The usual way of doing things is placing an attenuation circuit in a preamp, or, if we do not need gain, in a buffer. Often at the input (e.g. B1), sometimes at the ouput (e.g. Aleph P1.7). Perhaps some preamps have it between the gain stages (if more than one is present).
From Nelson's P1.7 user manual, I understand that the reason he placed the attenuator after the gainstage was to avoid amplifying noise (from a degraded signal).
"There are good reasons for having such an arrangement:
The gain stage operates at a constant level regardless of the setting of the master level control, and so the sound of the this circuit will not alter at various level settings.
...
Any noise characteristic of the preamp circuit is attenuated along with the signal, unlike circuits where the volume control is before the input."
But don't we amplify noise in all poweramps? Or is the signal now so robust that attenuation no longer (audibly) degrades it?
I know there will be many practical problems implementing attenuation after the power amp, such as getting rid of a lot of heat (!), but practical things aside, would it sound better if your pre- and poweramp are able to amplify the full, unattenuated, signal?
Why not place the attenuator circuit at the output of the poweramp?
How much of the sound gets lost in an attenuation circuit in a preamp? How much would be lost if the attenuation circuit were placed after the power amp?
Has anyone experimented with this?
From Nelson's P1.7 user manual, I understand that the reason he placed the attenuator after the gainstage was to avoid amplifying noise (from a degraded signal).
"There are good reasons for having such an arrangement:
The gain stage operates at a constant level regardless of the setting of the master level control, and so the sound of the this circuit will not alter at various level settings.
...
Any noise characteristic of the preamp circuit is attenuated along with the signal, unlike circuits where the volume control is before the input."
But don't we amplify noise in all poweramps? Or is the signal now so robust that attenuation no longer (audibly) degrades it?
I know there will be many practical problems implementing attenuation after the power amp, such as getting rid of a lot of heat (!), but practical things aside, would it sound better if your pre- and poweramp are able to amplify the full, unattenuated, signal?
Why not place the attenuator circuit at the output of the poweramp?
How much of the sound gets lost in an attenuation circuit in a preamp? How much would be lost if the attenuation circuit were placed after the power amp?
Has anyone experimented with this?
Fair enough. Had not thought of that. Thanks!
Would placing it between the two stages in say an F5 be better?
A good Power Amplifier does not need further noise attenuation.
Think of the sound at your listening position from your speakers.
A very high SPL (domestic) speaker will get to around 110dB at your ear.
A power amp that has 120dB S/N ratio will result in noise 10dB below audible limit if the amp can just get the speakers to that high SPL level.
More ordinary (domestic) amplifier/speaker systems attain 100 to 105dB at the listening position.
The Power Amplifier only has to achieve a S/N ratio of 105dB to get the noise below audible level.
But you have another 20 to 30dB on your side. Domestic surroundings are rarely, if ever, down at the 0dB, inaudible SPL level. More usually we have a non constant din of around 20 to 30dB from the wind, the heating, the fridge/freezer, the lighting, the clock, etc......
It's the stages before the Power Amplifier that benefit from noise attenuation.
Think of the sound at your listening position from your speakers.
A very high SPL (domestic) speaker will get to around 110dB at your ear.
A power amp that has 120dB S/N ratio will result in noise 10dB below audible limit if the amp can just get the speakers to that high SPL level.
More ordinary (domestic) amplifier/speaker systems attain 100 to 105dB at the listening position.
The Power Amplifier only has to achieve a S/N ratio of 105dB to get the noise below audible level.
But you have another 20 to 30dB on your side. Domestic surroundings are rarely, if ever, down at the 0dB, inaudible SPL level. More usually we have a non constant din of around 20 to 30dB from the wind, the heating, the fridge/freezer, the lighting, the clock, etc......
It's the stages before the Power Amplifier that benefit from noise attenuation.
Thanks Andrew, I think I understand what you mean, but am still wondering about the effect on sound.
A thought experiment.
Let's take a two stage amplifier like the F5. Assume a 2V source and 5V at the speakers.
