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plord
Joined: Dec 15, 2007
Posts: 21
Location: Charlotte, VT
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 Posted: Thu May 16, 2024 11:23 pm Post subject:
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| fonik wrote: | vactrols are used to apply voltage control. actually the diode of the vactrol is part of the cv circuitry, the resistor part of the audio path. some samples:
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Sorry to raise this zombie thread from the dead, but I'd like to add the vactrol crossfader from this post to a dual Oakley ADSR/VCA I'm building. I think I understand the circuit, except for a few things: 1. Why a diode in between the +15 and the signal 2 vactrol, but no similar diode on the other vactrol? 2. What is going on at the ^ symbol of the 22k resistor in the bottom left corner? Is that just ground? and 3: Does it matter what op-amp and transistors I throw at this? I've got plenty of TL072s and BC549s in the parts bin...
Thanks for any tips. After years of building kits with idiot-proof instructions (Many thanks to Blacet and Oakley), I want to be able to dig into these kind of schematics! |
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The Peasant
Joined: Nov 13, 2009
Posts: 112
Location: Sunny Alberta
Audio files: 1
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| Posted: Tue Jun 25, 2024 2:57 pm Post subject:
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Hi plord,
The only reason for the diode that I can see is to "bias" that end of the vactrol down to around 13 volts instead of up right at 15. Judging by the number 2 and small arrow next to it, I would guess that it's intended to be a LED with a voltage drop of 2 volts. This may be intended to compensate for an offset in the differential pair. You could always experiment and see how much difference it makes with and without it.
The ^ symbol represents the CV input, only points marked "0" go to ground.
Op amp type shouldn't matter and most regular NPN transistors should probably be fine, but you may want to match them.
I have never built this, so answers not 100% guaranteed.
Take care,
Doug
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The Electronic Peasant
www.electronicpeasant.com |
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plord
Joined: Dec 15, 2007
Posts: 21
Location: Charlotte, VT
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| Posted: Mon Jul 29, 2024 7:52 pm Post subject:
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| The Peasant wrote: |
The only reason for the diode that I can see is to "bias" that end of the vactrol down to around 13 volts instead of up right at 15. [...]
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Ah, hmm, ok. Not sure I grok why one vactrol has it, and the other doesn't...Is it "just" to create an offset that the circuit can use to pull the signal around or something like that?
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The ^ symbol represents the CV input, only points marked "0" go to ground.
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Oh, duh, of course, I didn't even realize I lost track of the CV input, but there it is.
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Op amp type shouldn't matter and most regular NPN transistors should probably be fine, but you may want to match them.
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Easily done, thanks. Appreciate the feedback! |
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The Peasant
Joined: Nov 13, 2009
Posts: 112
Location: Sunny Alberta
Audio files: 1
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| Posted: Tue Jul 30, 2024 10:07 am Post subject:
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| plord wrote: |
Ah, hmm, ok. Not sure I grok why one vactrol has it, and the other doesn't...Is it "just" to create an offset that the circuit can use to pull the signal around or something like that?
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I am guessing that there is some offset in the differential pair formed by the two transistors, resulting in slightly different behavior for each vactrol. The diode attempts to correct that difference.
As I said, you should experiment with and without it to see what effect it has on the crossfading.
Take care,
Doug
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The Electronic Peasant
www.electronicpeasant.com |
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