Volume control of a differential signal - diyAudio
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Old 24th August 2009, 02:55 PM   #1
berni8k is offline berni8k  Slovenia
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Default Volume control of a differential signal

I have a DAC chip with differential voltage output to drive a D class chip with differential voltage inputs.

Now i need to find a way how to properly attenuate the differential signal with digital pots to provide volume control.

So whats the easiest proper way to get volume control on such a analog signal.

(oh and sorry for such a dumb question)
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Old 24th August 2009, 03:02 PM   #2
AndrewT is online now AndrewT  Scotland
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a variable resistor (potentiometer wired as a VR) between the Hot and Cold balanced signal lines.
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Old 24th August 2009, 03:55 PM   #3
berni8k is offline berni8k  Slovenia
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Well if i do that i exceed the DACs minimal output independence of 10K (Its a voltage output DAC not current output)
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Old 24th August 2009, 04:25 PM   #4
AndrewT is online now AndrewT  Scotland
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wire a pair of accurately matched 10k resistors into the signal lines before the VR.
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Old 24th August 2009, 05:54 PM   #5
berni8k is offline berni8k  Slovenia
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Well i was hoping for also some kind of linear control on it.Because i had the idea to be able to limit the amp down to a certain power (Setting the linear control to 1/2 would turn the 240W per channel in to 120W per channel)

I like to avoid digital attenuation in the DSP chip thats driving the DAC because it causes loss of bit depth.
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Old 24th August 2009, 06:38 PM   #6
paulb is offline paulb  Canada
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If you're using it as a volume control, you don't want linear. See
http://sound.westhost.com/project01.htm
Also, you can't "turn down" the maximum power of an amplifier, it has a fixed gain. The max power would strictly be determined by the maximum DAC output and the power amp gain.
Finally, reducing the gain by half would reduce the power by one-quarter.
What are you really trying to do with this control? Volume control or speaker protection?
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Old 24th August 2009, 06:40 PM   #7
rdf is offline rdf  Canada
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Use a stereo linear pot, tie the two low ends together and input/output plus and minus to/from each section independently.
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Old 24th August 2009, 09:45 PM   #8
berni8k is offline berni8k  Slovenia
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Okey after some tough and a few simulations i figured out that this configuration should work for linearly controlling the output power.If the puts used here are log and there high side connected in the middle (reverse of what such a pot should be used)

Do you guys agree with my theory?
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Old 24th August 2009, 11:14 PM   #9
AndrewT is online now AndrewT  Scotland
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twin pots will ruin the balance of the impedances. Without near perfect balance between the Hot and Cold lines the balanced output is almost worthless in improving the rejection of interference.
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Old 24th August 2009, 11:17 PM   #10
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The design Andrew gave would be the best to use and has the advantage of only needing a single gang pot. With careful selection of series resistor and pot values, your desired halfway setting could give 10dB attenuation (half power). I have done this for an unbalanced line before.
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