Hey I recently built this simple LM13700 based VCA and got it to work for the most part. The problem with it is that instead of opening up the audio signal when the gate signal goes through, it basically closes. In other words, whenever the audio is going through and a gate signal hits it, it closes. I’m not sure if this is what it’s actually supposed to do or I did something wrong with it, I will link the video and schematic to it, would anyone be able to help?
(Dropbox - Schematic_13700-Dual-VCA.pdf - Simplify your life)
13700 Based Dual VCA - Modular in a Week 2.3 - YouTube
With 0 V on the CV input you should get no output, and with a large enough positive CV (probably +5 V) you should get basically full output. If that’s not what’s happening, there’s something wrong.
Update: I actually found the same issue on the forms here and it’s just the circuit itself that’s the problem, and it really isn’t the best vca to make
@dud has had success with this one
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If that’s not what’s happening, there’s something wrong.
I’m not sure how that circuit can even work (EDIT: with reasonable CV signals), since the IABC input is current driven and for the LM13700 the reference voltage is two diode drops above the negative rail, not 0 V. See the discussion here:
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My simple LM13700 VCA looks like this:
It’s not perfect but it works, it’s in my DIY synth. You may want to adjust R7/R14 for your final gain, mine’s pretty hot, something like 16V peak to peak.
Cheers
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Hi all,
I also need help with my dual LM13700 VCA. It is build around @HAGIWO circuit. The problem I have is this: one half of the VCA outputs signal even if there is no CV and the pot is at minimum. And when I mean it outputs signal it really does, its not a bleed through audio signal. What is even stranger is that this behavior is temporary: if I unplug my synth and leave it for a while, it goes back to normal. The other half of the VCA works ok every time.
Did you swap out with another LM13700?
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actually Im afraid to swap it because I only have one left and Im afraid to damage it. I need it to build an VCF. Texas Instruments don’t produce the through hole version of it anymore and, in my little corner of the world I cant find more to buy. If its the last resort I will try this as well but I hoped you guys have some ideas before I burn a second and last in my stock - 13700
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You mean you didn’t put it in a socket? That’s unfortunate.
After visual inspection of all solder joints and cross-checking with the schematic (which I assume you have done), swapping the ICs would be my next step. You can check the voltages on each pin, see if they are what you’d expect them to be. But even that wouldn’t be amazingly useful, as the pin you’re really interested in (Iabc) is a current input. Maybe check the output of the topright opamp (the TL074). Does that go to zero properly?
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Hi @TimMJN if you’re in the UK pm me as I’m sure to have spare LM13700’s. They’re not too expensive here (about 70 pence)
As for burning out your last chip. Use a chip socket and test for ground and shorts from the empty socket. Crack on for JFDI accreditation.
Can I just add that in my experience of LM13700s
It takes a fair bit to kill them and though I always replace them for any instability in other people’s kit I have on occasion applied power only and then tested again. The diodes in the tca can “stick”. (It’s about now that engineers freak out and start hunting me down)
But if in doubt swap it out.
Oh and embrace the blue smoke of failure. We’ve all been there and kept going
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