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 Forum index » DIY Hardware and Software » YuSynth
minimoog vcf - odd pot behavior
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defog



Joined: Aug 24, 2011
Posts: 113
Location: Philadelphia

PostPosted: Mon Apr 30, 2012 5:28 pm    Post subject: minimoog vcf - odd pot behavior Reply with quote  Mark this post and the followings unread

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.

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defog



Joined: Aug 24, 2011
Posts: 113
Location: Philadelphia

PostPosted: Tue May 01, 2012 7:21 pm    Post subject: Reply with quote  Mark this post and the followings unread

Also running at +/- 12v with resistor mods suggested on another post.
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defog



Joined: Aug 24, 2011
Posts: 113
Location: Philadelphia

PostPosted: Sat Jun 02, 2012 6:55 pm    Post subject: Reply with quote  Mark this post and the followings unread

bump - any insight will help me out here. I haven't moved on since this problem. Thanks.
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hoyager



Joined: Feb 16, 2010
Posts: 27
Location: New Zealand

PostPosted: Wed Jun 06, 2012 5:54 am    Post subject: Reply with quote  Mark this post and the followings unread

I'm having the same issue here... (just the 'res' signal seems to be being filtered)

Here's the stripboard layout I've done, based on the Yusynth version but using a LM741 as a differential amp as per the SH2000, SH3 and EFM VCF2

I'm thinking the input resistors going into the input and output amps are too large? Input signals are line level and general keyboard outputs, no modular signals here.

I'll try 10k's instead of the 120k and 56k

(edit) have just put 22k and 10k in those spots, which brings more of the signal through and the res knob seems to work better, but nowhere near optimal, the opamps gains (trimpots) are cranked just to get anything coming through (input is def overdriving), but still no self oscillation

I've also tried taking the res signal from a few different places, the overall signal gets loudest when taken from the - input of the output amp...

Can anyone see what the issue could be?

Andy


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defog



Joined: Aug 24, 2011
Posts: 113
Location: Philadelphia

PostPosted: Wed Jun 06, 2012 9:15 am    Post subject: Reply with quote  Mark this post and the followings unread

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.
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hoyager



Joined: Feb 16, 2010
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Location: New Zealand

PostPosted: Wed Jun 06, 2012 4:02 pm    Post subject: Reply with quote  Mark this post and the followings unread

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

PostPosted: Wed Jun 06, 2012 5:32 pm    Post subject: Reply with quote  Mark this post and the followings unread

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.
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hoyager



Joined: Feb 16, 2010
Posts: 27
Location: New Zealand

PostPosted: Wed Jun 06, 2012 5:57 pm    Post subject: Reply with quote  Mark this post and the followings unread

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
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Location: Philadelphia

PostPosted: Wed Jun 06, 2012 6:24 pm    Post subject: Reply with quote  Mark this post and the followings unread

Attemping to run two mfos vcos into it. I've tried other signals too though and have had identical response.
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hoyager



Joined: Feb 16, 2010
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Location: New Zealand

PostPosted: Wed Jun 06, 2012 9:51 pm    Post subject: Reply with quote  Mark this post and the followings unread

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

PostPosted: Thu Jun 07, 2012 12:44 pm    Post subject: Reply with quote  Mark this post and the followings unread

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.
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hoyager



Joined: Feb 16, 2010
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Location: New Zealand

PostPosted: Thu Jun 07, 2012 5:53 pm    Post subject: Reply with quote  Mark this post and the followings unread

Well, its either a strange beast or I'm having quite a fail. At different voltages the controls behave completely different. At 18v the res doesn't do anything, freq rolls off highs. At 15v the res adds a lot of signal but the filter cutoff doesn't affect the resonance.

I've changed the diff amp on this one with 100k resistors going in and 1.5M from the + input to ground and 1.5M in the feedback loop, as in the SH3a and SH2000 which *seems* to help

I've tried BC547, 2n5089, BC550 and 2n3904 in the ladder, I'll match the bc550's in there atm.

The caps are the weirdest thing, I can pull them without there being any change in the sound...


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hoyager



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PostPosted: Thu Jun 07, 2012 8:55 pm    Post subject: Reply with quote  Mark this post and the followings unread

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: 1233
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PostPosted: Sat Jun 09, 2012 11:29 am    Post subject: Re: minimoog vcf - odd pot behavior Reply with quote  Mark this post and the followings unread

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.

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defog



Joined: Aug 24, 2011
Posts: 113
Location: Philadelphia

PostPosted: Wed Aug 15, 2012 5:00 pm    Post subject: Re: minimoog vcf - odd pot behavior Reply with quote  Mark this post and the followings unread

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!

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