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Old 1st July 2004, 09:18 PM   #1
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Default Low cost PWM volume control. Could it work?

I have once read in a book (which of course I can't find anymore) about a direct PWM volume controller.
The idea is that you simply take the audio and chop it with a high frequency, variable duty cycle signal. Then you use some type of low pass filter, to get rid of the high frequency component, and that's it: the output signal equals the input signal (theoretically no distortions), scaled by the PWM ratio.
The schematic could be something like the atachement.
The advantage is that you can add any number of channels on the same PWM signal (I need 5), and they are completely synchronized.
I would like to implement such a volume controller, do you think it could work? (if anyone knows a similar design, I would appreciate it, no point in reinventing the wheel.)
I was thinking of using a 555 for the PWM generator, but I don't think it's a good idea, because I don't have the full range 0-100%. Especially the lower limit, there must be a setting on the potentiometer that gives 0 V on the PWM output, no audio is getting through, silence.
Also, what do you think about the output filter? Choosing values that give no more than 3dB drop at 20 KHz, gives a 22 dB reduction of the 200 kHz signal. Strictly speaking, the 200 KHz signal is exactly 22 dB below the audio signal. If the 200 KHz signal gets into amps and speakers, is there any danger? Because then I would have to use a more complex filter, and i want a low cost solution.
Should I aply any bias on the 4066, to get low distortions?
Thank you.
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Old 2nd July 2004, 08:32 AM   #2
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I'm pretty certain that John Linsley-Hood showed this in one of his books. I'll have a look for it.

It would be one way of getting a multi-gang volume control to go on the outputs of a Behringer 2496 digital crossover, so that all the analogue outputs had a common gain control. (You don't want to use the main volume control on the input of a DSP-based XO because you lose digital resolution as you turn the gain down).
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Old 2nd July 2004, 09:05 AM   #3
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This principle has been proposed in the "active fliter cookbook" for the tuning of state-variable filters. So it should basically but I don't know what quality one might achieve.


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Old 2nd July 2004, 09:08 AM   #4
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I've just done some SPICE simulations, and the technique works well, but obviously as it's a sampled-data system, the usual problems occur with filtering out the switching frequency.

You'll need to use fast, and high quality FET switches. Perhaps the DG411 type of switch will be OK. I think I'll breadboard this and give it a try.
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Old 2nd July 2004, 09:43 AM   #5
djQUAN is offline djQUAN  Philippines
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I just looked at the TL494 datasheet and it can handle a max freq of 200kHz.

might be a good alternative for a PWM source.
although I think (I read somewhere) the 555 could go up to 1MHz.

I have a scrap DG211,


and four AD7510's

the specs for the AD7510's are

SPST 4ch Latch-up Proof, 75ohm on resistance, 10nA channel ON leakage,

might be a good candidate...........not sure if they're fast enough though. but they work with +/-15V supplies.
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Old 2nd July 2004, 09:54 AM   #6
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You really need a switch with fast (and symmetrical) ON/OFF times, and also with low charge injection. Perhaps a MAX4501?

Although I bet even a simple 74HC4316 might do the job adequately.
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Old 2nd July 2004, 09:56 AM   #7
mskeete is offline mskeete  United Kingdom
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JLH publsihed a fully working circuit in Electronics World

Attenuator, Low-distortion for hifi
John Linsley Hood

April 1995, p320

With a distortion figure of just
0.005% over most of the audio
range, this switch-mode attenuator
design removes the transistor
matching problems normally
associated with high-performance

You can obtain a copy from the following website

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