Mutron MICRO V - working but wobbly

[half_smith]

Hey Gang! I'm back inside the first pedal I ever built, a Mutron Micro V - Envelope from GGG

It's always worked ok. I think it's not quite dialed.

There's a lot of turbulence in the sweep. Gurgly.
Kind of reminds me of my Mutron Octave Divider (from BYOC) when the "stabilizer" is turned OFF.

And I would like the sweep itself to be more pronounced.

I don't know where to begin. Voltages look pretty good on the ICs and Qs

[ElectricDruid]

"Gurgly" sounds like poor filtering in the envelope detector. Not enough sweep might also suggest something up with the envelope detector. I'd check that part over carefully and see what you can see.

[half_smith]

Hi Electric Druid!
Can you help break down the area of the circuit you mean by "envelope detector"

is it the area near Q3?

http://www.generalguitargadgets.com/pdf/ggg_microv_sc.pdf

[antonis]

Almost everything around IC1b..

http://ww1.microchip.com/downloads/en/AppNotes/01353A.pdf
(focus on fig.19)

[half_smith]

I see I have some reading to do, thanks Antonis

[antonis]

IC1b..
(edited)

[half_smith]

Any clues to what I would be looking for in that Wave Rectifier Area? For the gurgling.

Is this a classic case of unwanted DC? maybe caused by a bad Cap?

[Mark Hammer]

Ripple in an envelope follower is often perceived as being gurgly or even as "distortion".

Typically, ripple in a full-wave rectifier tends to be less objectionable than a half-wave rectifier, the reason being that full-wave rectification provides two positive-going "blips" for every one that half-wave provides (hence more likely to result in filter jiggling that falls within hearing range).

There are generally two "cures" for such ripple.  One is to increase the value of the smoothing/averaging capacitor, and increase the "hold time" of that cap, such that all the blips are smudged together.  For instance, increasing C11 to 10uf, and R15 (the resistance that "bleeds off" the charge stored in C11) to 100 or 220k, would do that.  The negative aspect is that the decay time is extended beyond what you may find useful.

The other strategy is to go the complete opposite way, and aim for as fast a response time as possible, so that one doesn't notice the ripple because it comes and goes too fast.  That would involve reducing R14 andC11, such that C11 charges up faster and doesn't hold as much charge (so it bleeds off faster).  Here, you might wish to drop R14 and C11 by half (e.g., 330R and 2.2uf).  The resulting sound will often feel more "synth-like", although it may not dovetail with the musical style you wish to use it with.

[half_smith]

Mark Hammer, thanks for that explanation!
I was guessing the Q3 loop would be involved, I've tried various different PNPs there with some change in the gurgle.

I may socket R14, R15, C11 and just see what happens

Any idea of how to deepen (strengthen?) the scoop in general?
I'm wondering how much of what I want to change is just a fact of this modified circuit to run off a 4558 and the LM13700...

I remember seeing a demo on Youtube of this pedal and the designer (M. Beigel perhaps? not sure if it was him) was in the comments saying how the clone sounds nothing like the original and it's a sham. He was pissed!

[ElectricDruid]

Schematic here, for those following along:

http://www.generalguitargadgets.com/pdf/ggg_microv_sc.pdf

Further to what Mark said, I don't see the full wave rectifier. I see the precision halfwave rectifier based on the op-amp, but I don't see any following mixer that would turn it into a fullwave rectifier. Instead we've just got a transistor, Q3, producing a current for IC2/LM13700/the OTA. I might be wrong (hey, it won't be the first time... ), but that's my reading of it currently.

HOWEVER if the pedal was designed with a halfwave rectifier and sounded decent then there's no reason why that shouldn't work here as well. I doubt the other changes would make any odds. The 4558 op-amp looks like it might be original from the schematics I can find of the Mutron Micro V (basic standard part - nothing special, but pretty typical in 70s/80s pedals) and the LM13700 (dual OTA instead of the CA3080 single OTA, but otherwise extremely similar) isn't going to alter anything radically. There's one change between the CA3080 and the LM13700, which is the volt-drop on the Iabc control input (two diode drops on the 13700, only one on the 3080) but I don't see that making a big difference here. There's also the "doubled OTA" but that's an unorthodox technique that I wouldn't like to comment on further!

There's a schematic (traced, presumably) of the original here, if you'd like to compare:

http://www.pisotones.com/MuTron_Micro-V/imgs/MutronVEsquema.jpg

(For non-Spanish speakers; "con" and "sin" is "with" and "without" true bypass. The rest is obvious)

I haven't checked this over for any discrepancies. YMMV, etc!