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What is Gain Structure?

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About the Author

Michael
Mardis

Michael Mardis is a moderator
at diyAudio. His interests include large speakers, small amplifiers, Igor
Stravinsky and Polynesian pop. Michael earns a living as a video projection
engineer and consultant.

What is Gain Structure?

Michael Mardis
,

29th March 2011

Gain structure (AKA Gain Staging) is a concept that gets talked about a lot
in pro audio, but most home audio folks have never heard of it. Understanding
gain structure can help you get the cleanest signal possible out of your system
and avoid some nasty things. Things like noise and clipping, which might sound
cool from a guitar amp, but not from a Hi-Fi system!

What's gain?
Basically it's amplification of the signal. When we increase the voltage level
of the signal, that's gain. Current gain can also be important, but we'll mostly
be talking about voltage gain here. The "structure" part of gain structure is
the various voltage levels throughout your audio system and the gain it takes to
get them to those levels.

How is gain expressed? Typically either in
amplification factor (times or X) or in decibels (dB). So if one volt goes into
an amp and two volts come out, that's a gain of 2X, or 6dB. The dB is a
logarithmic function as opposed to a linear one and is often seen on VU meters
and other audio scales. Take a look at this month's column by Jan Didden for
more about decibels.

What is the overall gain of a typical home audio
system? Let's start with an extreme example and assume that you're a vinyl lover
using a moving magnet cartridge. Your speakers are inefficient and need a lot of
power so you have a 300 watt power amp. How much overall voltage gain do you
need to get that tiny signal coming out of the phono cartridge up to the 300
watts (50 volts) coming out of your Ear Buster amp? A lot! A gain somewhere in
the neighborhood of 13000X or 82dB, sometimes more. Imagine a microscope with
that kind of magnification, and all the little specks and dirtballs you might
see clouding the image. Similarly, imagine all the noise your system might pick
up along the way with so much gain. If you have a moving coil cartridge, its
even worse!

SET amplifier fans can gloat, as they might have an overall
gain of only 2000X. If they are running a CD player into a flea power amp, that
might mean a gain of only 4X. But since flea power owners probably use more
sensitive speakers, they can still run into noise and clipping problems.

Typical
gain structure of a home audio system.

Where does the structure
part come in? It's about how much gain (or loss) each section of the system has.
The phono preamp will have a lot just to recover that tiny signal from the
cartridge. The preamp or line stage will add a bit more, and then the power amp
will have gain also. Keeping the levels reasonable throughout the whole chain
gives us good gain structure.

Adding up how much gain each section has
gives us the overall gain of the system. Now look over at your preamp or
integrated amp with that big knob in front that "goes to eleven". What you are
looking at is gain's twin brother from an opposite universe, attenuation. All
that volume knob does is attenuate the signal that comes before it by dividing
the signal and reducing the voltage. Typically in the 12 O'clock position the
volume control attenuates by 20dB, cutting the signal to 1/10th of what it was.
But it has not changed the gain of each section, only divided the signal,
(attenuated it) at a certain spot.

Now
we know the two sides of the structure, gain and attenuation. Both are
important. Most simple home audio systems have only one point along the signal
path to control gain, the main volume control, which controls only by
attenuation. The gain of each section does not change, you've just divided the
signal at one point. So that extreme system with an overall gain of 13000X will
still have an overall gain of 13000X, it's just that somewhere along the path
you've used a voltage divider (your volume control) to attenuate the signal. And
the sections downstream from the volume control will amplify everything just as
they always did. But now they are amplifying a smaller signal, the signal you
attenuated with the volume control.

So how about a practical example?
Suppose some evening you're with that Special Lady. You've poured the wine, lit
the candles, and now it's time for that famous Barry White CD. Mmmmm, mmm. Bring
on the love, baby. Being the smart and smooth audio dude that you are, you know
that Mr. W sounds his sexy best played at 2 watts average power on the speakers
in your "Love Den." So what voltage levels, gains and attenuations will you need
to bring out the best in Barry? Right now you don't care, But Hey! Snap back to
reality and look at the sexy graph below.

