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Pano 29th March 2011 10:57 AM

What is Gain Structure?
 
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

hifidaddy 14th April 2011 08:00 AM

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

Pano 14th April 2011 01:50 PM

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.

AndrewT 14th April 2011 02:01 PM

Can we look forward to Part 2 ?

Pano 14th April 2011 02:10 PM

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.

AndrewT 14th April 2011 02:28 PM

does MM=Pano=Panomaniac?

Pano 14th April 2011 04:12 PM

Correct. :checked:

adason 14th April 2011 06:31 PM

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...

killerfishes 14th April 2011 08:21 PM

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."

Pano 14th April 2011 08:29 PM

Exactly. :devilr:

danielwritesbac 14th April 2011 09:05 PM

Question: Doesn't higher gain, "in many cases," also mean decreased negative feedback and that also happens to affect/adjust stability of an amplifier?

Question: Why, when the gain setting is compromised in favor of all other factors, does the gain usually arrive at an average figure of 40? This is too much gain. But reducing it to half (20) or less, "in many cases," seems to make either too much or not enough of everything else.

My first thought on this seems to be fairly important (and isn't): Facilitating clipping with too much available gain (your choice with the volume knob), is a far out different thing than exacerbating clipping by design (most often seen in power amplifiers with line level input). But, I think that its nonsense, because the first problem is voltage and the second problem is current, so like an "apples and oranges" comparison.

Thank you for your low gain illustration involving the high current output buffer to use along with a preamp or flea-power amplifier.
I see two major effects:
1). The preamp can more easily deal with gain, possibly because all of the current is low current.
2). The high current output buffer is sister to the capacitive multiplier and its clean effect on audio applications is well known.

So, Kudos!! Keeping the big current jolts far away from the gain device really does seem like a fine idea. Its similar to avoiding using the recording studio during earthquakes. :)

It would be nice to see a chart or photographic demonstration of amplifiers that use different gain structures, along with some footnotes for the audible consequences of each (via averaged historical usage data, if necessary). Both theory and application together is the only time it makes sense to me.

---------- cart before horse? --------
It doesn't seem to do a lot of good to work so hard on gain structures if perchance the input circuit doesn't have the same scrutiny.

Problems when applying the volume control are persistent and commonplace in both production and diy amplifiers. So a supplementary article on "how to apply the volume control and not goof" would be a great thing for the entire industry.

And why on earth didn't Pioneer put resistors in series to a potentiometer for a control won't facilitate clipping no matter how far the knob is turned? Although not really line level spec, Technics of the same production time-frame always regulated the volume pot. So, that Pioneer example is the carelessness typical of retail products--Its similar to modern digital examples that just plug in a chip assuming a total solution, even though that assumption doesn't usually work as well as advertised.

Another terrible trick with the volume pot happens when its (wrong) resistance goes in series to your input cap, often causing a dull presentation except for when the volume is "almost all the way" up.
Four fixes:
1). (crude) Put the input cap and rf filter capacitor load all on the RCA jack along with a small additional resistor load onto the RCA jack.
2). (more elegant) Buffer the volume pot.
3). (weird) By reputation, succeeding where potentiometers don't and also giving an "almost buffer effect," that's Lightspeed Attenuator; however, I'd rather see a documented and plausible method of operation that's do-able without either pixies or opto-isolators.
4). Either a different value of potentiometer load or adding or subtracting a resistor load to one side or the other of the existing potentiometer.

Know what I'm saying? While low gain might be fine for many reasons, running low gain to have less of wrong series resistance at the potentiometer isn't a good enough reason but rather its a problem at the input circuit that needs to be repaired before working on amplifier gain.

So, although I like your article very much, it does seem to need a "prequel" article. Please assume that all of this post is either an observation or a question. And, thank you for the fine article.

AUDIODH 14th April 2011 11:44 PM

Gain Staging
 
Hi All,
Nice article Michael..
The way we look at things in the Pro Sound world is as follows:
1. At the Mic Preamplifier, take as much gain as possible, because that's where the minimum noise is, the very best amplification in the audio chain, and we want to start with the best S/N ration as possible.
2. Run the mix bus around 0dBV, that's referenced to 1 Vrms or also sometimes referred to as 0dBu when no reference is stated.
3. From that point run unity gain (around +4 dBV) through the signal chain all the way to the amplifiers or powered loudspeakers. Sometimes more depending on the quality of gear and cable lengths.
4. I typically turn down the power amplifiers to obtain required SPL...most all Pro amps will be specified as having 26 - 34 dB of voltage gain.
Sensitivity is typically 1.2Vrms for rated power, but may handle +24dB or 16Vrms, which is limited due to the input stage voltage rails. The amp inputs level control's (volume) job is to reduce the level to the input level to meet the input sensitivity specification or the desired output power level.

