice an increase in hum in your amp even when all of the controls are turned down. 

They're also used between the stages in your preamp to couple the audio through.  When they start failing there, your sound tends to start getting thinner.  In tube amps, your gain is all off because the tube stages are getting voltages there weren't designed to take (the technical term is "incorrect bias").

These things aren't the cheapest things in the world ($5 to $10 each, depending on the value and size), but if your tube amp is over 10 years old or solid state amp is 15 years or more, it's probably time to change these things out.  If you let them completely go, they can eventually short out and take out a lot of other stuff in the amp.

Hey Man...You Got Any Scratchy Pots?
Noisy potentiometers (you know...those knobs on the front of the amp) are a very common complaint.  Now, if you take your amp in for service, a good technician will clean these for you while he has the amp open (and I'm a good technician).

But whether it's on your amp, one of your guitars, or an effect this is something you can take care of yourself if you're so inclined.

First, I'm guessing that most of you don't know how a pot actually works.  Well, it's not really that complicated.  And it doesn't matter if it's servicing as a volume, tone, balance, or whatever control...the hardware is pretty much the same.

This is what's going on inside of potentiometer...or as they're also know a variable resistor:

Not the greatest picture in the world, but the important parts are that black semicircle and the wiper arm.  The semicircle is called the resistive element.  It can be carbon, conductive plastic, or a number of other proprietary things.  The wiper arm is what actually moves when you turn the knob.

If you get a bunch of dust / dirt / nicotine / body fluids on either the resistive element or the wiper arm at the point of contact, you get that oh-so-annoying scratch when you turn the knob. 

So, how do you clean it?  Well, the good news is that most pots have an opening in them, like this one:

That slot right under the terminals is where you can shoot some cleaner in.  And speaking of that, my favorite hands down is DeOxit D5.  You'll pay about $15 for a can of the stuff (Guitar Center sells it, along with Fry's, and some Radio Shack stores).  Squirt a little in the slot, then turn the knob all the way back and forth to distribute the cleaner.  If it's really dirty, shoot it again and repeat.

If you're doing this in an amp, make sure it's been off for a while and unplugged.  DeOxit isn't conductive, but you shouldn't be monkeying around in a powered amp unless you're looking for trouble (literally).

If you have slide pots (like on a mixer or EQ), just shoot it in the front and move the slider.  There, that was easy.

If your pot is still scratchy after this, it's likely that the resistive element has worn out (it happens).  At that point, you're looking at replacing the part.

DO NOT USE WD-40!  I've made a small fortune replacing pots (especially the slider type) because someone thought it was a good idea.  Don't get me wrong, WD-40 is a great product...on case casters.  But not electronics for two reasons.  First, it attracts dirt, and that's what we were trying to fix in the first place.  Two, it's reactive with some modern conductive plastics.  I once replaced 32 very expensive slide pots (these were $50 each my cost) on a guy's mixing console thanks to WD-40.  What's sad was he knew he was supposed to use something like DeOxit, but the WD-40 was "so much cheaper".  Yeah, right.  I think that total bill was almost $2,400.  Tell me again how much you saved?

Speakers....not usually.
Here's something that doesn't wear out unless you blow them, and that's speakers.  Sure, the cones can be damaged.  Sometimes the glue holding them together fails.  And although you typically don't see speakers with foam surrounds in pro audio gear, occasionally I come across one where the surround has rotted away.  In all those cases however, the speaker can be saved with a recone.  They take the frame and give you a new cone, voice coil, and dust cover where applicable.

But here's what cracks me up, and that's when someone claims that their speaker's magnets have lost their power.  I'll ask them if they've had an MRI done on their amp, or if they've been close to a nuclear detonation (in which case we wouldn't be having this conversation).  In every case so far, then answer has been "no". 

Good, it ain't the magnet.

In full disclosure, I used to wonder about this too.  So I called a friend at Electro-Voice, another at Meyer Sound Labs, and emailed an engineer with Celestion in the UK.  I got the exact same answer from all three, which I will paraphrase here.

 - When you initially power a speaker up, it will loose about 1% of it's magnetic flux
 - It will loose another 1% during the first year or so of operation
 - It will take about 100 years of continuous operation before it looses another 1%!

Yeah...100 years!  So OK, maybe Keith Richards' amps are getting close, but none I've come across.

Here's The Good News
I can say that audio techs and amp repair guys are (in general) the most honest service people out there.  TV Repair, automotive, appliance...I've had attempted rip-offs by all of 'em, but never another amp tech.  I guess I attribute that the fact that most of them, like myself, are musicians.  They understand how import your gear is to you, and know that 99% of us are not swimming in money. 

A good tech will walk you through everything they've done, and hand you the parts they pulled out when you pick up your gear.  Just things to keep in mind.

Next week ought to be interesting.  We'll talk about fixing stuff, but not equipment.  Let's talk about taking care of ourselves.  I've been fighting shoulder and arm pain in my right arm since 2009, and haven't found answer for what's going on.  Recently, I've been very lucky to meet a musician who is an Orthopaedic Physician's Assistant by day.  I'm going to see him next week, and want to share what we find out.  Because after all, you can have the best gear in the world...but if you ain't working....

Until next week, keep the meters out of the red moving!

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