Typical voltage levels and gains

Barry's voice is going to be
recorded at about 16dB (average) below the maximum level possible on the CD.
That is a standard mastering level. If your CD player or DAC is standard, then
that's going to mean an average level of about 0.32 volts coming out of the RCA
connectors on the back. That 0.32 volts will then be amplified 3X by your preamp
and 30X by your power amp. But that's 29 volts out of the power amp – over 100
watts! Not going to set the mood, is it? No, Special Lady has run for the
door!

That's why we have a volume control in there - to reduce the signal
to a reasonable level. You can see that the system has too much gain for the
evening's festivities, but that's OK, just turn it down. Now Barry is crooning,
not shouting. Throw away some signal in the middle of the chain so that it
doesn't get too loud at the end. In this case the volume control has divided the
voltage level by 7, (0.14X) or -17dB. So tomorrow when you want to rock out with
AC/DC for your victory lap, you can just turn it up and unleash the power. So
far, so good.

But if we look back at the system, we see that for most
situations, we have more gain than we need. Maybe 15 to 20dB (10X) more gain
than we really need. And that can lead to noise. Why? Because any noise that
occurs after the volume control does not get attenuated. In fact it gets
amplified. You've cut down the signal from the CD player by 17dB, so now it's
17dB closer to the noise in every circuit that follows. Any noise from the
preamp, the cables, bad connections, etc. will be also be amplified 30X by the
power amp. You took a medium level signal of 320mV (0.32 volts) and divided it
down to 44mV so its now much closer in level to all the noise living in the
bottom of the system.

Too much gain or bad gain structure not only gets
us into trouble with noise and clipping, it can be a pain for practical reasons,
too. I remember a big old Pioneer integrated amp from the 80's, a massive, heavy
thing. Big transformer, VU meters, clip lights, serious knobs and switches. Did
maybe 75 honest watts into 8 Ohms. You could connect any standard source like
CD, radio, tape, phono, and you barely had to crack the volume knob to get a big
blast of music. "Wow, this thing's got power, you hardly have to turn it up at
all!" But did it really have tons of power? No, it just had too much gain. The
volume knob would not get past 9:00 before the amp was clipping, so the useful
range of the volume control was from “Nothing” at 7:00 to “Clipping” at 9:00.
That sure made adjusting the volume very touchy. Stupid design, far too much
gain. Despite all the other good aspects of the amp, the primary user interface,
the volume knob, was a pain in the backside.

Let's return to the signal
path to see where things might be done better, or where they are often done
wrong. Generally speaking we want to run amps and preamps at a fairly high
level. That means that the signal (the music) will be at a much higher voltage
than the noise so we have a higher signal to noise ratio, S/N. That's a good
thing. But just how high a signal voltage do we need to run? Usually we want the
peaks of the loudest signals to be about 3dB-6dB below the maximum that device
can do without distorting for the best s/n ratio. That can be hard to determine
unless you've designed, built or measured the amp. Power amplifiers might give
you a clue in their specs, but preamps and phono stages usually don't. How much
signal does it take at the input to drive the device into clipping? Knowing that
will tell you where your gain should be all through the chain.

There
aren't any set standards as to what the input signal should be for a preamp or
power amp to reach its maximum level, but there are some conventions. Input
levels that will drive a device to full output can range from 0.77 volts to 2 or
3 volts in the consumer market, and even higher in pro audio. So you may have a
preamp that will hit maximum output when it gets a 0.7V input signal. That would
be a problem with standard CD players, as they output a maximum of 2V, but the
preamp has a volume control, the voltage divider we talked about before, that
attenuates the incoming signal. This attenuator is often the very first thing in
line after the input selector. Sometimes there will be a buffer circuit before
the volume pot, but that is more common in pro gear than consumer equipment. Our
2 volt signal coming from the CD player may need to be attenuated before it ever
hits the preamp circuits or it will overdrive them. A typical preamp will
amplify the signal by 2 or 3X after it has passed the volume control. This is
then passed on to the power amp.