It is my preference to run hot signals from the source to the amplifier. Most amplifiers have an input potentiometer of say 10,000 Ohms. So, if you drive a signal down the wires and through the low noise sections with a 100dB S/N ratio, the amp input pot in a good design, will indeed divide down the signal to the desired level, while reducing the noise by the same ratio. If you run a low level signal to the amp and boost there, you have sacrificed the "low noise" pre-amp stages for that of the amplifier - which just wants to amplify the world, noise and all!

The point is always take the gain up front where the low noise circuits live as to provide the very best S/N ratio.

Pano 15th April 2011 12:31 AM

Thanks.

Well I like to run a pretty strong line level signal, as AUDIODH has mentioned above. That comes from my live sound days in very electrically noisy environments. Even then, I find that the power amps have too much gain, they have to be dialed way back.

Keeping that line level signal high means that it will ride far above all the noise found in the circuits and cables. That's a good thing.

In a home rig, cable runs are short and levels are lower but keeping the level up still helps. Most sources these days can supply 2V as a nominal line level. That's great. Unless you attenuate it by 20-40dB because your power amp has too much gain! If your speakers are efficient, you may not need more than 6-10dB overall gain. Of course a little more wouldn't hurt headroom.

The design and sonic merits of low gain amps are outside the scope of this article. I just want readers to be aware of level matching and the ways to keep a clean signal path. Turning it way down before it hits the power amp in not a good way. :eek:

AUDIODH 15th April 2011 12:55 AM

Gain Staging
 
Pano,
Yes....
however, turning down the amp is better than turning down the pre...just sayin'
If the system is designed to work as such, then all is fine.
Home and Pro signal level mixed systems, will typically require the most attention.
I owned a complete McIntosh component system....it was awesome...still is!
I'm not here selling esoteric gear, rather the engineering of the "system", of any size, is the very best place to start.
No Harley wheels installed on a Hayabusa (はやぶさ :eek:
-Dave

nigelwright7557 15th April 2011 01:01 AM

My mixer has a gain stage before the main amplification stage.
I found if the pre-gain is down the sound tends to be washed out.
I have to turn it right up for my guitar to see right.

AUDIODH 15th April 2011 01:24 AM

Niegel,
some of the best sounding NEVE consoles I've heard, have the NE5532 op-amps running at voltages that exceeded manufacturers spec's +/- 22 VDC...but they have tons of headroom, gain, but run hot as a match!
-Dave

hifidaddy 15th April 2011 07:50 AM

I would like to add a quite simply "pain-free" way to solve the gain problem, a way which is compatible to standard hifi configurations.

vinylsavour, also a member of diyaudio.com, uses a linestage which has the gain block before the volume pot. This way no low level detail gets lost, because it is amplified first, then attenuated. Of course it is necessary to have very big headroom for the amplification stage. vinylsavour uses a 801 tube/valve for amplification.

Hartmut

AUDIODH 15th April 2011 01:22 PM

Dear Hartmut,
I'm not quite sure of how the system works you are describing. Do yo mean the preamp has a level control (electrically) at the end of it before the amplifier stage?

In a simple answer to gain staging, if you have an AC voltmeter, do the following:
I will assume a typical stereo system
1. Turntable to Preamp output, get the most level without distortion
2. Set the preamp for a high level with the lowest distortion, after any equalization.
3. Measure the voltage at the pre amp outputs (for conversation lets' assume 1V), now any components that follow should be set so they output the same 1V signal, do this all the way to the power amp input. (note if you own a receiver that is all-in-one, this is the levels are set by the manufacturer) Use the power amplifier to set the maximum desired level...unless you attended more than three AC/DC concerts near the loudspeaker stacks.
I hope this is what you were asking for.

