Modifying DOD Vibrothang LFO

[Passaloutre]

I have a DOD VobroThang whose tone I really like, but when turned up it makes some hiss and a "tick" everytime the LFO cycles.

Schematic:



Wondering if there's anything I can do to cure the "tick" and possibly reshape the LFO waveform (would be nice to make it a little more square). Maybe one operation might help the other?

[Passaloutre]

Perhaps the answer is something like the last paragraph of this article: https://mimmotronics.com/blog/talk-theory-to-me/tttm4/

The resistor R6 in the article is not present in the DOD pedal. I will try something like that. Any ideas for getting a square wave?

[Passaloutre]

Or maybe not. On second thought, it looks like the VibroThang's R51 is doing that job...

[Kipper4]

Could someone explaint to me what this ciruit topology is.
Phaser into a tremolo?
I find ota ciruits a pain to read.
thanks
Rich

[Passaloutre]

Yes it's a phaser/tremolo controlled by a single LFO

[Kipper4]

Looks like they rectified the lfo output for the tremolo.

[Passaloutre]

Could the tick be from that diode turning on and off?

[Kipper4]

Maybe U4A went open loop. Maybe the lfo biasing needs more power smoothing caps.

Even if the diode went open, you've still got some resistance and adjustment between the lfo driver and the OTA, inc ~12k series.
Sometimes layout can play a role in how the lfo interacts with the audio or the power rails.

Can of worms but most likely detectable.

[Passaloutre]

I guess a plan of attack could be probing it before and after U4A to see where the tick is present? Could a small (pf range) cap to ground somewhere in this region filter the LFO a bit? I guess that would make it less square...

[ElectricDruid]

Does the ticking come in from the phase stages or the tremolo VCA?

I'd audio-probe the outputs of each in turn to see if there's a particular area where the ticking appears.

Obviously if the LFO is yanking the power rails around the answer might be "everywhere", but you never know. Might get lucky.

In many ways, I'm not surprised it ticks. My experience of 13600/13700 is that they have significant offsets and control feedthrough, so getting them not to thump or tick when you feed them sharp edged control waveforms is tricky at best. Or maybe I'm just using them wrong - The last project I tried with one finished up with a vactrol-based filter in it instead because I never did manage to make the 13700 version work well enough to be satisfied.

[Passaloutre]

It seems to only happen with the tremolo turned up past halfway. It doesn't tick with just the phaser on. I can try some more specific settings when I get home tomorrow.