Establishing proper gain structure in a sound system is important. You can sometimes “get by” if the various devices are mismatched in gain. But when you need extra level from the system, or additional headroom to keep distortion down to an absolute minimum, or a very low noise flow for recording, a mismatched system will not perform properly. Ideally, each device in the system, and that includes Input Faders, Sub Masters, and Output Faders should be set to unity gain as a starting point. That is, 1 volt applied to an input should equal 1 volt at the output. Of course adjustments to faders have to be continually made for musicality. It's perfectly OK to have a given fader at the low end of its scale – if the instrument or voice it's related to is also low in level. Some engineers will turn up the mic pre gain so that the fader can be set to the unity gain mark, but this is not necessarily a good practice, unless the mic is abnormally low in output. In any case, cranking the mic pre upwards is likely to result in increased noise and cause the preamp to overload on transients. Mic pre amps are much more inclined towards becoming noisy when set too high, than the line level signals that run throughout the rest of the console.
Most modern outboard equipment, and this includes amplifiers, can handle pretty high drive levels, so the issue of matching should not be very difficult. That is unless a given product or products is misstated, or has been downright measured wrong by the designers (there's a classic case of this with a very expensive parametric equalizer from a well-known company). Another problem, much more common, is when an input level on an amplifier, equalizer, limiter, or other line level device is turned down too far. A detailed description as to ‘why,' follows.
A Brief Background
In the early days of audio it was common to see 600-ohm matching transformers installed in inputs and outputs, often in tube pre-amps and tube amps. This led to the belief that all connections should be matching. The industry progressed and it was determined that it would be better to send a low impedance pre-amp output to a “bridging” input of 20K ohms or even as high as 50K ohms. While this was a better approach that did not unduly load the pre-amp's output stage, it left operating engineers (as opposed to electrical design engineers) in the dark. What to do?
Today, all preamp inputs are scalable with adjustable gain to accommodate a wide range of input sources. Microphones deliver as little as six (or more) orders-of-magnitude lower voltage than line level sources, and the line levels that are used in the console itself.
There's a difference between consumer levels for decks and players that use -10 dbv as their standard max level, while pro gear can usually provide +18 dbv to +24, or greater. This is a gain factor of more than 20X.
Input Attenuators (volume controls)
Virtually all modern pro and semi-pro devices have balanced inputs. It's problematic for the designer to place the input attenuator (volume control) at the very front of the circuit. It would have to be a dual-gang pot, one gang for each side of the balanced line, laser trimmed so that both gangs track extremely closely. Otherwise, any adjustment will reduce the Common Mode Rejection Ratio (CMRR) of the balanced line, as well as changing the impedance of the input. So it's basically just not done. Not to mention that a highly accurate dual-pot in which the gangs track perfectly is a very expensive component, and one that's hard to get. Plus, it would have to be of the linear taper type for accuracy, not the usual log taper of a typical volume control. Many may find this difficult to use. A linear-taper pot reduces level by -6 dB in the first half of its rotation, then all the remaining gain reduction, down to infinity, in the second half. Audio taper pots are much more intuitive in use.
So what is done is the input goes to an op-amp, usually through DC blocking capacitors, with four resistors establishing the input impedance (usually 10K to 20K ohms) This de-balances the signal. Then, a normal, inexpensive pot is placed next in the circuit. It can be audio taper or linear taper, or any kind of taper. It doesn't matter because this will be a single gang pot with no need to accurately track a second gang.
From there, the circuit does whatever it's supposed to do, then the output stage, typically of low impedance, consists of another op-amp connected almost in reverse to the input op-amp. Here, as in the input, DC blocking is very good practice, so some large-value capacitors are usually the last components in the circuit, expect for the output connector itself. Just after the output of the op-amp and before the DC blocking capacitors, a resistor sets the output impedance, usually of a value of 50 ohms. The resistors and capacitors together form a high-pass filter, so it's important for the designer to make sure the values of the components do not roll-off low frequencies prematurely.
What all this means for the operator and/or the system commissioner, is that gain pots on the input of amplifiers, speaker processors, bus limiters, outboard equalisers, and so on, should be set to maximum. Otherwise, at low settings, the devices upstream will be cranking out voltage, possibly overloading the amplifier's front end op amp (or other device), and still there may not be enough gain in the system to achieve the acoustic levels that are desired. Input pots should be used for minor adjustments, let's say -3 dB or so, but never turned down to half or less. Why do they even exist? On some amplifiers they don't but on those where they do, small gain changes can balance the left and right loudspeakers to match in level, or perhaps to subtly adjust delay speakers. Plus, almost every amp has one so it would be difficult for a design engineer to convince marketing not to include them.
Balanced versus Unbalanced
Here's where things get just a little tricky. A balanced line delivers 6 dB greater voltage to a balanced line receiver than an unbalanced line. If you are working with balanced and unbalanced connections in your system, you should be acutely aware of this. A balanced source can drive an unbalanced load, and an unbalanced source can drive a balanced load. But in both cases the gain will be -6 dB less than a balanced-to-balanced connection. That's a significant loss of headroom and should be avoided on the main outputs, if at all possible. It's probably not an issue for an aux feed to a small cry room, where the loudspeaker is likely to be operated at a very low level. But when it comes to those hefty mains...you want all the headroom you can get, especially for a dynamic and powerful worship band.
Tying Pin 3 to Ground on an Unbalanced Output
This is a practice that is questionable at best. It will come close to shorting the pin 3 side of the op-amp. It may cause the other side driving pin 2 to go into oscillation. This is to be avoided unless there are special circumstances that are beyond the scope of this paper.
On an input, it's perfectly alright and may reduce the tendency for the otherwise ‘floating' pin 3 side of the op-amp to cause noise or other interference.
Limiters, compressors and make-up gain
Limiters and compressors are in the business of being “triggered” at a given threshold, one that's adjustable. Once the threshold is exceeded, they reduce gain by a factor that is also adjustable. If the gain reduction is significant, the output level of the limiter (also sometimes called “make-up” gain), can be used to restore the gain through the device to something approaching unity. The price to pay is greater noise in the system. This is equally true whether the compressor/limiter is built-in to the console, or if it is an outboard device. If contained within the console's bag of tricks, at least you don't have to think about balanced vs. unbalanced.
Now that we have discussed optimizing gain, it may be time to start from scratch and re-set all the controls in your system from mic inputs, to console outputs, to the rest of the gear. It may take a re-orientation period if the gain settings have been sub-optimal in the past, but it will be a worthwhile exercise in terms of lower noise, greater headroom, and the ability to achieve the full operating level of your loudspeaker system.