Pano 15th April 2011 03:17 PM

The main advantages to good gain structure are:
  1. Maintaining a high S/N ratio.
  2. Preventing overdriving that causes clipping and distortion

By keeping you signals at the "correct" levels, you can keep them well above the noise floor, but not overdrive the circuits. That's the golden rule. The more devices you have in the signal path, the more you need to pay attention to this.

My rule of thumb is to keep the signal about 6dB below clipping. That can often be impossible or impractical, but it's a goal to shoot for. You don't want to go above that level - unless you're looking to add distortion. Dropping too far below that means you'll pick up noise.

That's it in a nutshell.

Pano 15th April 2011 03:22 PM

Sorry Dave, I missed your
Quote:

however, turning down the amp is better than turning down the pre...just sayin'
Absolutely! I had hoped to make that clear in the article. Maybe I didn't.
Keep the signal hot as long as you can. If you have to permanently turn it down to get the SPL you need, then the power amp is the place to do it.
Either with volume controls or other voltage dividers at the amp inputs, or with a lower gain amp. :up:

AUDIODH 15th April 2011 03:58 PM

Quote:

My rule of thumb is to keep the signal about 6dB below clipping. That can often be impossible or impractical, but it's a goal to shoot for. You don't want to go above that level - unless you're looking to add distortion. Dropping too far below that means you'll pick up noise.
That's it in a nutshell.
Pano,
well said!
The 6 dB headroom is a very good margin to shoot for. If your a live player and/or sound man....that is a big part of your job.
In home theater/stereo systems, that is taken care of in the mastering process. Often times to the point of destroying dynamic range....can you say 6dB overall headroom!
Pano, I see we completely agree on this subject and I hope it helps all the DIY members when gain staging their system.

AndrewT 15th April 2011 05:23 PM

Going back to the first Gain diagram.
If the X4 gain stage were switchable to bypass (X1) or to +6dB (X2) then the system as it was could be adjusted to suit virtually all sources

AUDIODH 15th April 2011 05:55 PM

Quote:

Going back to the first Gain diagram.
If the X4 gain stage were switchable to bypass (X1) or to +6dB (X2) then the system as it was could be adjusted to suit virtually all sources
Hi Andrew,
that is a true statement.
In an ideal world for us tweaks, a continuously variable gain pot is optimum, preferably with an signal/clip indicator for optimum level in the associated device. However, in many systems, that just means more $ to the end user. There is a good argument for a matched system where that engineering has been done up front and isn't a concern.
I sometimes run through as many as ten devices in the signal chain through both analog and digital paths, I have to relay on indicators and my ears to optimize the gain staging.
-Dave

adason 15th April 2011 06:43 PM

Talking about the gain strcture and the levels accross the signal chain...I miss those days where almost any device had its own VU meter, so you knew where is your signal. Reel-to-reels and cassette decks all had nice analog or digital meters, to keep the signal as far from the noise, yet safe from overload...even good pre-amps had the signal meters, not to mention power amps. Its a pitty that those nice fluorescent VU meters dissapeard, they were so much fun to watch, especially the fast ones. If you see complete systems these days, there are barely any VU meters!

adason 15th April 2011 06:48 PM

see what I mean? nothing moving to the signal, no meters...
http://www.diyaudio.com/forums/every...-diy-rack.html

AUDIODH 15th April 2011 07:20 PM

Adason,
yes I use to have a visual on the old McIntosh system with those beautiful blue meters and lights while listening to some Frazier Texas Bull Horn loudspeakers. That system rocked! It was in 1970 when I was young enough to truly sit and listen to really good music!
Your system looks nice. I could see you had Bryston power, and from the looks of it, Bryston preamps too! They design great stuff and the whole gain stage blog is a nonissue with quality matched gear as that you have! I'm sure you don't experience noise, clipping, etc with that system.
-Dave

danielwritesbac 15th April 2011 10:52 PM

It would be interesting indeed to see some other examples of amplifiers that have gain strictly in proportion to output potential. The majority of examples like that may turn out to be high power models (prosound scale).

What are some of the more modest (home scale) exceptions that actually work well?

Are there more good examples of high current output buffers (purpose made for driving speakers) to compare and read up on for reference?

Conrad Hoffman 15th April 2011 11:25 PM

Excellent article. I do have one small complaint- the term has always been and will continue to be "gain distribution".

Pano 15th April 2011 11:49 PM

Never heard that one, Conrad. In the pro audio I've always heard it called "gain structure."
Maybe it's regional. I'll look into it.

