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 Forum index » DIY Hardware and Software » Thomas Henry designs
calibrating VCF-1 for good 1V/oct tracking
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Matt C.



Joined: Dec 22, 2015
Posts: 5
Location: Minnesota

PostPosted: Mon Jan 11, 2016 12:49 pm    Post subject: calibrating VCF-1 for good 1V/oct tracking Reply with quote  Mark this post and the followings unread

I'm looking for some advice about Thomas Henry's VCF-1 design. I've built it and generally it seems to be working great, but I'm having trouble calibrating it for good 1V/oct response.

I know that for many applications it's not that important, but I'd like to be able to crank up the resonance and basically use it as an extra oscillator. Not that it needs to be as accurate as my main VCO, but close would be nice.

With the circuit built following the schematic exactly, I couldn't get very close to a 1V/oct response. After tweaking some values (changing the R6 V/OCT trim pot from 100r to 500r, and changing the V/OCT summing resistor R19 from 100k to 68k) I could get it much closer, but I found that even if I got it right for one specific octave, it wouldn't respond evenly across the whole keyboard range.

anyone have any tips on how to tweak this thing? Is it reasonable to expect a (close to) accurate 1V/OCT response from this design? I've attached the relevant portion of the schematic for reference.

Thanks!


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elmegil



Joined: Mar 20, 2012
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Location: Chicago
Audio files: 16

PostPosted: Mon Jan 11, 2016 3:47 pm    Post subject: Reply with quote  Mark this post and the followings unread

Are you certain you have a 2K (not 1.8K, not 1K) tempco? I've seen the tempco muck with this sort of tracking in the past....


What you're looking for is an 18mV change in the value at the wiper of the trim pot for every 1V of change on the V/Oct input (assuming all others disconnected).

You should be seeing ~20mV per volt at the output of the summing amp, the divider is what brings it down to the 18mV range. If you're not seeing that, then your tempco or something else on the summing side is messed up.

Edit: Also worth noting is whether you're using a 15V or 12V supply. I could see IC4's response being relevant, and -12V vs -15V....
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Matt C.



Joined: Dec 22, 2015
Posts: 5
Location: Minnesota

PostPosted: Tue Jan 12, 2016 9:50 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks, that must be my problem. I don't even have a tempco installed there, just a plain 2k resistor. I knew this would make the circuit more sensitive to temperature changes, but since this issue didn't seem to be temperature related, I assumed the problem was elsewhere.
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elmegil



Joined: Mar 20, 2012
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PostPosted: Tue Jan 12, 2016 12:29 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hm. Don't jump to conclusions. If the resistor really is 2K it shouldn't have so much drift that it causes you this much grief. Unless you're testing it in a windy room Smile

If it's 2.2K (which is more common than 2K), that could certainly be a problem.
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Matt C.



Joined: Dec 22, 2015
Posts: 5
Location: Minnesota

PostPosted: Tue Jan 12, 2016 1:23 pm    Post subject: Reply with quote  Mark this post and the followings unread

cool. I'm pretty sure that resistor is in fact 2k, but I'll double check. your tip that I should be looking for an 18mV change at that wiper is helpful, I'll do some more tests and see what I come up with. thanks.
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Matt C.



Joined: Dec 22, 2015
Posts: 5
Location: Minnesota

PostPosted: Wed Jan 27, 2016 10:31 am    Post subject: Reply with quote  Mark this post and the followings unread

quick update, I returned the circuit to stock schematic values, and I was able to get the 18mV/oct on the trimmer wiper as elmegil suggested, but the response still was not a reliable 1V/ot.

The only other thing I could see contributing to this is that I did not include the two offset trimmers going to the 3080 ICs. Could that be the issue here? When I tried to install them I couldn't figure out how to properly calibrate them because, IIRC, the DC output on those chips changed depending on the position of the cutoff frequency pot. Is this where I'm going wrong? If so, can anyone give me tips on how to set these two offset trims? Or could it be something like a faulty 3080 chip?

Thanks!
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elmegil



Joined: Mar 20, 2012
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PostPosted: Wed Jan 27, 2016 11:57 pm    Post subject: Reply with quote  Mark this post and the followings unread

3080 chips and such things usually either work or they don't. That doesn't mean they work or they're DEAD mind you, but if it doesn't work, it won't be a filter.

I don't think I could explain why the offset trimmers would be a problem with the tuning, but I would expect it's important that they be there. If you're getting 18mV per volt at the base of the transistor, the only other thing I can think is that the two transistors are not matched well.
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