The power amp is going to behave much
like the preamp, it has a certain amount of gain (30X is typical) and it will
take a certain voltage at the input to drive the amp to its maximum power. How
much voltage? Again, we may not know. You might find it in the amp's specs, or
you might know because you designed or measured it. Either way, at some input
voltage level, the amp will reach full power. Here is where we often find a
difference in consumer power amps and pro audio power amps. Pro amps have a
level adjust on the inputs, high end consumer power amps often do not. They may
not need it in simple systems, but if they don't have an input level control
they will apply full gain to everything coming in. The result? You have to turn
down the preamp volume to keep the power amp from getting too loud. Turning down
the preamp will attenuate the signal near the beginning point of the preamp
circuits, leaving any and all noise from the following circuits to be fully
amplified by the power amp.

Because we have attenuated the music signal
at the input of the preamp, it's now closer in voltage to the noise in the whole
system downstream. We’ve destroyed our good S/N ratio.

A good digital
source like a CD player, DAC or high quality sound card will have a signal to
noise ratio of 90dB or better. But that ratio is the maximum signal over noise.
Music isn't recorded at the maximum level, its average level may be down 16, 18
or 22dB below peak, at least on well mastered CD's. But the noise floor of the
device doesn't change, so effectively there is a "Music to Noise" ratio of only
74dB or less. In other words, the noise coming out of your speakers will be 74dB
below the average music level. That's still very good and most of us can live
with that and never hear it, but there is trouble brewing...

Looking
at the flow chart above, we see two systems. On top is our system with an
overall gain of 90X. Below it is a system with a lower gain of 20X. Both start
out with a musical signal of the same voltage -0.32 volts and both end with 2
watts at the speaker (4 volts). But along the signal path we see big differences
in the signal voltage at corresponding points. The 90X gain system has to reduce
the CD output by a large amount or it will be overdriving the power amp and
speaker. The 20X system uses only moderate attenuation of the signal because the
subsequent gain is much less.

Now imagine that we pick up 1mV of noise
right after the volume control. In the 90X system, that will reduce the music to
noise ratio to 33dB. Not great. In the 20X system 1mV of noise at the same spot
would reduce the ratio to 46dB, a 13dB noise advantage for the low gain
system.

Picking up 1mV of noise at a less sensitive spot like at the
inputs of the power amp would result in a 42dB ratio for the 90X and 52dB for
the 20X system, a 10dB difference. The above example is simplified for clarity.
In reality noise would be picked up all along the signal path and be amplified
to various degrees, but starting out with a higher signal voltage still helps at
all points along the path.

Things can get worse. What if you use a piece
of pro gear like the DCX2496 crossover? It's meant for the higher signal levels
of the pro world. To drive it to maximum we need 7 volts RMS! It will take lower
levels, of course, but remember that those lower input levels are much closer to
the DCX noise floor. Our CD player won't drive it high enough with its 2V RMS
maximum output. Our preamp might just get us close. It has a gain of 3, so with
the volume wide open we'll get 6V into the DCX. That's enough to keep it happy
and keep the signal up out of the noise, but then what happens? The pro
crossover now outputs 6 volts as well. That level is so hot it's going to drive
our precious power amps into severe clipping. Six volts into our power amp with
a gain of 30 means 180 volts out of the speaker terminals. Not going to happen
unless it's a 4 kilowatt amp! Again, too much gain. If the power amp reaches its
maximum output with a 1V input, we have no choice but to turn down the preamp.
So we turn down the preamp until its output is 132mV as seen in the 90X system.
If the crossover has an optimistically good S/N ratio of 95 dB, that still means
0.2mVof noise added to the signal, so we are at an S/N ratio of only 56dB coming
out of the crossover. What to do?