Daniel. Yes, if someone has some good examples from right here on diyAudio, that would be great.

sandyK 16th April 2011 12:13 AM

Michael
What you are saying is highly desireable.
However, until STBs and DTVs etc. use a gain stage to make up for the <-20dB level used with DTV broadcasts,
( and the level of the L and R from 5.1 Surround broadcasts is even lower)
then many of us are stuck with the need for additional gain, even though it will degrade other sources.
The DTV level via SPDIF is also low, and you really need to take extreme measures to avoid further S/N degradation,
otherwise low level ambience information is severely degraded.
Alex

Conrad Hoffman 16th April 2011 12:37 AM

Pano, it may well be. It seems that audio people, pro rf people, amateur hams, general ee types, electricians and physicists all have their own special vocabulary to describe exactly the same things! Interestingly, their fundamental understandings of how things work are also often different.

AUDIODH 16th April 2011 01:11 AM

I've been in audio for over thirty years..as well as electronics. I have always heard and used gain staging...never the others mentioned. Perhaps it's the EE schooling, or maybe it's geographical...but I understand either term, whether or not I agree what is right or wrong -I only have one each. :mischiev:

t3t4 16th April 2011 02:24 AM

Absolutely yes!

Quote:

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.
Gain can be/will be the death of us all... Less is better on all counts, but use the power of the amp, not the pre-stage input!

It's a tough game to play, I know. But the more you add component wise, the more distortion you "will" receive. The same is true of gain! Less is more in most if not all cases...

Until we deal with diamond based structures and/or components, we will always have loss and/or distortion. But also until then, keep your gains low with as few a component as possible!

My $0.50 worth.

t3t4

danielwritesbac 16th April 2011 02:39 AM

sandyK
Moosefet from Classic Valve Design is a dandy inexpensive little project with a small gain and some triode like harmonic filtering ability to increase intelligibility of digital TV, computer audio and the like. I think you can use it only for sources that need a boost so that you don't have to run high gain on everything.

Some people use a compressor limiter device on the TV to bring down the loud commercials and bring up the whispering voices.

I use the Moosefet on the TV because that TV already has AGC to defeat loud commercials. And I use 6n3p current buffer to filter Sony's HD radio just so that radio doesn't sound so awful and clip itself.

So, I'm saying that its okay to have specific preamps for specific sources. Its also inexpensive and its fun. :) You can sort of tune in each one to your own personal preferences.

fotios 16th April 2011 03:51 AM

That is a good article, very well focused on the issue. I have to add only a comment:
Gain is a restriction factor of Bandwidth according to my experience. Stages with unity gain have excellent bandwidth, instead when the gain of a stage is going increasing its bandwidth restricted proportionally.

Pano 16th April 2011 03:52 AM

Interesting. A quick Google shows both terms being use for the same thing.
May need to edit the article to include the "staging" term, if it's in common use.

fotios 16th April 2011 06:10 AM

Another one issue of big importance, is the volume control element (of any type: carbon track pot, opto-resistor, IC etc). When it works like a voltage divider at the input of a stage, it causes non-linearities in high frequencies. When is placed at full (i.e. 0dB thus no signal attenuation) it works like a simple resistor establishing thus a "constant" input impedance combined with the Zin of the following stage, where does not affects the high frequencies. In the past (working for more than two decades with pro audio equipments) i had not noticed this phenomenon, because pro audio signal processing devices are implemented by 99% with ICs which have limited bandwidth. Working the last two years with discrete implemented stages, I have to admit that the volume control part IS A REAL NIGHTMARE to me. Discrete stages with JFETs at input, seems to be not infected from the voltage divider established by a volume control. Instead those with BJTs at input are easily offended by the volume control. It seems to be an issue of the Zin of the stage that follows the volume control, because JFETs present very high Zin compared to BJTs (i am working exclusively with BJTs to the present). I did a lot of experiments on this issue, using always the same test setup: A 10KHz square wave as stimulus and a DSO connected at the output of the stage. Up to now, i have ascertained that a small resistance of the volume pot (up to 10KΩ as much, 1KΩ is "a must") can resolve the problem of high frequencies infection (overshoot or excess rounding of square wave rising edge).

