=20
Gain structure (AKA Gain Staging) is a concept that gets talked about =
a lot=20
in pro audio, but most home audio folks have never heard of it. =
Understanding=20
gain structure can help you get the cleanest signal possible out of your =
system=20
and avoid some nasty things. Things like noise and clipping, which might =
sound=20
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=20
of the signal, that's gain. Current gain can also be important, but =
we'll mostly=20
be talking about voltage gain here. The "structure" part of gain =
structure is=20
the various voltage levels throughout your audio system and the gain it =
takes to=20
get them to those levels.
How is gain expressed? Typically either =
in=20
amplification factor (times or X) or in decibels (dB). So if one volt =
goes into=20
an amp and two volts come out, that's a gain of 2X, or 6dB. The dB is a=20
logarithmic function as opposed to a linear one and is often seen on VU =
meters=20
and other audio scales. Take a look at this month's column by Jan Didden =
for=20
more about decibels.
What is the overall gain of a typical home =
audio=20
system? Let's start with an extreme example and assume that you're a =
vinyl lover=20
using a moving magnet cartridge. Your speakers are inefficient and need =
a lot of=20
power so you have a 300 watt power amp. How much overall voltage gain do =
you=20
need to get that tiny signal coming out of the phono cartridge up to the =
300=20
watts (50 volts) coming out of your Ear Buster amp? A lot! A gain =
somewhere in=20
the neighborhood of 13000X or 82dB, sometimes more. Imagine a microscope =
with=20
that kind of magnification, and all the little specks and dirtballs you =
might=20
see clouding the image. Similarly, imagine all the noise your system =
might pick=20
up along the way with so much gain. If you have a moving coil cartridge, =
its=20
even worse!
SET amplifier fans can gloat, as they might have an =
overall=20
gain of only 2000X. If they are running a CD player into a flea power =
amp, that=20
might mean a gain of only 4X. But since flea power owners probably use =
more=20
sensitive speakers, they can still run into noise and clipping =
problems.
Where does the = structure=20 part come in? It's about how much gain (or loss) each section of the = system has.=20 The phono preamp will have a lot just to recover that tiny signal from = the=20 cartridge. The preamp or line stage will add a bit more, and then the = power amp=20 will have gain also. Keeping the levels reasonable throughout the whole = chain=20 gives us good gain structure.
Adding up how much gain each = section has=20 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=20 looking at is gain's twin brother from an opposite universe, = attenuation. All=20 that volume knob does is attenuate the signal that comes before it by = dividing=20 the signal and reducing the voltage. Typically in the 12 O'clock = position the=20 volume control attenuates by 20dB, cutting the signal to 1/10th of what = it was.=20 But it has not changed the gain of each section, only divided the = signal,=20 (attenuated it) at a certain spot.
So how about a practical = example?=20 Suppose some evening you're with that Special Lady. You've poured the = wine, lit=20 the candles, and now it's time for that famous Barry White CD. Mmmmm, = mmm. Bring=20 on the love, baby. Being the smart and smooth audio dude that you are, = you know=20 that Mr. W sounds his sexy best played at 2 watts average power on the = speakers=20 in your "Love Den." So what voltage levels, gains and attenuations will = you need=20 to bring out the best in Barry? Right now you don't care, But Hey! Snap = back to=20 reality and look at the sexy graph below.
Typical voltage levels and gains
Barry's voice is going = to be=20 recorded at about 16dB (average) below the maximum level possible on the = CD.=20 That is a standard mastering level. If your CD player or DAC is = standard, then=20 that's going to mean an average level of about 0.32 volts coming out of = the RCA=20 connectors on the back. That 0.32 volts will then be amplified 3X by = your preamp=20 and 30X by your power amp. But that's 29 volts out of the power amp =96 = over 100=20 watts! Not going to set the mood, is it? No, Special Lady has run for = the=20 door!
That's why we have a volume control in there - to reduce = the signal=20 to a reasonable level. You can see that the system has too much gain for = the=20 evening's festivities, but that's OK, just turn it down. Now Barry is = crooning,=20 not shouting. Throw away some signal in the middle of the chain so that = it=20 doesn't get too loud at the end. In this case the volume control has = divided the=20 voltage level by 7, (0.14X) or -17dB. So tomorrow when you want to rock = out with=20 AC/DC for your victory lap, you can just turn it up and unleash the = power. So=20 far, so good.