To
fix this gain structure problem we put an attenuator on the inputs of the power
amps to reduce that 6 volt signal to a usable level, or we build amps with low
gain. Preferably both. Or we find a crossover that works in a range closer to
the signals provided by the CD player and preamp.

Obviously the more
complex the system gets, the more we need to worry about gain structure. Using a
simple system with only a CD player and integrated amp, we can usually just spin
the Barry White disc, set the volume, and get down to business. But with a more
complex system Barry may get lost in a fog of noise before you do. That's going
to spoil the evening.

© Panomaniac 2011

Comments on the article "What is Gain
Structure?"

14th April 2011

hifidaddy

diyAudio Member

Michael,

your
article is great. I found exactly the same. For some years I am using this gain
structure with power amp set at low gains (1x to 4x, depending on sensitivity of
loudspeaker components), while using a digital xover for my loudspeaker. This
gives a huge leap in overall transparancy compared to the common power amp gain
settings at (10x to 30x).

Low gain in the power amp is the way to get
line stage noise considerably down, too.

Another plus is, that you have
higher signal levels in amplification blocks with less regulated (sophisticated)
power supplies, which further enhances S/N ratio.

The first time I did
this was in 1986, when I modified a (cheap) Dual compact turntable/integrated
amp combo (of my girlfriend at that time) to power amp gain of 4x, and made
other changes to the overall topology. This tiny thing then gave more musical
pleasure than my (expensive and high rated) Linn/Naim active setup. This gave me
some food for thought.

Note that Hiraga "Le Monstre" power amp has only
gain of 10x while Hiraga "Class A" and "Le Class A" have higher gains. I presume
that some people are preferring the LeMonstre power amp just for having lower
gain.

One of my friends who also uses a digital xover takes it even
further: he uses the volume pot before the linestage just for having the right
input level for the xover, while he has an analog 6-ch volume pot behind the
xover to set the listening level.

best regards,
Hartmut

14th April 2011

Pano

diyAudio Moderator

Thanks for the kind
words.

Yes, a main component of gain structure is that we almost always
have too much gain. It's there "just in case" but too often leads to trouble. We
often just don't need much.

As Mr. Hiraga was one of my teachers, you
could say I come from the Low Gain school of audio. And working in pro audio for
years taught me that getting it right is important.

Take the Speaker Voltage Test!

14th April 2011

AndrewT

diyAudio Member

Can we look forward to Part
2 ?

regards Andrew T.
Sent from my desktop computer
using a keyboard

14th April 2011

Pano

diyAudio Moderator

LOL! Maybe. The article was
originally about 3X longer. It got cut down so as not to bore you guys to
tears.

What would you like to see in Part 2? If this article raises
questions or something isn't clear, I'd be happy to tackle that in a second
part.

Take the Speaker Voltage Test!

14th April 2011

AndrewT

diyAudio Member

does MM=Pano=Panomaniac?

regards Andrew T.
Sent from my desktop computer
using a keyboard

14th April 2011

Pano

diyAudio Moderator

Correct.

14th April 2011

adason

diyAudio Member

Nice article Panomaniac. I
felt for a long time that all we need is good buffers, rather than high gain
amplifiers. With highly efficient speakers working at few hundred milliwatts at
normal listening level, occasionally up to 1 watt, all you need is to privide
current capability to your 2V source...

14th April 2011

killerfishes

diyAudio Member

I think you are all looking
at this wrong, we are diy freaks or we would not be here. What I got from this
is that im justified having a low gain set up with tube output for the Berry
White times and a high gain SS for the AC/DC. Now I can tell the old ball and
chain "shut up, I read this is the right way now go away im building a new gain
clone for the bathroom."

14th April 2011

Pano

diyAudio Moderator

Exactly.

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