blondamps 16th April 2011 09:16 AM

I love all you guys! As a fiddler who will NEVER stop trying out different things to keep "scratching the itch" I'm so happy that so many clever, resourceful, motivated people can STILL be doing what so many of us have been doing for 40 years or more and still want to keep doing it. As an oldie who works with youngsters whose only comprehensive understanding stretches to 'digital' audio, to see something that is so clear and commonsense about starting at the input and getting the right result at the output is refreshing indeed. Young hero musicians, PLEASE take note, and many MANY thanks, Panomaniac!

chaimovitch 16th April 2011 04:01 PM

Hi, I like this thread. In fact I think that I've got some problem with my "gain structure".
One question I'd like to raise is: does one need different gain structures to playback rock/pop/compressed CDs versus acoustical/classical/non compressed CDs?
To put things into practice, using Adobe Audition, a rock track (My Wife/ Who's Next/The Who) has a Peak amplitude of 0dB and an Average RMS amplitude of -11.5dB. A classical song (own recording of a Mozart's Berceuse) has a Peak amplitude of 0dB (the last chord) and an Average RMS amplitude of -42dB during 99% of the track. This makes a difference of 30dB on average perceived loudness with both the tracks peaking at 0dB.
As it's uncomfortable/dangerous to one's ear system to listen to the Who at more than 95dB for long, it is absolutely safe to listen to peaks of a few milliseconds during a acoustical track. Does it mean that one should have a system with a dedicated gain structure to play back compressed music and a different system with a dedicated gain structure to play back acoustical (not compressed) music?

Pano 16th April 2011 06:04 PM

Darn good question, Chaim. My first thought would be "No" but I'm open to criticism on that point. That Who track is super hot, it's in the category of over compressed mastering that is so disliked today. It's so loud all the time, how would you ever hear any noise? As long as it's not clipping anywhere, you likely won't hear any problems.

The Mozart on the other hand is rather low. -22dB is where I see a lot of classical CDs mastered. But a lullaby should be quiet, after all. That's the one that going to be a challenge. You want to keep that -42dB out of the dirt, but not clip the system or blow out your ears on the final chord. It can be done, but needs some care. The average music level is 133x lower than the peak. Meaning its around 0.015 volts coming out your CD player.

One way to figure out the levels along the path is to work backwards. Let's take an example with the Berceuse. How loud is the peak? Once you have it set were you want it, you could measure a 0dB sine wave at the speaker. Since 0dB is as loud as you're going, that's your reference. And if your CD player is typical, 0dBFS (full scale) is going to be 2 volts RMS or 2.8V peak. Working back from the speaker, to the amp inputs, to the preamp, to the CD player will give you an idea of how much gain (or attenuation) each section has. You'll probably find that you've actually dropped the signal somewhere along the path.

What can be done to optimize it? Without changing any of the components, you might find that you can get a stronger signal out of the preamp and attenuate more at the power amp. Is the power amp far below clipping at the max SPL you want? If so, maybe a lower gain or lower power amp would serve you better.

The first step is to measure and know what the signals are along the path. That will help you get a handle on how well balanced the gain is at each stage.

Conrad Hoffman 16th April 2011 07:54 PM

I can't recall ever building a power amp with a gain much higher than 10X. Going over 10X seems like the first step towards the troubles described. OTOH, trying to go too low can lead to stability problems; notice how many high bandwidth opamps have a minimum gain of 5X, or you have to add phase compensation networks to avoid building an oscillator.

Signal to noise performance is set at the first stage of any amplifier. It can get no better than that downstream, though it can certainly get worse, as the article shows. Complexity is bad; I've no experience with true pro audio, but it seems obvious that complicated (many stages) systems get a huge benefit from standardizing on larger signals. It's a wonder modern PC stuff works as well as it does, given 5V power rails.

[begin dumb idea] We have an environmental chamber at work with a refrigerator and a heater. It controls the temperature by running the fridge full blast all the time, and simply changing the heater power to get the desired temperature. Maybe the ultimate answer is to run the audio system wide open, just below clipping, and put a power attenuator right at the speaker! The numbers work out well unless efficiency is one of them. [end dumb idea]

Pano 16th April 2011 08:42 PM

Ha! Actually I was going to suggest just such a thing, if only as an example. :D

Run it all flat out (barring clipping) and turn it down at the speaker with an L-Pad or transformer attenuator. Inefficient, and probably won't sound good, but it should keep the S/N ratio pretty high.

