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Old 27th August 2012, 10:37 AM   #1
elmura is offline elmura  Australia
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Default Is Variable Gain a better choice?

In modifiying an Opamp audio circuit, for the volume control, there are a few ways to go about it.

1. Potentiometer before the gain stage (noise is amplified)
2. Potentiometer after the gain stage
3. A low impedance Pot generating Variable Gain but not in the negative feedback loop.

I'm looking at options 2 and 3.

My preference for option 3 is that rather than having the gain being constant and attenuate the gained signal of option 2, we are able to have low gain (and its benefits at low volume levels) and only get more noise etc when increasing the gain past unity.

The concept is this: The feedback loop has a fixed resistance, but the division to ground is varied. Thus the feedback loop is not dirtied.

What are your thoughts on this?
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Old 27th August 2012, 11:28 AM   #2
DF96 is offline DF96  England
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If the pot varies the feedback then the pot is 'in' the feedback loop. Don't believe some of the nonsense people say about 'signal path'.

If possible, avoid circuits which give maximum gain when the wiper lifts off the track.
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Old 27th August 2012, 01:38 PM   #3
AndrewT is offline AndrewT  Scotland
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The opamp must be stable for low gains and for gains of less than 1.

Will the signal quality be the same when the gain is +10dB, or 0dB, or -10dB, or -40dB?
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Old 27th August 2012, 04:41 PM   #4
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I wouldn't do that, because I believe one always should use as short tracks as possible for feedback, as well as use high quality resistor. Well, for NON-Inverting it might even work, as you said, it is just to set ratio, but for inverting version - I wouldn't recommend.
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Old 27th August 2012, 05:27 PM   #5
Davey is offline Davey  United States
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You could consider the classic, inverting configuration. A linear-taper potentiometer can yield a linear db attenuation characteristic.

Cheers,

Dave.
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Old 27th August 2012, 05:57 PM   #6
johnr66 is offline johnr66  United States
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What AndrewT said. Some op amps may not be stable at lower gain settings. Audio power amps (which are really power op amps) are probably not stable at low gain.
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