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-   -   What is Gain Structure? (http://www.diyaudio.com/forums/diyaudio-com-articles/186018-what-gain-structure.html)

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:


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