But then you run into problems such as trying to mix consumer and pro levels, such as the DCX2496 example in the article. If you are using the analog inputs, your CD player signal may still be a little weak. Use the digital input and you'll get more than enough level out to run any amp.

AndrewT 17th April 2011 11:38 AM

The F5 or F5x (+16dB) will be must first Power amp with a gain of less then 20times (+26dB)
Not many Power Amps will perform well if the gain is significantly lowered cf. what the designer intended.

roscoeiii 19th April 2011 04:26 AM

What a great article and thread. One of the most useful I have read here. Wish I'd been more aware of this a year or so ago. After moving to a tube amp (an OTL Atma-sphere S-30), I was delighted that I was able to get jumpers to replace 2 of the 6SN7s and reduce the gain of my amp. Speakers are much quieter as a result and I get a greater range of travel in my preamp.

To get more fine-tuned control from my preamps attenuator, I have also lowered the output voltage from my DAC to 1.35V. But after reading this thread, I feel that the sound quality might benefit from bumping it back up (options are 5.48, 2.74, 1.78 or 1.35V output). With the lowered amp gain, maybe I will have an acceptable sensitivity in my volume control and improved sound quality from having a hotter source?

AUDIODH 19th April 2011 05:21 AM

GAIN
 
I guess I am confused here about at all the post regarding Amp gain....did I miss something all my professional career???
Did your favorite XYZ bands, sound company's/Pro audio designers, recording studios, etc. miss the collective boat?
WHY is amp gain such a big issue - did the rest of the world skip a beat???? Did the DIY counsel find something the rest of the audio engineering world missed??? Help me out here...please! Dang these industry pro's sheesch
- Dave

Pano 19th April 2011 06:09 AM

Why? Because of the costs of gain. Why have 40X voltage gain if you only need 4x?
The DIY crowd tend to be frugal. Frugal with their money and also their designs. Many like to uncomplicate things.

The costs of more gain than you need include things like noise, distortion, higher parts cost, more complicated circuits, etc. While I can't swear that low gain systems are great just because they are low gain, many of the best systems I've heard were low gain. Certainly noise is low in such systems.

And there is a certain elegance in "enough, but not too much." Nothing wrong with that as a goal.

Pano 19th April 2011 06:16 AM

Quote:

Originally Posted by roscoeiii
I was delighted that I was able to get jumpers to replace 2 of the 6SN7s and reduce the gain of my amp.

.
:up: That's cool that you were able to do that.

Quote:

With the lowered amp gain, maybe I will have an acceptable sensitivity in my volume control and improved sound quality from having a hotter source.
That's been my experience. A source that is a little hotter than it "needs to be" somehow sounds better. I don't know why, but to me it does. Going to 2.74 volts is an increase of 6dB. The change is worth a listen.

KaffiMann 19th April 2011 12:25 PM

So basically, you should have a power amp with short signal path, that you could adjust the psu voltage on...?

So I could use for example 2x3-30VDC power supplies for the power amp (in series, middle to ground), and replace the pot for voltage adjust with a stereo lightspeed attenuator, or just a regular stereo pot (one part for each psu).

But what kind of power amp would handle such a variable psu?

Edit:

I noticed that this has been discussed before:
http://www.diyaudio.com/forums/solid...e-control.html

Pano 19th April 2011 01:03 PM

Let's not get bogged down in amplifier design, it's not the subject of this article. There are many threads that cover the subject.

The idea is good management of signal levels all along the path. Choosing devices with modest gain is often a good way to do this for a home system. Finding the right amount of gain and attenuation is what you really want.

KaffiMann 19th April 2011 01:27 PM

Yes, the right amount of gain in the right place.

I was just thinking how you could apply that in a practical way. You have to be able to adjust the gain somewhere, I have no intention of listening to music on 10 different audio systems, pending on the volume i desire.

The consensus seems to be that high gain sources are nice, so that would translate to voltage buffers - stepped attenuator (because a lot of potmeters cannot really cope well with more than a few volts) - current gain stage.

But then the gain would be higher in one stage than the other, and regulated by variable resistance on top of that. Isn't some of the idea that the gain should be somewhat balanced throughout the entire chain?

Unless you have some way of adjusting the gain directly, by for example adjusting the supply voltage of the circuits, provided that they can handle that without going into "crappy snr mode".

I may be lost in some chain of thought here, but the article got me thinking, which is completely foreign to me.


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