Recording - March 2015 - 58

Part 2: Gain
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By M
Gain is a regular part of our audio gear vocabulary-preamp
gain, gain reduction, master gain, maximum gain, make-up
gain, gain riding... the list goes on. Since gain is all about levels and usually involves decibels, learning about it will be a
good followup to last month's article.
There's gain and then there's gain
"Gain" is our jargon for amplification, a way of expressing the
ratio of output to input level of a device. A mic preamp that gives you
5 volts out for an input of 5 millivolts has a gain of (5V / 0.005V)
= 1000. Saying that it has a gain of one thousand is perfectly legitimate, but since the decibel is a ratio, we often express gain in decibels. As we explained last month, decibels let us write very large or
very small ratios of numbers in a compact form. Fire up your scientific calculator, take the logarithm of the ratio (the log of 1000 is 3),
multiply the log by 20 (since it's a voltage ratio), and you have what's
likely a more familiar gain of 60 dB.

River

important to know how far it can be turned down. The difference
between maximum and minimum gain is sometimes specified as
gain range.
A guitar or bass pickup generally has about 20 dB more output than a microphone, so an instrument (DI) input on a preamp
will have about 20 dB less gain than the mic input. Electronic
instruments usually have a line level output, so no gain is needed, although line level inputs often have a small amount of gain
that is adjusted to best match the source.
How do we measure gain?
Like most specifications, gain has some strings attached. One
is that it must be measured under realistic operating conditions.
When measuring gain, the input level must be low enough to
assure that the device doesn't clip. You need a clean sine wave
at only a few millivolts RMS to measure the maximum gain of a
mic preamp.

Like most specifications, gain has some strings attached.
With modern audio gear, we're nearly always talking about
voltage gain, so the adjective "voltage" is usually dropped, but
there are such things as current gain and power gain as well.
Since transistors amplify current rather than voltage, current
gain is important to circuit designers, but not so much to end
users. What about a power amplifier? As a rule, we put in volts
and get out watts, so voltage, current, or power gain could
describe them. However, I'll discuss how it's usually and properly done in our next installment. Stay tuned.
How much?
The amount of gain that a device or a circuit provides is often
related to its function. In order to boost a low-level mic signal to
line level, gain on the order of 30 to 60 dB, or occasionally as
much as 75 dB, is necessary. Since mics and sound sources
vary widely, a mic preamp has to deal with a wide range of
input levels. Since we expect about the same level out of a preamp no matter what goes in, its gain is, by necessity, adjustable.
In addition to knowing a preamp's maximum gain, it's also
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RECORDING March 2015

Gain is usually measured at 1 kHz and is assumed to be constant within a fairly small tolerance over the device's full operating frequency range, unless of course it's a device where gain
varies with frequency (a filter or equalizer) or amplitude (a compressor). It's hard to cheat on a gain measurement, but sometimes there can be some surprises-see the sidebar for a realworld example I encountered some years ago.
Measuring gain of an amplifier is easy as long as it has connectors for attaching a signal generator and meter to its input and output, respectively. Suppose, though, that you want to measure the gain
of the mic preamp inside a mixer or interface, where the preamp's
output is hard-wired to the mix bus or A/D converter and never sees
the outside world. If the spec sheet says that mic preamp's gain is 56
dB, you'll have to trust the manufacturer unless you plan to open the
case and put your meter on the correct point in the preamp circuit.
However, if there's a direct output or insert (send), it usually comes
directly from the mic preamp, so it's possible to check the preamp
gain at that point. A block diagram helps here, though an actual
schematic diagram may be more accurate.

Recording - March 2015

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