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Relay stepped attenuators vs pots, volume control, and measurements

jaytrinitron

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So the other day I was reading the MassDrop THX AAA 789 amp page and one of the engineers at THX was explaining that the pot on the 789 only has -85 dbfs of attenuation, and so you can still hear the music with really sensitive IEMs with the volume pot turned all the way down - described as "pot bleed". He said the only way to stop this would be a relay stepped attenuator (as present in the Benchmark THX HPA-4) but that was prohibitively expensive for the 789. All fair enough. But I am wondering whether this would affect the output on the 789 negatively. As far as I am aware, Amir measures only the max output and/or line out of an amp when doing his tests, but hardly anyone listens at these volumes. So I am wondering, does the attenuation of the pot negatively affect the measurements to a significant degree? If it does, are the effects of pot attenuation vs not using a pot audible? I believe not, since as I understand it the amp measures at its worst on full/max output, and only gets better from there (I think). Is this true?
 

RayDunzl

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If you have "really sensitive" IEMs, why do you need an amp?
 

RayDunzl

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So I am wondering, does the attenuation of the pot negatively affect the measurements to a significant degree? If it does, are the effects of pot attenuation vs not using a pot audible?

Somebody measured a pot a couple of days ago here.

They don't necessarily harm the signal, but a mechanical two-gang pot tends to attenuate the two channels a little bit unevenly as you twist the knob.

Recent investigation: https://www.audiosciencereview.com/...-dig-or-not-to-dig-that-is-the-question.5951/
 
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RayDunzl

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As I understand it the amp measures at its worst on full/max output, and only gets better from there (I think). Is this true?

The signal to noise ratio is worst when there is no signal. If noise stays the same, increasing the signal level increases the SNR (better).

Harmonic Distortions - worst as the amplifier approaches whatever level of distortion you define as clipping.

I'm unsure how the value of THD changes with non-clipping signal level, different devices show different curves. The ratio of harmonics levels to signal level may change at different signal levels, and higher signal likely creates relatively higher harmonics, but I don't have the gear to test/verify that assumption with what I have here to play with.
 
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jaytrinitron

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If you have "really sensitive" IEMs, why do you need an amp?
I don't have IEMs, I was just explaining that the poster heard "pot bleed" with his IEMs and was wondering what it was, hence the explanation by the THX engineer.

Thank you for your other responses as well, they were very elucidating, particularly the post comparing digital and analog volume control.
 
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