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Old 29th April 2012, 03:41 PM   #1
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Default Attenuation circuit. What is ideal location?

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?
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Old 29th April 2012, 03:45 PM   #2
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Attenuating the audio after the power amplifier will reduce the ability of the amplifier to control the loudspeaker. The damping factor will be very low.
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Old 29th April 2012, 03:48 PM   #3
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Quote:
Originally Posted by Frank Berry View Post
Attenuating the audio after the power amplifier will reduce the ability of the amplifier to control the loudspeaker. The damping factor will be very low.
Fair enough. Had not thought of that. Thanks!

Would placing it between the two stages in say an F5 be better?
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Old 29th April 2012, 04:34 PM   #4
AndrewT is offline AndrewT  Scotland
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Default 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.
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Old 30th April 2012, 12:13 PM   #5
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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?
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Old 30th April 2012, 12:42 PM   #6
OllBoll is offline OllBoll  Sweden
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Isn't the ideal to have no attenuation circut at all but to do the attenuation digitally before sending it to the DAC?
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Old 30th April 2012, 12:44 PM   #7
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Quote:
Originally Posted by OllBoll View Post
Isn't the ideal to have no attenuation circut at all but to do the attenuation digitally before sending it to the DAC?
Possibly, but not the question in this case.
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Old 30th April 2012, 07:59 PM   #8
AndrewT is offline AndrewT  Scotland
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the F5 is your Power amplifier. You can fit attenuation before it, or after it. Not in the middle.
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Old 1st May 2012, 08:52 AM   #9
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Quote:
Originally Posted by AndrewT View Post
the F5 is your Power amplifier. You can fit attenuation before it, or after it. Not in the middle.
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?
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Old 1st May 2012, 09:56 AM   #10
DF96 is online now DF96  England
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Option 1 will have more noise. Option 2 will have more distortion. You choose which you prefer. All electronics, like all engineering, is a matter of compromise.
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