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Reverse-engineering my speakers' API to get reasonable volume control

(jamesbvaughan.com)
287 points jamesbvaughan | 5 comments | 25 Jul 24 15:11 UTC | HN request time: 0s | source
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rahimnathwani ◴[26 Jul 24 23:52 UTC] No.41083383[source]▶
I know it is tangential, but this about his old system caught my attention:

  With that system, I could set the amplifier’s analog volume knob such that the max volume out of the streamer corresponded to my actual maximum preferred listening volume, giving me access to the full range of Spotify or AirPlay’s volume controls.
Assuming an analog input, this might result in a noticable quality reduction at low volumes.
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1. AceJohnny2 ◴[27 Jul 24 00:11 UTC] No.41083495[source]▶
Tangential fun fact: amps have a fixed gain, because it's hard to make a variable gain without distortion [1]. The volume knob doesn't control amplification, in fact it controls an attenuation stage, because it's easier to make variable attenuation with low distortion.

[1] that's why there were so many different "classes" of amps, they're all making different tradeoffs about how they're doing the amplification.

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2. jamesbvaughan ◴[27 Jul 24 00:16 UTC] No.41083522[source]▶
That helps explain why the "volume" as represented on disk in the debug bundle as "attenuation" and was measured in negative dB.
3. rahimnathwani ◴[27 Jul 24 01:38 UTC] No.41083876[source]▶
According to Claude, the attenuation stage is before the power amp stage. Does that mean worse SNR whether the volume is controlled using the volume control or via the input?

(Ignoring the additional quantization issue with a scaled digital input.)

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4. kevin_thibedeau ◴[27 Jul 24 01:55 UTC] No.41083940[source]▶
Class D doesn't have any attenuation. This is a big factor in their greater efficiency.
5. dreamcompiler ◴[27 Jul 24 05:18 UTC] No.41084601[source]▶
You could put the attenuation stage after the power amp stage but it would require big beefy resistors that could absorb a lot of power. They'd get hot and the whole thing would be very inefficient.

But hey, very low distortion.