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allochthon
Joined: Aug 20, 2014 Posts: 2 Location: United States
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Posted: Wed Aug 20, 2014 4:06 pm Post subject:
CV voltage offset circuit |
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I am attempting to offset a CV voltage being sent from a MIDI-to-CV converter to an analog synth. I need the voltage to increase 0.58V consistently for every note value (e.i., voltage).
There's a six year old post on this forum HERE. Since it's so old, I thought I'd refresh the topic.
I built this circuit (almost - I used a TL082 and a slightly different cap value) and it I am able to adjust the a 0V source to 0.58V. However, if I change the input voltage, the resulting output voltage does not change. Well, it does to a very small extent over many octave ranges, but nothing like it should (e.g., follow a sequenced pattern).
Can this circuit do what I'm trying to do, or am I barking up the wrong tree? I'm not an electronics whiz by a long shot, so I may be missing something fundamental.
I am assuming that the commercial versions of this device like the A-183-2 and the Q125 are designed for this very function. Please correct me if I am wrong.. on any of this.
Next I'll try building the circuit that Doepfer has on their DIY page - see #3. Fingers crossed.
Cheers. |
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blue hell
Site Admin
Joined: Apr 03, 2004 Posts: 24079 Location: The Netherlands, Enschede
Audio files: 278
G2 patch files: 320
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Posted: Wed Aug 20, 2014 5:16 pm Post subject:
Re: CV voltage offset circuit |
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allochthon wrote: | Can this circuit do what I'm trying to do, or am I barking up the wrong tree? |
Yes, should do the trick ... the gain pot should be set about midway, but it needs fiddling to get it exact.
Oh, and _________________ Jan
also .. could someone please turn down the thermostat a bit.
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allochthon
Joined: Aug 20, 2014 Posts: 2 Location: United States
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Posted: Mon Aug 25, 2014 2:24 pm Post subject:
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Well, I've built the circuit several times over trying to get it "right", and I think I've done it correctly, as it will adjust the voltage of the CV signal. However there is one big problem with it - I can dial it in for any given voltage, but if the input voltage changes, the output does not change accordingly.
Example: Input voltage of 0V = output of 0.58V; input of 1V = output of 0.64V.
So if a sequence is sent in, the output sounds like a warbled mess, as the voltage is just changing slightly, rather than changing at fixed offset. I have twiddled with gain and offset endlessly, and have replaced all the components to be exactly as shown in the diagram... any ideas welcome. |
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PHOBoS
Joined: Jan 14, 2010 Posts: 5591 Location: Moon Base
Audio files: 705
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prgdeltablues
Joined: Sep 25, 2006 Posts: 222 Location: UK
Audio files: 12
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Posted: Tue Aug 26, 2014 5:47 am Post subject:
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I'm not quite sure what you are trying to do. Midi-CV should be putting out 0.083V per note (1/12 V, so 1 V per octave). If for some reason you want to increase that to 0.58V per note, you simply need to amplify, not offset. Or do you simply want to offset every note by 0.58V - in which case you don't need to amplify the incoming signal.
I'd suggest adding resistors either side of the offset pot - you've got a swing there from V+ to V-, which is surely far more than you need, and won't be helping you to calibrate. Try disconnnecting the offset pot, and getting the amplification to as close to 1 as possible (assuming that's what you want). Then reconnect the offset, and only adjust that pot.
Note that you won't get this accurate with a TLxxx, since they produce significant (in this context) voltage offsets, but they should be Ok to get the circuit working roughly.
Peter |
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