Option 1: Pot before input.
Option 2: Pot between voltage and current gain stages. (please disregard impedance issues, this is just to make a point).
Let's assume that I have the pot at 10 o'clock position in both cases for the low to moderate sound level I wish to listen at (at that time).
With option 1, the 2V source is attenuated to, say, 0.5V and then amplified by the poweramp to 5V.
With option 2, the 2V source is amplified to 20V by the first stage of the poweramp and then attenuated to 5V (by the pot).
Assuming levels are matched to within whatever accuracy is possible, will there be any difference in the sound coming from speakers from options 1 and 2?
A thought experiment.
Let's take a two stage amplifier like the F5. Assume a 2V source and 5V at the speakers.
Option 1: Pot before input.
Option 2: Pot between voltage and current gain stages. (please disregard impedance issues, this is just to make a point).
Let's assume that I have the pot at 10 o'clock position in both cases for the low to moderate sound level I wish to listen at (at that time).
With option 1, the 2V source is attenuated to, say, 0.5V and then amplified by the poweramp to 5V.
With option 2, the 2V source is amplified to 20V by the first stage of the poweramp and then attenuated to 5V (by the pot).
Assuming levels are matched to within whatever accuracy is possible, will there be any difference in the sound coming from speakers from options 1 and 2?
Let's assume it is a BA then. Same question.
Let's take a two stage amplifier like the BA (1, 2 or 3). Assume a 2V source and 5V at the speakers.
Option 1: Pot before input.
Option 2: Pot between voltage and current gain stages. (please disregard impedance issues, this is just to make a point).
Let's assume that I have the pot at 10 o'clock position in both cases for the low to moderate sound level I wish to listen at (at that time).
With option 1, the 2V source is attenuated to, say, 0.5V and then amplified by the poweramp to 5V.
With option 2, the 2V source is amplified to 20V by the first stage of the poweramp and then attenuated to 5V (by the pot).
Assuming levels are matched to within whatever accuracy is possible, will there be any difference in the sound coming from speakers from options 1 and 2?
option 1 is the same as having a aleph p1.7 and move the pot to the next chassis.(power amp)
Ideal Location For POT
Your assumptions are "illogical" or the logic is "illegal"
It is better if you put one simple question after another, to clearly show which small part that you try to understand. But anyway I will try to read between the lines...
# Noise may come from the circuit itself, not only from the input. If you put POT after an amp the circuit noise is attenuated by the POT. Notice how power supply quality plays its role.
# Noise in a preamp is more critical than in an amp because the signal is low and along with the noise they will be amplified by final amplifier. Many of us cannot accept even the best preamp because of this nature.
# If you think about sound quality, you cannot just assume that impedance has no effect. The real answer to your question depends on the exact design of the used preamp and amp. There are too MANY possibilities even if the effect of impedance is neglected, which I think we cannot. The benefit of the amp to amplify full signal (assuming that it is capable of doing so) is actually related to impedance.
# Depends on the design. What if the amp clips, overloaded, or whatever with full 2V signal? To make it simple, what if 1V input gives 0.001% THD but 2V input gives 1% THD?
# Attenuation is relatively not an issue. What is more critical is gain. Most preamps of course tend to have less gain than power amps.
# You cannot make such assumption. You cannot put a POT there and assume it is an F5 (because it will not work). You forget about global feedback (taken from the second stage and feed forward to the first stage). Also, a high output current cannot be created without high voltage after the first stage.
# You must have an exact working design without assumptions, and the answer depends on the design. For example, a HEXFET has a high Ciss, it needs strong current drive. Putting a POT in front of it will worsen the situation. Another example, a SE preamp has low PSSR or the effect of power supply noise is big. If the supply is conventional then putting the POT at the output will be beneficial. And of course, the more important issue is the gain and distortion characteristics related to signal input.
Your assumptions are "illogical" or the logic is "illegal"
It is better if you put one simple question after another, to clearly show which small part that you try to understand. But anyway I will try to read between the lines...# Noise may come from the circuit itself, not only from the input. If you put POT after an amp the circuit noise is attenuated by the POT. Notice how power supply quality plays its role.
