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Old 12th June 2008, 06:01 PM   #1
preiter is offline preiter  United States
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Default Control gain by changing supply voltage?

Since a class-D amp works by switching on and off the supply voltage, would it be possible to control the gain by changing the supply voltage?

Would this be a viable form of volume control?
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Old 12th June 2008, 07:54 PM   #2
luka is offline luka  Slovenia
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Hi

I wouldn't do that, there would be danger of clipping...isn't it more easy just to lower gain, or input signal?
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Old 12th June 2008, 10:02 PM   #3
preiter is offline preiter  United States
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Clipping? If I understand Class-D amps (and it's quite possible I don't) the output device is either on or off. The supply voltage affects the gain, but not whether it will clip (as long as the supply voltage isn't affecting the amplitude of the modulation signal).

The reason to do this rather than just attenuating the input signal would be to take the volume control out of the signal path.
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Old 12th June 2008, 10:11 PM   #4
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Yes, I guess it has been already done (Tact or something?)
But the limitation of this approach is dynamics of a typical volume control, one would demand volume adjustment in range of at least 50dB, lowering the power supply voltage by 50dB is damn impractical.
So it shouldn't still be the only way to control volume.
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Old 12th June 2008, 11:18 PM   #5
luka is offline luka  Slovenia
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Hi

Anything is possible, but is it practical is question here. It could work/it does, but why control volume by supply voltage, how will you control supply then? it is just nothing simpler then using pot at input of fixed amp, at least as far as I can see
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Old 12th June 2008, 11:50 PM   #6
jkeny is offline jkeny  Ireland
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Panasonic SAXR range of digital amps uses this - they call it Power Supply Volume Control (PSVC) & you'll find the details in a TI document SLEA038.
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Old 13th June 2008, 01:41 PM   #7
fredos is offline fredos  Canada
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Control triangle wave amplitude insted...More easy if you whant to control amplifier gain by external device!
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Old 13th June 2008, 04:39 PM   #8
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One thing varying the power supply voltage will do is allow you to safely get max power into different loads. If you have 8 ohm loads you can increase the voltage. If you have 1 ohm load you can lower the voltage. In either case you can get stable operation at full power into the load. This could be good for pro-audio applications where you may be adding or subtracting drivers based on the venue.
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Old 13th June 2008, 05:52 PM   #9
preiter is offline preiter  United States
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Quote:
Originally posted by fredos
Control triangle wave amplitude insted...More easy if you whant to control amplifier gain by external device!
Sending the triangle wave through a pot has the same issues as sending the signal through a pot.

That TI document is pretty interesting. They used power supply voltage for part of the range of the volume control and digital attenuation for the rest.

Interesting discussion, guys. Thanks
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Old 13th June 2008, 06:36 PM   #10
banana is offline banana  Hong Kong
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isn't the idea of PSVC only works for open loop design??
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