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CV voltage offset circuit
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allochthon



Joined: Aug 20, 2014
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Location: United States

PostPosted: Wed Aug 20, 2014 4:06 pm    Post subject: CV voltage offset circuit Reply with quote  Mark this post and the followings unread

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
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PostPosted: Wed Aug 20, 2014 5:16 pm    Post subject: Re: CV voltage offset circuit Reply with quote  Mark this post and the followings unread

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 welcome

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allochthon



Joined: Aug 20, 2014
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PostPosted: Mon Aug 25, 2014 2:24 pm    Post subject: Reply with quote  Mark this post and the followings unread

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



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PostPosted: Tue Aug 26, 2014 5:38 am    Post subject: Reply with quote  Mark this post and the followings unread

hmm, I took a look at the ciruit and it should indeed work. With both Offset and Gain pots at the center position
the output voltage should be the same as the input. What supply voltage are you using and what's the value of the capacitor ?

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prgdeltablues



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PostPosted: Tue Aug 26, 2014 5:47 am    Post subject: Reply with quote  Mark this post and the followings unread

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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