geofex variable stuttering pedal

[moosapotamus]

Anyone tried this?
http://www.geofex.com/FX_images/stutter.pdf

I've got it on my breadboard right now and it stutters great. The duty cycle control is cool. I like maybe an 0.47uF or smaller cap on the integrator for faster speeds. Smaller (faster) will send it into ring mod territory.

Replacing Rsource with a 1M pot to control the "off" signal level works great, but all the control is restricted to about 1/4 of the pot's rotation, so 100K or 250K might be a better choice.

RG's idea from a while ago (previous thread) about throwing in another opamp to invert the input signal, sending it to another JFET switch out of phase with the first one and coupling it to the existing output buffer would probably sound awesome.

It's a little noisy, tho. Shunt switching is definately quieter than series switching. But, even with shunt switching, the ticking bleeding through is just enough to be a little bothersome.

I've got all the components for the audio path on one side of my breadboard, and everything else for the LFO on the other side. I tried increasing some of the cap values, tweaking the bias voltage, and messing with the gain in the input and output buffers, but none of that stuff seemed to make much difference.

Anyone have any more ideas about quieting bleedthrough in this circuit?

Thanks
~ Charlie

[Paul Perry (Frostwave)]

Try making the 1nf cap at the diode a .047, might take the edge off the switching? just a guess. Depends where the 'tick' is coming from, the other possiblity is to isolate the LFO section with a 220 ohm resistor to the power rail & bypassed by a 200mf electro to ground on the LFO side of the resistor.

[R.G.]

Yeah, what Paul said plus maybe:
- make a second reference voltage for the LFO separated from the signal reference
- now that I think about it, change Rseries to 10K or lower. This lowers the general impedance of the signal path, so that any coupled "ticks" have a harder time moving the signal path around. You could go as low as 4.7K with a TL series opamp, I think.

Now that I think more, isolate the problem. Change that 100K at the output of the LFO to 10M (which makes it totally ineffective at switching the JFET) and see if you still have ticks. If you do, the ticks are coming through the power supply, ground, and/or reference voltage. If not, it's the JFET ticking, and other methods apply.

As we all know, ticks are the bane of bang-bang integrator LFOs. This one is no exception.

[jrc4558]

Or use a optoisoltaor, like in Anderton Tremolo Or Tremulus Lune.

[moosapotamus]

Thanks for the suggestions, guys!

I pulled out the 100K at the output of the LFO and the ticking is still coming through. So, I'm going to have at the power supply and try isolating the LFO. Thing is, I'm currently using a TL074. So, I'm going to have to pull it apart an put in a pair of TL072's... which, if I had actually given it a bit more thought, I probably should have done from the start. :wink:

Regarding the other ideas...

Try making the 1nf cap at the diode a .047

Increasing that cap slows down the rate, but doesn't really quiet the ticking.

isolate the LFO section

Yup.

make a second reference voltage for the LFO

This didn't sound like it made any difference with the TL074. But, I'll try it again after redoing it with a pair of TL072's.

change Rseries to 10K or lower

This actually did reduce the ticking a tiny bit. I think 4K7 is what I'll stick with. Anything smaller and the chop doesn't completely mute the signal.

use a optoisoltaor

Optos are not really fast enough to give a decent sounding chop, yes/no?

I'll let y'all know how it goes with the pair of TL072's.

Thanks!
~ Charlie

[jrc4558]

Well, yes, the're won't be such abrupt stuttering, but with an opto you're not letting any signal from the LFO into the audio path.

[R.G.]

pulled out the 100K at the output of the LFO and the ticking is still coming through. So, I'm going to have at the power supply and try isolating the LFO.

Hey! That's very, very good. That means that the JFET switch itself has a good chance of being quieter.

Are you using the input-jack power switching scheme? That is a disaster for coupling in current changes on the input of an effect. The full current of the effect is in series with the input ground wire, and the IR of current changes shows up at the input.  Try hard-wiring the power if you don't have it hardwired already.

Try separate references. Or buffer the reference.
Try separage ground wires from the LFO and audio path to battery -.

[R.G.]

Well, yes, the're won't be such abrupt stuttering, but with an opto you're not letting any signal from the LFO into the audio path.

The problem is that he's getting ticking with the path to the JFET disabled entirely - there can't be any ticking coupled in that way at all.

However, yes, if that were the ticking path, you could use an H11F1 LED/photoFET and get really, really uncoupled from the LFO path. Doesnt' help the power, ground and reference paths, and might make them worse since the current switched in the LED is much bigger than the thing does now.

[moosapotamus]

Made a few more changes...
Not sure if this is as good as it can get, but it's definately an improvement. Here's a couple of clips...

http://www.moosapotamus.com/stutterslow.mp3

http://www.moosapotamus.com/stutterfast.mp3

- Replaced the TL074 with a pair of TL072's.
- Made separate references, each individually buffered using another TL072. This probably had the biggest effect on reducing the ticking.
- Isolated the LFO section from the 9V power rail, like Paul suggested.
- Replaced the 1nF cap at the diode with a 5.6nF (it does seem to take a teeny bit of edge off).

Try separage ground wires from the LFO and audio path to battery -.

Maybe I'm thinkin' too hard, but if the LFO and audio grounds connect at the battery (-), what's the difference?

Are you using the input-jack power switching scheme?

No, I've got it all on a breadboard with the power plugged right in. That's good to know, tho. Thanks, R.G.!

A couple other minor tweaks...
- Either 10K or 50K pot to replace Rsource (seems like 50K is plenty to go to no noticeable chop).
- Probably needs at least two different caps to switch between on the integrator to get full range speed control, like 0.47 and 0.1, maybe... I'm still tinkering with that.

So, the ticking isn't gone. I can still hear it in my headphones. But, I guess it's not too bad on playback. Probably would be even less noticable in a mix.

So, mucho appreciado for all the suggestions, y'all! If anyone's got any more, I'm still willing to give'em a go, tho.

Thanks
~ Charlie

[Paul Perry (Frostwave)]

Try separage ground wires from the LFO and audio path to battery -.

Maybe I'm thinkin' too hard, but if the LFO and audio grounds connect at the battery (-), what's the difference?

The difference is this: if you have a very high current spike (like from these LFO circuits) then, as the spike flows to or from the battery, the PCB trace acts as a small resistor.. big enogh though, to have a voltage spike develop across it from the current!! and the spike is on the power connections near to the LFO op amp.
I actually had to solder some 1/8 inch copper wire to a PCB trace to get over this once.. to search for info about this, the keyword is "star grounding". Covered in R.G's excellent PCB 'how to lay out fx' book, of course :oops:

[RDV]

Hey Moose, that thing sounds an awful lot like a Kay Tremolo circuit.

RDV

[John Lyons]

Hey there

Mooseapotamus
I was looking into building the variable stuttering pedal at GEO. I read some old posts about your tweaking it but was curious what became of it...Did you ever build it and get everything worked out? If you have a layout or PCB you could share that would be great.

Thanks

John

[Nasse]

an idea...

I need to test my ideas, sorry

[moosapotamus]

Never took it any further. But, I did refresh the links to those two audio clips, above, that I recorded when I had it on my breadboard.

~ Charlie

[John Lyons]

Thanks for renewing those sound clips. I built the Pulsar and I liked it after I did a couple mods to make the intensity mellow out when played clean...and more intense when played through a distorted amp.

I'd like to give this one a shot but I'll have to look at the circuit and maybe just bread board it then perf when I get it going. I would be nice to have a PCB for it...but....

anyone else build it?

John