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Old 5th July 2012, 06:34 AM   #441
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by DouglasSelf View Post
I thought I had demonstrated in Post #401 that nothing bad happens in the active gain control at very low volume settings.
My thoughts on that are that 300mv output is still huge. That would give around 4.5 watts RMS when fed into a (typical) power amp and 8 ohm.

What about at 20 or 30 mv output which might realistically equate to around 50mw into 8 ohm when fed into a typical power amp. Also at even lower levels still. 20vrms in and say 1 or 2 mv output is a totally valid condition too.

I don't know the answers to all that as I haven't the means to make such measurements but it would be interesting.
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Old 5th July 2012, 06:41 AM   #442
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Originally Posted by owdeo View Post
Apologies for interrupting an interesting technical discussion that I hope continues. I've been doing some.................. .
An interesting read, thanks for putting it all down.
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Old 5th July 2012, 07:41 AM   #443
Mooly is offline Mooly  United Kingdom
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Originally Posted by mrlincoln61 View Post
Hello Mooly

I presume he measured his volume control with an input signal of 20V rms because the unit at hand had a 5k pot rather than a 1k pot. This is to get a hefty class AB current flowing from the op-amp output stage. What matters is this and the level of attenuation. If no distortion components were visible from the noise at a 300mV output, I'd question if there would be anything to see at 30mV output, let alone 1 or 2mV.
The class ab output stage of an opamp generates its distortion when supplying current to a load.

If the level of signal voltage (opamp output) is very small, the opamps "distortion" is still at the same level as it was for a much higher voltage output. The opamp supplies the same current into the feedback network regardless of the feedback resistor value. As the feedback resistor value goes down the voltage goes down keeping the current the same.

Noise (another analogy, think wanted signal vs noise (opamp distortion))... its like saying that 1 mv of noise riding on 100 volts of signal isn't very much. Attenuate the signal with a normal pot and the noise and signal go down together. Attenuate with a set up that keeps the noise (read distortion) at 1mv and only reduce the signal and then as signal goes down the "noise" (distortion) becomes more dominant.

What do others think to that ?
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Old 5th July 2012, 10:53 AM   #444
Mooly is offline Mooly  United Kingdom
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Originally Posted by mrlincoln61 View Post
................ then why should one be worried about the distortion?
Because the conventional resistive divider type of volume control attenuates signal and noise and distortion and everything else progresively down to zero.

The active gain stage does not achieve the same overall result.
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Old 5th July 2012, 12:01 PM   #445
forr is offline forr  France
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Mooly
Because the conventional resistive divider type of volume control attenuates signal and noise and distortion and everything else progresively down to zero.
The active gain stage does not achieve the same overall result.


In digital format, percentages of distorsion increase with lowering levels.
Hower absolute values of distorsion do not, just like the Baxandall active gain control, I think.
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Old 5th July 2012, 03:27 PM   #446
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properly applied dither, Delta-Sigma DAC give linearity below the noise floor - verified by averaging
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Old 5th July 2012, 04:18 PM   #447
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Quote:
Originally Posted by Mooly View Post
Because the conventional resistive divider type of volume control attenuates signal and noise and distortion and everything else progresively down to zero.
Correct, the noise and gain of the first stage determines the noise factor of the entire system. If the first stage gain is zero and the stage is purely resistive the noise can be approximated by KTBR, (K= Boltzmann Constant; T = Temperature; B- Bandwidth; R is resistance. This is common knowledge to the communication/RF engineers.
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Old 5th July 2012, 11:57 PM   #448
owdeo is offline owdeo  Australia
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I believe I understand the proposed mechanism, but have not Mr Self's measurements proven that even at high levels of attenuation the distortion is buried in the noise? Is anyone suggesting this phenomenom would not show up under these test conditions? Or that distortion at a level below the noise floor could be audible?

I think there is something different about the way this configuration sounds compared with a conventional passive volume control. But seems as though it must be something else. I have no idea what though
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Old 7th July 2012, 04:16 AM   #449
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Quote:
Originally Posted by Mooly View Post
The class ab output stage of an opamp generates its distortion when supplying current to a load.
I see the stark fist of removal has been applied, I can't believe it was not obvious from the start.
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Old 7th July 2012, 04:50 AM   #450
qusp is offline qusp  Australia
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how is distortion attenuated with a passive control? its level remains the same, as a percentage of the signal being attenuated does it not? that seems the only useful way to represent distortion to me, as a percentage of the signal, not vs fullscale. noise? perhaps, noise in the signal anyway, but the noise created by the resistors, lower CMRR by resistor mismatch in balanced systems, non-linearity from contacts etc etc, i'll take a constant vanishingly low noise and perfectly matched levels, with cheap and handy means to control it perfectly over these any day.

besides, modern digital attenuation as part of software, DSP or the dacs themselves, is operating at 40 and 64bits float, I havent studied how this relates to the interpolation/dither of the signal though, anyone got any info or links on that?

Last edited by qusp; 7th July 2012 at 04:53 AM.
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