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Old 9th August 2011, 12:07 PM   #1
Elvee is offline Elvee  Belgium
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Default Electronic volume control idea

Here is an idea for a voltage-controlled volume.

It is based on PWM, and has the potential to be more linear than conventional VCA's.

The PWM signal is generated by by comparing the control voltage against an exponential waveform, yielding an inherently logarithmic function.

The circuit shown in example is just a (working) proof of concept: to make it usable in the real world, the swtching frequency would need to be increased, by a factor 5 or 10 at least.
In this prototype, the frequency is only 133KHz and is limited by the LM339.
The switch would also need to be a better one than the CD4053: at least a CD40HC4053, but preferably a modern, low charge injection one like the LTC1043.

With quality components, there would be no sources of non-linearity in the signal path, except for the intermodulation products between the signal and the sampling frequency, and they fall very rapidly if the sampling frequency is pushed higher.

In this circuit, the control voltage varies between -6 and +6V, giving a nice and smooth 0 to 100% logarithmic variation.

The particular arrangement of ground, control voltages and supplies is just an example, one could adapt it to obtain a 0 to 5V control range for instance.
Attached Images
File Type: gif VolContr.GIF (20.2 KB, 210 views)
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Old 9th August 2011, 12:12 PM   #2
forr is offline forr  France
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Linsley-Hood used the same idea for a compressor/expander and a volume control.
If interested, I may still have some details.
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Old 9th August 2011, 07:54 PM   #3
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by forr View Post
Linsley-Hood used the same idea for a compressor/expander and a volume control.
If interested, I may still have some details.
Yes, that would be interesting: knowing the take of a great mind on a problem is always enlightening.

Note that I certainly do not claim any originality in the concept: it is part of the common knowledge body of electronics.

I think that the use of an exponential reference waveform is particularly relevant in this case (perhaps not original either, I didn't research the subject).

Here are some pics: the first waveform is the exponential signal.
The second is the output signal with a LF triangle input, and the volume set at 50%. Note the incomplete filtering of the carrier.

Following pics show the prototype board.
Attached Images
File Type: jpg Exponential.JPG (239.3 KB, 184 views)
File Type: jpg WFM.JPG (261.9 KB, 147 views)
File Type: jpg VolCtl1.JPG (461.0 KB, 140 views)
File Type: jpg VolCtl2.JPG (286.0 KB, 123 views)
File Type: jpg VolCtl3.JPG (221.3 KB, 27 views)
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Old 9th August 2011, 08:24 PM   #4
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The Inovonics model 250 compressor also uses this method. The Inovonics 250 used to be a popular broadcast audio processor, it is now still used by local stations that buy it second-hand.
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