# Noise in a preamp is more critical than in an amp because the signal is low and along with the noise they will be amplified by final amplifier. Many of us cannot accept even the best preamp because of this nature.
# If you think about sound quality, you cannot just assume that impedance has no effect. The real answer to your question depends on the exact design of the used preamp and amp. There are too MANY possibilities even if the effect of impedance is neglected, which I think we cannot. The benefit of the amp to amplify full signal (assuming that it is capable of doing so) is actually related to impedance.
# Depends on the design. What if the amp clips, overloaded, or whatever with full 2V signal? To make it simple, what if 1V input gives 0.001% THD but 2V input gives 1% THD?
# Attenuation is relatively not an issue. What is more critical is gain. Most preamps of course tend to have less gain than power amps.
# You cannot make such assumption. You cannot put a POT there and assume it is an F5 (because it will not work). You forget about global feedback (taken from the second stage and feed forward to the first stage). Also, a high output current cannot be created without high voltage after the first stage.
# You must have an exact working design without assumptions, and the answer depends on the design. For example, a HEXFET has a high Ciss, it needs strong current drive. Putting a POT in front of it will worsen the situation. Another example, a SE preamp has low PSSR or the effect of power supply noise is big. If the supply is conventional then putting the POT at the output will be beneficial. And of course, the more important issue is the gain and distortion characteristics related to signal input.
I know.
I am trying to find out at which point in the cascade of stages it would be ideal to put the attenuation circuit. I feel that straight after the last voltage gain stage would be the best spot, but so far I get answers that focus on the example (B1, Aleph P1.7, F5 or BA) rather than the concept.
Perhaps my question should have been different.
For what it's worth:
Where, in the cascade of stages between source and speaker, would it be best to place an attenuation circuit?
I also see that Jay has just answered. I will reply to that after I post this.
Ideally, source-amp-speaker should be designed as one system for best result, but we know that it is not the common practice. If we design them as a single system, we might want to put the attenuation in the digital domain.
Your assumptions are "illogical" or the logic is "illegal"
Please see post #15. I think my assumptions were logical but were taken literally instead of as intended, purely as an example.
I was quoting Nelson from the Aleph P1.7 user manual:
There are good reasons for having such an arrangement:
...
Any noise characteristic of the preamp circuit is attenuated along with the signal, unlike circuits where the volume control is before the input.
If you extrapolate then it would be best to attenuate "as late as possible", i.e. after the last gain stage....
Any noise characteristic of the preamp circuit is attenuated along with the signal, unlike circuits where the volume control is before the input.
The impedance remark was because I did not want the discussion to bog down into technical details (as it has ended up doing) because I was looking for a theoretical answer to a theoretical question.
I was looking for a "class A sounds better than class AB" type answer. I know there a very nice sounding class AB amplifiers and I know that if you screw up the design class A will sound horrible, but in general some (perhaps even most in the Pass Labs forum anyway) will agree that class A sounds better.
My answer was to a specific question you posed, but I also hinted at the generic case.albertNL said:
If you are worried about noise, put the control as late as possible. If you are worried about distortion and/or clipping, put the control as early as possible. If you want to get the best compromise, then learn about gain distribution and put the control somewhere in the middle. That is the concept. If this does not answer your question, then you are not asking the question you think you are asking.
There is no clear answer to the original question because the question is vague and not specific ( 'best place' - best what?).
As noted before, as late as possible improves S/N ratio, as early as possible improves distortion and overload performance.
You cannot decouple these considerations from a specific implementation (pre amp, power amp) you have in mind.
That is the reason that the discussion wavers back and forth - no clear target to hit!
jan didden
As noted before, as late as possible improves S/N ratio, as early as possible improves distortion and overload performance.
You cannot decouple these considerations from a specific implementation (pre amp, power amp) you have in mind.
That is the reason that the discussion wavers back and forth - no clear target to hit!
jan didden
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