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defog
Joined: Aug 24, 2011 Posts: 113 Location: Philadelphia
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Posted: Mon Apr 30, 2012 5:28 pm Post subject:
minimoog vcf - odd pot behavior |
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Hi,
I'm having some trouble with my build. For whatever reason, I get very little output until the resonance is maxed, at which point it just acts like a normal filter with no res. added. It does not self-oscillate obviously. I do not have any cv going into the cutoff currently, so it is just wired as a voltage divider. I also. an hear bleeding of the oscillators into it even when nothing is connected to the input and I'm monitoring the output. Any help would be great. Thanks. _________________ The Phoenix Vertigo is my Electronic alias. Check out my songs on soundcloud : http://soundcloud.com/charles-stieg/ |
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defog
Joined: Aug 24, 2011 Posts: 113 Location: Philadelphia
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Posted: Tue May 01, 2012 7:21 pm Post subject:
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Also running at +/- 12v with resistor mods suggested on another post. _________________ The Phoenix Vertigo is my Electronic alias. Check out my songs on soundcloud : http://soundcloud.com/charles-stieg/ |
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defog
Joined: Aug 24, 2011 Posts: 113 Location: Philadelphia
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Posted: Sat Jun 02, 2012 6:55 pm Post subject:
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bump - any insight will help me out here. I haven't moved on since this problem. Thanks. _________________ The Phoenix Vertigo is my Electronic alias. Check out my songs on soundcloud : http://soundcloud.com/charles-stieg/ |
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hoyager
Joined: Feb 16, 2010 Posts: 27 Location: New Zealand
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defog
Joined: Aug 24, 2011 Posts: 113 Location: Philadelphia
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Posted: Wed Jun 06, 2012 9:15 am Post subject:
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I'm going to try processing another signal through it at 15+/- with the stock resistors, I have a feeling this is the result of something else, 3v shouldn't make that big a difference in the signal amplification. _________________ The Phoenix Vertigo is my Electronic alias. Check out my songs on soundcloud : http://soundcloud.com/charles-stieg/ |
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hoyager
Joined: Feb 16, 2010 Posts: 27 Location: New Zealand
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Posted: Wed Jun 06, 2012 4:02 pm Post subject:
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Yeah, mines running at +/-18v, so I'm almost certain the different voltages can't be the problem. |
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defog
Joined: Aug 24, 2011 Posts: 113 Location: Philadelphia
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Posted: Wed Jun 06, 2012 5:32 pm Post subject:
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What kind of transistors and ICs are you using? I've got BC547's and 557's, TL072, and CA3046s in place, following the parts list exactly. I've used these same parts in other PCBs and projects, and they work, so I know they're not fakes or anything. I'm wondering if there are just really wide tolerances on these CA3046's or transistors whereas if they aren't falling in the range of Hfe or gain that it just won't work at all. _________________ The Phoenix Vertigo is my Electronic alias. Check out my songs on soundcloud : http://soundcloud.com/charles-stieg/ |
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hoyager
Joined: Feb 16, 2010 Posts: 27 Location: New Zealand
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Posted: Wed Jun 06, 2012 5:57 pm Post subject:
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BC547's (matched, but not carefully, I think it should pass audio even if they're not matched, not sure about that though) BC558 / 2N3906 for expo converter (anything seems to work here) and CA3146, same pinouts and internals as CA3046, but higher voltages? or voltage ratings?
I haven't matched the caps but I can pull any of them out while its going and there's no change to the sound, or lack of sound with no res
IC's are 741 for the differential amp and 4558 / TL072 for the inout buffers
What signals are you running into the filter? |
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defog
Joined: Aug 24, 2011 Posts: 113 Location: Philadelphia
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Posted: Wed Jun 06, 2012 6:24 pm Post subject:
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Attemping to run two mfos vcos into it. I've tried other signals too though and have had identical response. _________________ The Phoenix Vertigo is my Electronic alias. Check out my songs on soundcloud : http://soundcloud.com/charles-stieg/ |
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hoyager
Joined: Feb 16, 2010 Posts: 27 Location: New Zealand
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Posted: Wed Jun 06, 2012 9:51 pm Post subject:
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I am getting better results at around +-15v, but have to crank the gain on the output amp and turn the output volume up? to get it self oscillating, although there's no change in pitch of the oscillation. I'm taking the res directly through a cap from the output of the diff amp, but turning the knob still seems to impact the ouput amp for some reason.
