Gain Staging?? Care to expand...

I wrote an article for Bass Player back in 1997 or 1998. I'll have to dig it out from a backup disk.

I wrote a dozen articles in my "Studio Time" column from 1997-1999.

 

I thought I recognized your name!

 

Thanks, Jimmy! When I was at Apple, one of the guys in another software group was reading BP on an airplane and saw my photo. Surprised him.

 

Here it is. It is probably still owned by BP, but what the heck.

I see the theme here was using a mixing board, but the concepts of gain still hold true for your signal chain and amplifier. We tend to set input gain just below clipping on loud notes.

Gain Staging [c 1998 Music Player Publications, used without permission; this ran in the Feb 1998 edition]

In my first column (“The Signal Chain,” April, 1996) I used the term gain staging, mentioning that it had to do with signal levels throughout the chain; I also felt it was important enough to merit a separate discussion. Now’s the time to talk about it. Quite simply put, gain staging is the process of minimizing noise while maximizing gain throughout the signal chain. It’s a fancy name for getting all the level controls set properly.

Oneness
In any discussion of gain, it’s almost impossible to avoid mentioning unity gain. A circuit provides unity gain when it neither amplifies nor attenuates a signal. Wire has unity gain, if you ignore the effects of cable resistance, capacitance, and inductance along its length. An amplifier circuit built for unity gain is sometimes referred to as a buffer; its purpose is usually to add additional current drive to a signal without amplification. Within a mixing board, many of the summing amplifiers mix incoming signals at unity gain.

Most circuits with a gain knob, level pot, or fader have a setting at which they provide unity gain, but these points aren’t always clearly marked. For most faders, a setting of 7 on a scale of 10 is generally considered the unity-gain point. The Mackie 1604 mixer even has a detent at the faders’ unity-gain point.

Because a unity-gain circuit isn’t amplifying the incoming signal, it also isn’t boosting the incoming noise with that signal. Any noise it adds to the signal is generally worse at higher gain settings. It also won’t help to overload the next stage. When we consider noise and distortion along with the signal on multiple tracks, we start the process of gain staging.

Passive Instruments
A passive instrument, one with simple volume-cut and treble-cut controls, operates at unity gain only when the controls are all set to their full-open settings. At any setting less than full, the pots create a negative-gain circuit, because they cut the signal level. (The tone control is also a negative-gain circuit, but in only the treble portion of the audio spectrum.) So playing with passive controls anywhere except full blast simply serves to cut your signal–and this means the gain may have to be boosted at the mixing board.

When you boost the gain at the mixing board, however, you’ll be amplifying not only the instrument’s signal, but all the hum and noise picked up between the pickups and the board–in the instrument cable, DI, mic cable, and even the mixing board’s preamp. If you were to take this scenario to the extreme and turn the instrument’s volume way down, you’d have the board’s gain cranked wide open and get almost pure background noise. For these reasons, I record passive basses up full and adjust the tone in the mix. When I play live, I set the amplifier so “wide open” is slightly louder than normal; I then roll the instrument volume control down a bit, keeping the last notch in reserve for the occasional volume bump.

Setting The Stage
Setting the gain sensibly at every stage of the signal chain isn’t hard, but it does take some thought and a little practice. In most cases, you can develop rules of thumb to set gains quickly and accurately. The general idea is to avoid setting gain controls very high (above 7 out of 10), because this is the region where the circuit will usually get noisy. To avoid doing this, you’ve got to set the gains before and after so that everything is in an optimal place. Whenever you adjust gain, you should check that your change doesn’t adversely affect other stages in the chain.

When setting up a mixer channel, start with the input gain down and the fader at unity gain (“7” if there’s no marking). With the EQ set neutrally, bring up the input gain to get the signal up to about 0dB on the meters. At this point, you’ve got the fader at unity and the input preamp providing enough gain to get the signal up where it belongs. There should be no internal distortion within the board, because the signal isn’t being amplified anywhere after the preamp. If you adjust EQ, though, remember that the EQ circuits will either boost or cut the signal, changing the gain structure already set up. A big EQ boost can cause distortion down the line, because the EQ is amplifying the signal too much; it’ll overload the next stage. It’s usually better to cut the frequencies you don’t want, then bump the overall gain–you’ll hear the same effect as a boost at the desired frequency. Don’t crank the midrange to hear the guitar: cut the low-end mud and turn the whole mix up.

Outboard Gear
Most signal processing units have input and output gain controls: here again, it’s important that they be set properly. Consider a reverb unit in an effects loop. We can probably adjust the signal level going out the loop, adjust again at the reverb input, adjust again at the reverb output, and one more time at the loop’s return point. The usual drill is to send a clean, hot signal out the loop, cut it as needed at the reverb’s input gain control, keep the reverb’s output at unity or a little hotter, then cut again at the loop’s return. This lets us always send hot signals down the cables, and by cutting at inputs we also cut noise pickup. By not cranking up any output-gain controls we avoid adding noise to the signal.

Proper gain staging is essential for clean, undistorted signals. It takes a little time to do it right, but the payoff is well worth the effort. When you’re mixing sixteen or more tracks, every little bit of noise adds up.

 

Uh-oh, now you did it! Here come the copyright police!

That's a terrific article that pretty well sums it up. No wonder you wrote for BP. I might add that this is EXTREMELY important when you're using home recording equipment, as a lot of it tends to be on the noisy side.

 

Thank you, sir, for the kind words. It was rare that they did any editing; I was surprised when I would read the articles in print.

Hopefully the print-copyright police aren't as nasty as RIAA...

 

Rick, why don't you post this over in the other forum? I'm sure they would appreciate it.

 

In the Eden forum, Al?

 

Many thanks...a nice, succinct summary of the issues.