But if we look back at the system, we see that for = most=20 situations, we have more gain than we need. Maybe 15 to 20dB (10X) more = gain=20 than we really need. And that can lead to noise. Why? Because any noise = that=20 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=20 17dB closer to the noise in every circuit that follows. Any noise from = the=20 preamp, the cables, bad connections, etc. will be also be amplified 30X = by the=20 power amp. You took a medium level signal of 320mV (0.32 volts) and = divided it=20 down to 44mV so its now much closer in level to all the noise living in = the=20 bottom of the system.
Too much gain or bad gain structure not = only gets=20 us into trouble with noise and clipping, it can be a pain for practical = reasons,=20 too. I remember a big old Pioneer integrated amp from the 80's, a = massive, heavy=20 thing. Big transformer, VU meters, clip lights, serious knobs and = switches. Did=20 maybe 75 honest watts into 8 Ohms. You could connect any standard source = like=20 CD, radio, tape, phono, and you barely had to crack the volume knob to = get a big=20 blast of music. "Wow, this thing's got power, you hardly have to turn it = up at=20 all!" But did it really have tons of power? No, it just had too much = gain. The=20 volume knob would not get past 9:00 before the amp was clipping, so the = useful=20 range of the volume control was from =93Nothing=94 at 7:00 to = =93Clipping=94 at 9:00.=20 That sure made adjusting the volume very touchy. Stupid design, far too = much=20 gain. Despite all the other good aspects of the amp, the primary user = interface,=20 the volume knob, was a pain in the backside.
Let's return to the = signal=20 path to see where things might be done better, or where they are often = done=20 wrong. Generally speaking we want to run amps and preamps at a fairly = high=20 level. That means that the signal (the music) will be at a much higher = voltage=20 than the noise so we have a higher signal to noise ratio, S/N. That's a = good=20 thing. But just how high a signal voltage do we need to run? Usually we = want the=20 peaks of the loudest signals to be about 3dB-6dB below the maximum that = device=20 can do without distorting for the best s/n ratio. That can be hard to = determine=20 unless you've designed, built or measured the amp. Power amplifiers = might give=20 you a clue in their specs, but preamps and phono stages usually don't. = How much=20 signal does it take at the input to drive the device into clipping? = Knowing that=20 will tell you where your gain should be all through the chain.
There=20 aren't any set standards as to what the input signal should be for a = preamp or=20 power amp to reach its maximum level, but there are some conventions. = Input=20 levels that will drive a device to full output can range from 0.77 volts = to 2 or=20 3 volts in the consumer market, and even higher in pro audio. So you may = have a=20 preamp that will hit maximum output when it gets a 0.7V input signal. = That would=20 be a problem with standard CD players, as they output a maximum of 2V, = but the=20 preamp has a volume control, the voltage divider we talked about before, = that=20 attenuates the incoming signal. This attenuator is often the very first = thing in=20 line after the input selector. Sometimes there will be a buffer circuit = before=20 the volume pot, but that is more common in pro gear than consumer = equipment. Our=20 2 volt signal coming from the CD player may need to be attenuated before = it ever=20 hits the preamp circuits or it will overdrive them. A typical preamp = will=20 amplify the signal by 2 or 3X after it has passed the volume control. = This is=20 then passed on to the power amp.
The power amp is going to behave = much=20 like the preamp, it has a certain amount of gain (30X is typical) and it = will=20 take a certain voltage at the input to drive the amp to its maximum = power. How=20 much voltage? Again, we may not know. You might find it in the amp's = specs, or=20 you might know because you designed or measured it. Either way, at some = input=20 voltage level, the amp will reach full power. Here is where we often = find a=20 difference in consumer power amps and pro audio power amps. Pro amps = have a=20 level adjust on the inputs, high end consumer power amps often do not. = They may=20 not need it in simple systems, but if they don't have an input level = control=20 they will apply full gain to everything coming in. The result? You have = to turn=20 down the preamp volume to keep the power amp from getting too loud. = Turning down=20 the preamp will attenuate the signal near the beginning point of the = preamp=20 circuits, leaving any and all noise from the following circuits to be = fully=20 amplified by the power amp.
Because we have attenuated the music = signal=20 at the input of the preamp, it's now closer in voltage to the noise in = the whole=20 system downstream. We=92ve destroyed our good S/N ratio.
