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cslammy
Joined: Apr 27, 2018 Posts: 206 Location: USA
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Posted: Sun Feb 03, 2019 9:49 pm Post subject:
Rene Schmidt MS20 filter: when I crank CV output goes silent Subject description: OTA VCF "lockup"? |
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Hey there, I've now built 2 rene MS20 filters: https://www.schmitzbits.de/ms20.html
First one works fine, second one built w/ same parts and PCB has an issue.
when I really push the cutoff CV (crank up the cutoff frequency pot, +/- 15V or that combined with CV modulation) the audio output goes dead.
When I crank the CV inputs back down the VCF output comes to life again after about a second, I can hear audio at its output again.
I can't see anything wrong w/ either circuit; the only difference is that I used smaller green LEDs in the one with the "lockup" problem to get more nasty "MS20 peak distortion".
Beyond that when I look with a scope etc. they look about the same until one's audio output dies.
The resonance settings doesn't affect this issue--the small LED one will get silent and conk out even with resonance set to 0.
I read about zeners used to protect against "lockup", is that what I am seeing here? I mean this: http://www.syntiac.com/syntiac_obvcf.html. What does this mean?
I thought maybe I was killing the 3080s by over currenting them but I have also found when you do this they die really fast, and if that's what's happening here, if the 3080's are getting knocked silly, these 3080s are coming back to life.
Any assistance would be greatly appreciated, this issue is driving me crazy! _________________ Visit my AUDIODIWHY blog and website |
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Grumble
Joined: Nov 23, 2015 Posts: 1294 Location: Netherlands
Audio files: 30
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Posted: Mon Feb 04, 2019 1:14 am Post subject:
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Could you post a picture with the not functioning module? (both sides when possible) _________________ my synth |
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cslammy
Joined: Apr 27, 2018 Posts: 206 Location: USA
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Posted: Mon Feb 04, 2019 2:53 am Post subject:
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Hi Grumble:
I have not added bypass caps yet. I usually do that once I am pretty sure the build was OK. I assume that's not the issue here, I use a stable power supply while testing (siglent 3303X)
As I said, I have another identical board that works without this issue (Same parts, bigger LEDs) which is the same layout, same PCB etc.
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_________________ Visit my AUDIODIWHY blog and website |
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wackelpeter
Joined: May 05, 2013 Posts: 461 Location: germany
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Posted: Mon Feb 04, 2019 8:52 am Post subject:
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does this appear also when you replace the 10k resistors at the OTA bias Inputs with larger ones say somethin between 15k or 27k?
The Filters behaviour then should be more tame and it's not working as in the original but perhaps to make sure the fault you're describing is connected to the OTA's or the bias Control Input.
I have this one on stripboard too, twice and both switchable between HPF and LPF... but i wasn't so much satisfied with my results, so i ended up replicating the whole original circuit with most additional parts until i was happy enough with my results... _________________ https://soundcloud.com/bastian-j |
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cslammy
Joined: Apr 27, 2018 Posts: 206 Location: USA
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Posted: Mon Feb 04, 2019 9:25 am Post subject:
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wackelpeter wrote: | does this appear also when you replace the 10k resistors at the OTA bias Inputs with larger ones say somethin between 15k or 27k?
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Hi Wackel, thanks for the suggestion, no, I haven't tried that.
I haven't tried any troubleshooting yet....
If I was going to shotgun this I think the first thing I'd try is removing the LED's to see if that's the issue, since I have the same design already that works with larger LED's.
My hypothesis is that the small LED's cause the resonance op am via the 1nF cap to sink all the current coming from the first OTA to ground, so no audio makes it to the rest of the circuit.
If the problem goes away with the LED's out, I know more about the problem I figure?
But really, I want to understand why this doesn't work, that's how I learn things....
So here's a more general OTA question I know some of the readers here will probably just know: I thought (I could be wrong) that if the OTA +/- inputs see too much voltage (over 35mV? something like that?), or the control current goes beyond something like 2mA, you fry the 3080 immediately. No passing go, no collecting $200, poof, the 3080 is toast.
I have seen the too much control current quick-fry 3080s several times, but I am not sure about the inverting and non-inverting inputs on the OTA, does anyone know if overvoltage there causes the OTA to get stupid for a bit then, if the offense isn't too great, the OTA comes back to life when the voltage drops again?
If so then I agree i need to drop the voltage into the OTA stages (probably by decreasing the 220 ohm resistors' size).
Ah! And I just thought of something. On both units I built this weekend, one has this issue, one doesn't, I didn't have 220 1% so I used 270 ohm 1%.
But that's so close I doubt that's it....and it's not a problem on the same module I built with larger LEDs that also uses 4x 270 ohm 1%. _________________ Visit my AUDIODIWHY blog and website |
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Grumble
Joined: Nov 23, 2015 Posts: 1294 Location: Netherlands
Audio files: 30
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Posted: Mon Feb 04, 2019 2:21 pm Post subject:
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It looks like you have a cold solder joint somewhere, reflow all your solder joints, maybe that will cure your module, _________________ my synth |
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cslammy
Joined: Apr 27, 2018 Posts: 206 Location: USA
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Posted: Mon Feb 04, 2019 2:26 pm Post subject:
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Grumble wrote: | It looks like you have a cold solder joint somewhere, reflow all your solder joints, maybe that will cure your module, |
Thanks Grumble, i will try that today. _________________ Visit my AUDIODIWHY blog and website |
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cslammy
Joined: Apr 27, 2018 Posts: 206 Location: USA
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cslammy
Joined: Apr 27, 2018 Posts: 206 Location: USA
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