I think something else has gone awry as the freq pot isn't sweeping anything or much currently (have replaced the pnp and checked orientation).
I'm suspecting the diff amp in yours, maybe resistor value mixup?
Did you match the caps in yours? I'm using unmatched panasonic ecq films, (I have no matcher) but I figured these caps would be more matched than mylars and that it would at least work albiet not optimally |
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defog
Joined: Aug 24, 2011 Posts: 113 Location: Philadelphia
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Posted: Thu Jun 07, 2012 12:44 pm Post subject:
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I've got matched Panasonic ECQ greenies in mine. I did go through and check all of my resistor values too, but it never hurts to check again. This is why I put projects like this down for a while, it forces me to forget what I last though was wrong with it and start fresh. I socketed the TL072 so I can swap it out for other dual opamps and check. I too have the trimmers cranked to max on mine. _________________ The Phoenix Vertigo is my Electronic alias. Check out my songs on soundcloud : http://soundcloud.com/charles-stieg/ |
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hoyager
Joined: Feb 16, 2010 Posts: 27 Location: New Zealand
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hoyager
Joined: Feb 16, 2010 Posts: 27 Location: New Zealand
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Posted: Thu Jun 07, 2012 8:55 pm Post subject:
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There's very little difference between using matched and unmatched transistors with mine in its current state. Added a 1k trimpot instead of the 330R from pin 6 of the CA3146 to ground, nothing seems to be gained by tweaking that pot.
Levels seem to be getting through better now, but still no res until there's almost 1 Meg of resistance in the feedback loop of the output amp, and then it seems like its just the opamp oscillating, or something. the filter freq doesn't seem to have any effect on that resonance at least.
Swapped in a 100k antilog pot for res, behaves better but still just boosts the signal.
Currently have matched BC550's orientated as shown in the stripboard layout, and its running at +15v and -15.6v |
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yusynth
Joined: Nov 24, 2005 Posts: 1314 Location: France
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Posted: Sat Jun 09, 2012 11:29 am Post subject:
Re: minimoog vcf - odd pot behavior |
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defog wrote: | Hi,
I'm having some trouble with my build. For whatever reason, I get very little output until the resonance is maxed, at which point it just acts like a normal filter with no res. added. It does not self-oscillate obviously. I do not have any cv going into the cutoff currently, so it is just wired as a voltage divider. I also. an hear bleeding of the oscillators into it even when nothing is connected to the input and I'm monitoring the output. Any help would be great. Thanks. |
Hi
It sounds like there is some mistake in the wiring (VCOs bleeding is a clue).
As usual post photos of your PCB (component side and track side) with good resolution such that we can check for any error. _________________ Yves |
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defog
Joined: Aug 24, 2011 Posts: 113 Location: Philadelphia
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Posted: Wed Aug 15, 2012 5:00 pm Post subject:
Re: minimoog vcf - odd pot behavior |
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yusynth wrote: | defog wrote: | Hi,
I'm having some trouble with my build. For whatever reason, I get very little output until the resonance is maxed, at which point it just acts like a normal filter with no res. added. It does not self-oscillate obviously. I do not have any cv going into the cutoff currently, so it is just wired as a voltage divider. I also. an hear bleeding of the oscillators into it even when nothing is connected to the input and I'm monitoring the output. Any help would be great. Thanks. |
Hi
It sounds like there is some mistake in the wiring (VCOs bleeding is a clue).
As usual post photos of your PCB (component side and track side) with good resolution such that we can check for any error. |
Thanks, I'll check that out, I know I probably have something wired wrong there, it's the first diy synth built from components using switching I've ever done. This is how you learn though. Now I just need the time to complete it. Having a baby makes that next to impossible! _________________ The Phoenix Vertigo is my Electronic alias. Check out my songs on soundcloud : http://soundcloud.com/charles-stieg/ |
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