Setting up proper gain structure throughout a complex PA or recording rig can be a pretty difficult exercise for even a seasoned veteran. It often requires the use of meters, scopes, and analyzers the likes of which most people don’t have easy access to unless they know a good, professional technician. Today’s tip introduces you to a way to accomplish most of what a tech does with a relatively simple set of tools.
First, we need to understand conceptually what we are shooting for. That is having all of our audio equipment in the chain be set up with an equal amount of headroom. This means everything in the chain should clip or distort at about the same level. Once this is achieved the user can back the level down and more intelligently use the metering built in to the equipment during normal operation. Aligning your gain structure this way helps give you the maximum signal to noise ratio and best overall performance from each piece in the chain, but more importantly from the system as a whole.
All you need is a test tone oscillator that will generate a tone of somewhere between100 Hz and 1 kHz (400 Hz is ideal), and a piezo tweeter (these can be picked up at your local Radio Shack). The tone generator will serve as a constant signal source (you can even use a synthesizer to generate a sine wave tone like this) for you to inject into the system. The piezo tweeter will serve as our measuring device at various stages of output.
Theory of operation: A piezo tweeter will generally not respond very well to frequencies below 1 kHz, and almost certainly not much at all below 400 Hz. But what happens to a 400 Hz signal when it becomes distorted by some gain stage running out of headroom and clipping the signal? Clipping, squared off waves, excessive harmonic distortion (pick your term, they all apply) is the result. Your once pure sine wave is now some other waveform with energy at frequencies far above our 400 Hz. These upper frequencies will be reproduced by the piezo tweeter. So, only when your gear clips and produces distortion will you hear a significant output from the tweeter.
Here’s the procedure. Turn your power amps off and disconnect them from their source (we don’t need your speakers or amps for this test). Connect your signal generator to an input of your mixer. Connect the piezo tweeter to the output you intend to use to drive your system. Turn up the mixer’s output until you hear a buzz coming from the piezo. This is the harmonic energy created from distortion in your mixer. Now turn your mixer down until the tweeter stops buzzing. This is the maximum amount of output your mixer can produce without distortion. This particular part of the test is the most difficult to get right because there are a number of places in the mixer where gain staging problems and thus distortion can occur. You could have distortion at the preamp simply being passed on to the output, which is otherwise running cleanly. Be very careful and aware of what you are doing. Operating the various stages of a mixer with proper gain staging is a Tech Tip in and of itself (and one we’ve covered before in inSync so feel free to search the archives).
There is a way to set your internal mixer gain staging with this procedure. You simply need to first connect the signal generator to a microphone input. Then use your PFL to solo that channel. Attach your tweeter to the headphone output or a control room output where you can feed it the PFL signal from the channel. Turn up the preamp gain (make sure EQ is set flat or bypassed) until you hear the distortion from the tweeter (it’s also a good idea to note where you hear this in relation to the onset of any peak lights associated with that channel. Now you can set the channel fader to unity gain (which is usually marked on the mixer), as well as the master output fader and/or any subgroups you need to use. Finally you connect your tweeter to the output as described above and check the integrity of the signal coming out of this last gain stage.
From here it gets relatively easy. You now have clean, maximum output coming out of your mixer. Simply move your piezo tweeter to the output of the next device in the signal chain. Using the levels already established in your mixer turn up the input and/or output of this next device until you hear that familiar buzzing from the tweeter. Then back it off as before. This is now the maximum level available at this gain stage. Continue this procedure until you get to the last device before your power amps. Then reconnect them. If they have level controls turn them all the way down. If not, back down the output of the last device prior to them. Turn them on and begin to bring up the level until you feel it’s as loud as you would ever need (this part of the test is often best done with a music source instead of the tone). Stop the playback and then turn it up just a little more to give yourself some breathing room. Make sure your power amps aren’t clipping. They usually have meters for this. If not you have to use your ears, or you might even try the piezo tweeter (instead of your speakers) on the output of the amp to measure for distortion. If you amp has too much power it will blow the tweeter so be careful with this. Setting proper power amp levels becomes easier once you’ve done this procedure and used the system enough to get a feel for its overall performance and headroom (this topic has also been covered in past Tech Tips).