Would this work as a volume control??? - diyAudio
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Old 16th December 2004, 10:54 AM   #1
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Default Would this work as a volume control???

Preamble:
Some (20+) years ago I built a clone of a threshold (I think) solid state moving coil preamp.

consisted of 1 gain stage of 7 transistors in parallel and ran off 1 (-22v) supply rail.

To test, I connected it to a lab variable power supply, put signal generator on input, and oscilloscope on output
Could easily observe wave form etc and unit just sounded great!.


BUT

I varied power supply voltage and gain also varied..(can you see the question coming?)...


I can't design electronics hence the following question:..

Is it possible to design a simple amplifier stage of sufficient quality to give no gain but allow volume adjustement based purely on power supply voltage????
If so one could "easily" gang as many of these units as you liked and have them all syncronised in their attentuation

The intention is to be able to control outputs on DCX2496 dig XO


Any comments appreciated
cheers
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Old 16th December 2004, 11:37 AM   #2
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Bit of a loony idea, ain't it?


If you really cannot get a pot, stepped attenuator, TVC, digital pot, optocoupler or adjustable earplugs i guess this will also work. But with greatly reduced range of control (<15db?), volume dependent distortion and probably a dozen more probs.
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Old 16th December 2004, 11:44 AM   #3
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Yes quite possibly a loony idea!
As I said i cn't design this stuff , 15db no wher near enough but maybe there is someone out there who has some idea to make this work, in principle ist solves some probs , not least of which is the extreme cost of fancy six ganged pots.
DACT stuff for example is out of my price range here in oz..

Thanks for the reply
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Old 16th December 2004, 01:27 PM   #4
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i will design 1 for u...Will be using a cheap IC CD4007, It will have 4 channels of voltage controlled attenuator.By suitably modifying u can use it as a votage controlled attenuator...
I have a similiar need to control my HT setup @ home.
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Old 17th December 2004, 12:16 AM   #5
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Silly me

I think I've just described a voltage controlled amplifier (VCA)

told you i didn't know what i was doing!

any way- is there an idea we can use here?
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Old 17th December 2004, 02:01 AM   #6
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Quote:
Some (20+) years ago I built a clone of a threshold (I think) solid state moving coil preamp.
Please post the the circuit: using discrete transistors.
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Old 17th December 2004, 02:43 AM   #7
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The problem is that varying the rail like that changes the operating point of the transistor(s). Unless you design it with a current source, the bias current will change, too. At least as far as high fidelity goes, these are fatal flaws. For low-fi, it might be a workable kludge, but there are better ways to accomplish the same thing.
A normal circuit won't start doing much to the volume until you've cut the rail down to a really, really low value. By that point distortion is climbing rapidly. For instance, I did a little fiddling with a MOSFET circuit a while back and found that the distortion climbed rapidly below about 15V rails or so. I don't remember the exact numbers, but it was a pretty noticable rise.
If you want to keep going in this direction, you might want to look up JFET volume circuits.

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