Volume control vs. attenuation control

03-14-11: Herman
Elizabeth wrote:
"Technically an 'attenuator' cuts the signal before any amplification stages."

No, attenuate means to reduce, it doesn't matter where you do it. If it reduces the amplitude of the signal it is an attenuator no matter whether it is before or after the gain stage.

as a practical matter, when it comes to the way that preamplifiers actually work, elizabeth is correct. the way that preamplifiers actually work is that they are designed to a set input sensitivity level. however, some sources might be higher or lower than that input sensitivity level. sources that are higher are "hot" sources, so when you switch to a "hot" source, your volume setting may become too high. if the source is really hot you may have to turn the volume down really low. what an attenuator does is to allow you to better control for differing voltage levels from differing sources by applying differing levels of signal reduction at the input. any gain would then occur within the preamplifier from that point.

03-14-11: Herman
Elizabeth wrote:
"A volume control actually changes the active amplification of the signal, from well below less than what it comes in as, to increasing it."

That would be extremely unusual. As stated above, the gain of almost all active stages is fixed. The volume control is merely dividing the voltage so you get the level you want. It isn't possible for a typical volume control to increase the amplification of the signal.

your explanation is incorrect. gain is determined by the ratio of input signal amplitude to output signal amplitude. the output signal amplitude from the preamplifier varies in response to the volume setting of the preamplifier. therefore, the gain from the preamplifier does indeed vary as elizabeth suggested.

as an aside, i will add that it is incorrect to say that an attenuator and a volume control are the same thing, because they aren't. depending upon the volume control setting you do often get gain as a result of the volume control (i will let others add comments specific to passive preamplifiers as i am not addressing that device in my comments) but you, of course, never get gain from an attenuator. and, as i stated above, it is also incorrect to say that the gain of a preamplifier is fixed: the reason why people use preamplifiers is to get variably controllable gain.

Paperw8, I think you may have misread Herman's comment. Note that he did not say that the gain of a preamplifier is fixed. He said that "the gain of almost all active STAGES is fixed." That is a correct statement, as are the rest of his comments, IMO. The statement in his post that "the gain of these amps is fixed" was clearly a reference to amplifier STAGES, when considered in the context of the surrounding paragraph.

It is certainly possible to design an active stage whose gain can be directly varied by the setting of a potentiometer, and where the maximum gain setting is greater than 1. An example would be an op amp stage with a pot in the feedback loop. However, that "would be extremely unusual" (to use Herman's words) in a quality audio design.

Regarding preamps that provide independent level settings for each input, to allow similar settings of the main volume control to be used for sources having significantly different output levels, it obviously makes sense to not refer to those controls AND to the main volume control as "volume controls," which would lead to confusion as to what was being referred to. So it would make sense in that situation to refer to the controls at the input as attenuators or input level controls. But those controls and the main volume control are doing the same thing, just at different points in the signal path, and with different kinds of adjustability.

In Mark's case, I believe he is saying that there are three controls involved -- one in the power amp, a main volume control in the preamp, and an "attenuator" in the preamp, perhaps providing a few discrete attenuation settings that affect all inputs equally (i.e., the attenuator is "after" the source selection function in the signal path). To be able to utilize the main volume control in the preamp at higher settings, which was his original question, I don't think it is possible to say in general if it would be better to lower the setting of the control in the power amp, or to increase the attenuation setting in the preamp, or both, as there are many design-dependent variables and unknowns involved. In some cases it might not matter at all. Experimentation figures to be the only way to tell.

Regards,
-- Al

Everything I said is correct. The problem comes when trying to explain technical terms to non-technical people.. I'll drop it after this because it is clear that most who have contributed are using terms the way non-technical people use them. There is no way to educate them to a sufficient level in this forum that they will fully grasp all of the terminology. By taking technical terms and applying very narrow audiophile meanings to them we get confusion.

You are taking a technical term "attenuator" and saying you can only use it the way some manufactures use it, which by the way, is also often incorrect. How can they call a knob an attenuator when at some positions it results in the preamp having an increased output?

it is incorrect to say that an attenuator and a volume control are the same thing, because they aren't. depending upon the volume control setting you do often get gain as a result of the volume control

but you, of course, never get gain from an attenuator.

Perfect examples, you use gain in the non-technical sense which assumes an increase. In many cases the output of the preamp is less than the input which means it has a gain of less than 1. As long as the discussion mixes the meaning of terms we will just be going in circles.

The only way your statement correct is if you consider the entire preamp to be the "volume control." The knob you turn which is often called a volume control is indeed a voltage divider that can only reduce (attenuate) the level. Any gain (in the incorrect non-technical sense of increasing amplitude) comes from active stages either before or after this divider and that gain is indeed fixed. If you want to change conventional terminology and say a voltage divider plus an active gain stage equals a volume control then you would be correct.

On the majority of preamps the knob you turn labeled "volume" is a voltage divider at the input. It is an attenuator. It has no positive gain. A few put it at the output but it is also a voltage divider which reduces the voltage which again makes it an attenuator.

The other problem is you misquoted me.

it is also incorrect to say that the gain of a preamplifier is fixed:

I never said that. You are confusing the "active stages" I was talking about inside the preamp with the entire preamp. Once again, a confusion between what is technically correct and how non-technical people sometimes incorrectly use the terms. Although many preamps contain active stages they are not equivalent terms.

You insist an attenuator differs from a volume control even though they both attenuate, they both control volume, and depending on where they are set in a particular preamp they both might result in an increase in amplitude.

I'll stick by my explanation. It is technically correct.

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Ergo, my integrated amp has a "gain" setting which defaults at zero gain and which can be adjusted to plus or minus settings. So the zero "gain" setting is set at an arbitrary position by the manufacturer, and actual amplifier gain would be the highest "gain" setting. And "gain" is a misnomer (sp). Technically, the "gain" setting is an attenuator, a voltage divider. Right?

Not exactly. Gain is simply the ratio of output to input. the 0db setting isn't arbitrary, it is the point where out = in.

If the gain is less than 1 (output less than input) we say the preamp is attenuating the signal. With 2 volts in and one volt out the gain would be .5

If the gain is greater than 1 (output more than input) then it is amplifying the signal. With 2 volts in and 4 volts out the gain would be 2

If the output equal the input the gain is 1.

This is commonly expressed in dB which for voltage is 20 times the log of the gain. If gain is less than 1 that comes out as -dB, a gain of .5 is -6dB. A gain greater than one comes out +dB. a gain of 2 is +6dB. Unity gain when out = in comes out to 0dB so that isn't an arbitrary position.

Your preamp is attenuating the signal when you have it set to -dB, amplifying when it is set to +dB, and not changing the level when set to 0dB.

It accomplishes this with a combination of an active stage that amplifies and a voltage divider volume control which attenuates. The volume control can have a gain from between 0 and 1 and the active stage some positive amount. Multiply the 2 together to get the overall gain.

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