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Matt C.
Joined: Dec 22, 2015 Posts: 5 Location: Minnesota
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elmegil
Joined: Mar 20, 2012 Posts: 2177 Location: Chicago
Audio files: 16
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Posted: Mon Jan 11, 2016 3:47 pm Post subject:
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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
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Posted: Tue Jan 12, 2016 9:50 am Post subject:
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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 Posts: 2177 Location: Chicago
Audio files: 16
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Posted: Tue Jan 12, 2016 12:29 pm Post subject:
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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
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
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Posted: Tue Jan 12, 2016 1:23 pm Post subject:
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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
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Posted: Wed Jan 27, 2016 10:31 am Post subject:
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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 Posts: 2177 Location: Chicago
Audio files: 16
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Posted: Wed Jan 27, 2016 11:57 pm Post subject:
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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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