How can I prevent clipping when using a microphone with a tube amp into a PA?

Question

This weekend I played electric guitar through a 25 watt tube amp through a PA system. This was the setup (telecaster > tube amp > microphone > PA). The sound guys asked me to turn my tube amp down to prevent clipping. However, tube amps have to be played at a higher volume to get the best tone. I bought a low wattage amp so that I could turn it up and get that sweet tube tone. How can I prevent clipping when using a microphone with a tube amp into a PA, without turning the volume down so low that the tubes don't sound their best?

Answer 1

As already said, mic-preamp clipping cannot possibly be an issue if you use any amp that's legal to operate without a strategic weapons license, and the sound guys know what they're doing. I would add three possible things to joseem's list, that might be the real issue:

4. The sound guys were using the same channel for different purposes (e.g. before you for an acoustic guitar with low-output DI), and didn't want to recalibrate the gain. That boils down to lazyness on their part.

5. The sound guys were pure classical-music tonmeisters and are adamant that all distortion is wrong. Note that amp overdrive is in fact clipping too, only, tubes clip much more subtly and guitar cabinets are designed so this is no actual problem (indeed it's of course desired in many styles!). Still, if you considered a guitar amp a pure sound-transmission device, this distortion would be a technical error. This was certainly a problem many of the early psychedelic bands had: record companies would send back demos with extreme distortion and feedback as “damaged”. But... that any sound guy today – classical or otherwise – would still be oblivious of artistic use of distortion, seems highly unlikely.

6. Perhaps not quite so unlikely: the sound guys outright lied at you. They just wanted your amp quieter, and used the clipping pretence so you couldn't be personally offended at this. Granted, 25 W is on the low end of the amps power scale, but these can still be pretty loud. And as a sound engineer, you usually want the sound level on stage as quiet as possible. Perhaps you had a singer who struggled hearing themselves in the monitors? Or some hard-to-mic acoustic instrument?
   These are good reasons not to have even a 25 W amp turned full up bleeding in the mics, and it's quite possible that the overall sound did in fact benefit from turning the amp down. Tube-powerstage distortion may be the ne plus ultra in terms of guitar sound, but overall a cheap amp with transistor power stage may still serve a band better if it can be better balanced in the mix.

Answer 2

Get better sound guys.

Clipping is an electronic phenomenon when the input signal is too hot for the circuitry, so the tops and bottoms of the waveform are getting shaved off. (This is bad because speakers don't like constant voltages at anything other than 0V.) Microphone signals are far below the line level that the mixer is operating on, so the only way this could be happening is if the microphone preamp is set too high. Any sound guy who knows how a mixer works should be able to make the appropriate adjustment to bring the level down to within bounds.

The only other thing that could be happening is that if the microphone was being overdriven by sheer volume of output from your guitar amp. This would be a mechanical phenomenon (at the point of the element in the microphone that picks up sound vibrations from the air), not an electronic one, and would result in a bad sound from the mic no matter how low the preamp is set. But this shouldn't be called clipping, it should be called a broken or bad choice of microphone.

Sound techs routinely mic 100 watt cabinets turned up to 11 with a microphone placed right next to the speaker cone. There's only so much energy that can be transmitted over an air gap to a microphone capsule, and the mechanical limits of a microphone capsule should tolerate all reasonable ranges of this.

One more thing. It's possible that vibrations from your amp were being transmitted to the capsule through the floor and mic stand to the mic, or from the front grille of your amp to the capsule if they were physically in contact with one another. The air gap is important, and if the floor is a problem this can usually be solved with a piece of foam under the mic stand, but I've rarely seen this to be necessary. Some sound techs will actually suspend a mic in front of a speaker cabinet with the mic cable itself, just taping the cable to the top of the cab.

Answer 3

Clipping in this situation could be caused by:

1) the microphone not being the most appropriate to the task at hand, and not being able to capture your audio without, well, clipping. Positioning the mike further away from the amp could possibly lead to some working solution, but could bring other problems, like capturing other sources or ruining your sound any way (as mike distance and positioning in relation to the amp are very important factores in the overall resulting sound). So ensuring that a proper mike for guitar amping (the Sure SM57 would be the default) is used is the only sure proof solution for this part of the problem

2) the PA system is not very good and is not able to deal with the incoming signal. I find this highly improbable, as any mixer worthy of the name will have a combination of inputs for different levels of input signal and an input gain control for each chanel. If the input signal is to loud, you just lower the knob, that's what it's there for.

3) The PA guys didn't have a clue to what they were doing, didn't understand what you were trying to achieve, or didn't care (or any combination of these).

Answer 4

Two options not mentioned so far are:

• Run another amp and cab offstage and mic that one. This does require a signal splitter, so you can get the tone you want on stage, while doing something different offstage. This doubles the kit, so can double your costs, as well as requiring somewhere to mic up that other amp...
• Use an attenuator (often called Power Brakes) before your cabinet, so you still run your power amp hot, but don't drive the speaker cones as loud. These can be really good (look for Marshall Power Brake, or Gibson Power Stealth)

These both require a bit of extra kit - an attenuator isn't big, but it can cost a couple of hundred pounds (or dollars)

Answer 5

If your board has VU meters (input level meters), that would be a great way to determine if the mic is clipping your signal. If there is audible clipping before your meter indicates your signal is in the red or reaching 0dB then you know your mic is the problem, or the signal before the mic (which seems unlikely unless your amp is actually acting up). If the VU meter is actually in the red or above 0dB, then your gain is too high. Pull the gain back because everything the mixer does to the signal happens after gain.

Make sure your fader is at 0 or nominal, turn off all processing, effects, gating, compression, EQ, etc. before the test so you can hear clearly what the signal coming in to the board (plus gain) is like. Make your gain adjustments off the raw signal, then turn anything else back on once you have your source signal squared away. Keep playing as the board operator turns things back on so you can hear if something else is giving you trouble.

Edit: During your test you should play as hard as you expect the peak of your music to be so the gain structure is based off the peak signal, not the average.