DIY Minimoog Loudness Contour

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deaf-punk
Learning to Wiggle
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DIY Minimoog Loudness Contour

Post by deaf-punk »

Hello wigglers,

I have a little problem with my loudness contour circuit. As you can see from the screenshot, there is some sort of offset voltage (about 2.2V) that overlays the contour curve.
DS0001LOUDNESSOUT.PNG
Checking the resistor values and replacing Q2 & Q3 has unfortunately not brought any success. I noticed that the curve looks alright up to the base of the last transistor (Q2) before the output. But after this transistor, there is this strange offset voltage that covers most of the decay curve.
Btw this also happens if I omit Q2 altogether and connect Q3 directly to the output/reset section.

I tried to simulate the corresponding section with different simulators, neither of them showed this behavior. All other simulated voltages and real voltages look pretty similar I'd say. Any ideas what I might have missed?
Attachments
Schematic.pdf
(277.2 KiB) Downloaded 10 times
DS0001CAPOUT.PNG
DS0001EMITTER1OUT.PNG
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KSS
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Re: DIY Minimoog Loudness Contour

Post by KSS »

PCB? Breadboard?
Power supply used?
Does the Filter EG work correctly?
deaf-punk
Learning to Wiggle
Posts: 6
Joined: Wed Mar 24, 2021 5:44 pm

Re: DIY Minimoog Loudness Contour

Post by deaf-punk »

It's all PCBs and the standard power supply with the rectifier board. The filter gives a nice curve, that's what confuses me.
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KSS
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Re: DIY Minimoog Loudness Contour

Post by KSS »

Check the loudness attack pot element. It might be cracked or burned.

Compare the various electrical net voltages of the working VCF EG with those of the loudness EG.

In the top scope shot the attack offset appears aligned with the sustain level. :hmm:
Does the offset move with the sustain level?
Is the offset the same for all attack and decay settings?
deaf-punk
Learning to Wiggle
Posts: 6
Joined: Wed Mar 24, 2021 5:44 pm

Re: DIY Minimoog Loudness Contour

Post by deaf-punk »

Yes,the offset increases along with the sustain, however only for higher sustain levels (>6). As far as I can tell, the offset is independent of the attack and decay settings. Will replace the pots in the next step.

Apparently the filter contour section was also out of order, found a wrong resistor there. Unfortunately, this has rather made things worse. I have then made the PCB completely new again with the same result. So I guess the fault lies outside the board. Since the behavior is totally inconsistent now, I'll have to take a closer look at the error. Right now the attack and decay pots do little and the decay sometimes affects the attack control and also the sustain control. Strangely enough, the filter sustain can also increase the voltage on the loudness sustain quite a bit, but not the other way around. Anyway, thanks for your support!
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KSS
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Re: DIY Minimoog Loudness Contour

Post by KSS »

Don't be too quick to change the pots. Or to swap parts in general as a fix technique.. As you've seen here, this often leads to more problems instead of fewer.

The fact that the glitch follows the sustain is a valuable clue that takes an attack pot defect out of the equation.

Instead you're now looking for why and where the sustain is bleeding-feeding the main timing capacitor. <--Which suggests that whatever device is being used to switch off the sustain during A and possibly D,R is not closing fully. I haven't looked at the schematic yet.

Edit: Did you use 1N34A germanium diodes? Are they all pointing the correct direction? Compare with the correctly working filter EG.

Edit2: From your report above you have correct EG output at base of Q2, but not its emitter output? Are the collectors of Q2 and Q3 joined and only going to R15?
Treat utility modules as stars instead of backup singers.
Treat power supplies like Rockstars instead of roadies!
Chase magic sound, not magic parts.
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