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Intense oscillations inside slew circuit
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Intense oscillations inside slew circuit
gooooob
I recently started building building eurorack modules, because I need some utilities. Got my oscilloscope yesterday and checked the slew circuit, I built using breadboards.

It's based on the yusynth gated slew (http://yusynth.net/Modular/EN/DUALSLEW/index.html) without the gate.

I used two pots with the diodes, so rise and fall can be controlled separately, and replaced the original pot with a 1MΩ resistor, so the output can reach input level in log-mode. My reasoning was, that the diodes would prevent that once the voltage-difference got to small.

Other differences are TL074 instead of TL072 and 100k-pots.

I measured input, output and the output of U1a and it seems to work well, but...

In lin-mode, when input=output <=> not slewing, U1a oscillates like crazy: Vmin to Vmax at 2-40kHz depending on the slew-rate. (Frequency isn't really stable and pulse width varies with asymmetric slew.)

This makes sense to me, but I don't really know, what I'm doing. Didn't even know what an opamp is 2 months ago.

It doesn't seem to have any effect on the actual output. Might this interfere with other modules? Should I be worried to mess up everything with my first circuit?

My primary use will be smoothing changes from MIDI velocity into my VCA.

Thanks for any input. love
cygmu
This seems logical to me. When the op amp's inputs are at the same voltage the output should be 0V, but any bit of noise in the system will disturb that slightly and the op amp will respond. Because the negative feedback goes via the slew, the op amp has time to push its output quite far before it sees the effect, and then there is a bit of overshoot, and it responds in the other direction, and so on. Just like a regular oscillator.

If all is working well, U1A will only perform this oscillation as far as it needs to to keep the output where it should be, so you won't see anything nasty on the output.

There shouldn't be any significant impact on the rest of the system. After all, some people do have oscillators in their systems on purpose smile
slow_riot
you need some capacitance between u1b pin 7 and u1a pin 2
cygmu
slow_riot wrote:
you need some capacitance between u1b pin 7 and u1a pin 2


Haha, I was just coming back to add this. Silly me, didn't think when posting first that this is exactly the sort of thing that can be calmed down with a little cap.

Anything from 47pF to 1nF should do. I would go small, but use whatever you have.
slow_riot
Any circuit involving negative feedback using multiple active elements like 2 opamps, or a transistor inside the feedback path are known trouble makers.

I would also recommend values in the region of where Cygmu has suggested, big enough to quell the oscillation, but small enough to avoid interacting with the main feedback path (because a cap will short it out above it's operating region).
elektrouwe
this is a feature not a bug ;-) And I don't believe that you can calm down with a small C in the feedback path. And I also don't think it has to do with multiple opamps in the feedback path.
I think it's just the fact that you have a lowpass filter in the overall feedback, which means no feedback for high frequencies.
This will cause ringing U1a every time the input jumps (even from noise).
OT: That's why I developed a nice base drum from a slew limiter circuit :-)
The cure would be to add a small R (100R..1k) in series with C3 (680n) to establish a minimum feedback for high frequencies
Synthiq
The new capacitor between output and input of U1a will be more effective if a 10-100kohm resistor is also placed in the feedback from the second amplifier to the lin/log switch. To be effective, the impedance in the capacitor must be lower than the impedance of the signal from the second amplifier which now is the output resistance of the amplifier itself which is quite low at the oscillation frequencies you see.
gooooob
I think I got it worked out. Capacitors didn't help.

But I replaced the switch with a 100k-pot with an additional 4.7k resistor to UA2. No oscillation anymore. And I can blend between the two modes.

Seems to work fine. Will report back, when it's soldered.

Thanks everyone.
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