A good = digital=20 source like a CD player, DAC or high quality sound card will have a = signal to=20 noise ratio of 90dB or better. But that ratio is the maximum signal over = noise.=20 Music isn't recorded at the maximum level, its average level may be down = 16, 18=20 or 22dB below peak, at least on well mastered CD's. But the noise floor = of the=20 device doesn't change, so effectively there is a "Music to Noise" ratio = of only=20 74dB or less. In other words, the noise coming out of your speakers will = be 74dB=20 below the average music level. That's still very good and most of us can = live=20 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=20 overall gain of 90X. Below it is a system with a lower gain of 20X. Both = start=20 out with a musical signal of the same voltage -0.32 volts and both end = with 2=20 watts at the speaker (4 volts). But along the signal path we see big = differences=20 in the signal voltage at corresponding points. The 90X gain system has = to reduce=20 the CD output by a large amount or it will be overdriving the power amp = and=20 speaker. The 20X system uses only moderate attenuation of the signal = because the=20 subsequent gain is much less.
Now imagine that we pick up 1mV of = noise=20 right after the volume control. In the 90X system, that will reduce the = music to=20 noise ratio to 33dB. Not great. In the 20X system 1mV of noise at the = same spot=20 would reduce the ratio to 46dB, a 13dB noise advantage for the low gain=20 system.
Picking up 1mV of noise at a less sensitive spot like at = the=20 inputs of the power amp would result in a 42dB ratio for the 90X and = 52dB for=20 the 20X system, a 10dB difference. The above example is simplified for = clarity.=20 In reality noise would be picked up all along the signal path and be = amplified=20 to various degrees, but starting out with a higher signal voltage still = helps at=20 all points along the path.
Things can get worse. What if you use = a piece=20 of pro gear like the DCX2496 crossover? It's meant for the higher signal = levels=20 of the pro world. To drive it to maximum we need 7 volts RMS! It will = take lower=20 levels, of course, but remember that those lower input levels are much = closer to=20 the DCX noise floor. Our CD player won't drive it high enough with its = 2V RMS=20 maximum output. Our preamp might just get us close. It has a gain of 3, = so with=20 the volume wide open we'll get 6V into the DCX. That's enough to keep it = happy=20 and keep the signal up out of the noise, but then what happens? The pro=20 crossover now outputs 6 volts as well. That level is so hot it's going = to drive=20 our precious power amps into severe clipping. Six volts into our power = amp with=20 a gain of 30 means 180 volts out of the speaker terminals. Not going to = happen=20 unless it's a 4 kilowatt amp! Again, too much gain. If the power amp = reaches its=20 maximum output with a 1V input, we have no choice but to turn down the = preamp.=20 So we turn down the preamp until its output is 132mV as seen in the 90X = system.=20 If the crossover has an optimistically good S/N ratio of 95 dB, that = still means=20 0.2mVof noise added to the signal, so we are at an S/N ratio of only = 56dB coming=20 out of the crossover. What to do?
To=20 fix this gain structure problem we put an attenuator on the inputs of = the power=20 amps to reduce that 6 volt signal to a usable level, or we build amps = with low=20 gain. Preferably both. Or we find a crossover that works in a range = closer to=20 the signals provided by the CD player and preamp.
Obviously the = more=20 complex the system gets, the more we need to worry about gain structure. = Using a=20 simple system with only a CD player and integrated amp, we can usually = just spin=20 the Barry White disc, set the volume, and get down to business. But with = a more=20 complex system Barry may get lost in a fog of noise before you do. = That's going=20 to spoil the evening.
=A9 Panomaniac 2011
=20
=20
your=20 article is great. I found exactly the same. For some years I am using = this gain=20 structure with power amp set at low gains (1x to 4x, depending on = sensitivity of=20 loudspeaker components), while using a digital xover for my loudspeaker. = This=20 gives a huge leap in overall transparancy compared to the common power = amp gain=20 settings at (10x to 30x).
Low gain in the power amp is the way to = get=20 line stage noise considerably down, too.
Another plus is, that = you have=20 higher signal levels in amplification blocks with less regulated = (sophisticated)=20 power supplies, which further enhances S/N ratio.
The first time = I did=20 this was in 1986, when I modified a (cheap) Dual compact = turntable/integrated=20 amp combo (of my girlfriend at that time) to power amp gain of 4x, and = made=20 other changes to the overall topology. This tiny thing then gave more = musical=20 pleasure than my (expensive and high rated) Linn/Naim active setup. This = gave me=20 some food for thought.
Note that Hiraga "Le Monstre" power amp = has only=20 gain of 10x while Hiraga "Class A" and "Le Class A" have higher gains. I = presume=20 that some people are preferring the LeMonstre power amp just for having = lower=20 gain.
One of my friends who also uses a digital xover takes it = even=20 further: he uses the volume pot before the linestage just for having the = right=20 input level for the xover, while he has an analog 6-ch volume pot behind = the=20 xover to set the listening level.
best regards,
Hartmut