Corduroy question (trim pots)

[Pierre67]

Hello !
I have build the Corduroy fuzz, it sounds pretty good but I'm having a hard time dealing with the trim pots, I'm getting a high pitch squealing if I set the trimpots to 4.5V drain voltage...Squeals goes away at aproximately 6V- 6.5V. I have tons of J201s and I already tried replacing them...Am I missing something?

 

[xefned]

Chuck, you're a flippin' genius.
(I'm traveling atm, but I'll try this as soon as I'm back in town.)

Thank you so much for all the mods you've contributed.

 

[xefned]

Even a tiny amount of capacitance between the VOLUME control pads and that trace
is enough to sustain oscillations under worst-case conditions.

Um yeah, about that. My wiring could be contributing to the situation. It's a mess of wire with ins and outs traveling haphazardly to a relay bypass PCB. I could stand to follow paniagua's scheme of shielded ins and outs. That's what I aspire to.

 

[xefned]

Here's my quick & dirty fix. I lifted the ends of R2, R7 & C7 that are closest to the center of the board. I removed C6. …

Great news!!! This mod works! Thanks so much, Chuck.

I just cut the trace and added the bypass cap and now I can max the fuzz knob in big mode with no oscillation. All is well.
I'm shipping this off to my friend Aaron who is struggling with his band trying to make an OD sound like a fuzz.

I'm assuming I don't need to lift the components and cutting the trace is adequate (?) since you already lifted the parts to prove it works.

All trimpots have been reset to give 4.5v on the drain of each JFET.
If I try really really really hard, I can still achieve a small amount of squeal, but that is with all 3 knobs maxed out and only with hot humbuckers. If I split the coil on either humbucker, the squeal goes away. (It's also only at the very top end of the volume pot.) Only mentioning this for due diligence for future travelers. There is no musical scenario in which you'd max any of the 3 knobs, let alone all 3. (Well, maybe the fuzz you'd want to max for mad sustain, but that's been fixed.) Unity volume is around 9 a.m. Max treble is a tad harsh. Max volume is socially irresponsible.

Played for another hour this evening. My daughter appeared at the door and asked me to turn it down. Another successful build.

Thanks Chuck. Thanks all for being here in this land of sonic possibility.

 

[Chuck D. Bones]

I wasn't sure that cutting the trace would be good enough because even with that one leg removed, the trace still runs too close to the VOLUME pot's pads IMO.

Wire routing is important with high-gain circuits. Your routing looks good. The IN & OUT wires are far apart and routed agains the enclosure walls. I would not recommend shielding. Some people use it as a band-aid.

 

[xefned]

I wasn't sure that cutting the trace would be good enough because even with that one leg removed, the trace still runs too close to the VOLUME pot's pads IMO.

It definitely did the trick. But I'll build my next one with the legs lifted away from the volume pads for comparison with this one.
Somehow I convinced all my friends that they need a Corduroy. Or maybe they convinced me. I can't remember.
I will
report back...

Wire routing is important with high-gain circuits. Your routing looks good. The IN & OUT wires are far apart and routed agains the enclosure walls. I would not recommend shielding. Some people use it as a band-aid.

Thanks dude. I'm loving these relay bypass boards. I wanna retrofit all my favorite builds with them.
Appreciate all your advice.

 

[Chuck D. Bones]

I have it on good authority that the Corduroy board is in line for an update. Don't know when.

 

[xefned]

I have it on good authority that the Corduroy board is in line for an update. Don't know when.

Cool. I will compare lifted to non-lifted, experimenting with the distance to the volume pot to provide data to Robert for the redesign. (I already have 3 rev. 1 PCBs.)

Corduroy #2 now has an elevated mod (green wire below,) farther away from the volume pot than just cutting the trace near the pot's pads. I'll compare them and report back.

 

[xefned]

Here's a TL;DR graphic for anyone else experiencing oscillation with a rev. 1.3 PCB. It's Chuck's mod from above for people for whom words are hard. Connect the 3 red dots. Move C6 (100pF) up to the 2n5089.

 

[xefned]

Side-by-side Corduroys. All 6 FETs at 4.5v at their drains.

Left one oscillates with 3 dimes. Right one (with elevated mod) does not.
This tells me you should do the full elevated mod ABOVE-BOARD and not just cut some traces.
That is all.



Course, there are other differences: Intelligent Relay bypass. Not likely to be the cause.
Left one uses LM-386-4 rather than LM-386-1. Also unlikely to be the cause, since it's in the "tight" part of the circuit.

Still, I'll go ahead and mod the left one with an elevated wire. I was just being lazy because I didn't feel like unboxing it.

 

[Robert]

I have it on good authority that the Corduroy board is in line for an update. Don't know when.

It's done, waiting for PCBs to arrive to verify.

Anyone who has this issue, or has the PCB and hasn't assembled it yet can shoot me a message when the new revision is available and I'll make sure you get one, no charge.

 

[xefned]

Once the PCB is out of the enclosure, the mod only takes about 10 minutes.
I'm not sure why I feel the need to share this pic. But the takeaway is that I'm now certain that cutting traces is not enough. The offending trace is still near enough to the volume pot to pick up the oscillation.



Having said that, cutting traces is still good enough to keep the fuzz knob from oscillating which is the main thing we care about because nobody will ever max all 3 knobs. Dial back *any* of the 3 knobs, including the treble, or use a single coil, or back off your guitar's volume knob, and you won't hear any squeal. I'm also using high-output humbuckers, so I may be the only person with this “issue” if you can even call it that.

One last thing: starving one of the 3 FETs with a 7v drain as a quick fix is just as well. This pedal still sounds like a sex machine with an anemic 3rd FET. I've now played many hours with this pedal and it's addictive in any mod state including just cutting the trace that runs between the volume pot lugs, or starving a fet.

 

[Robert]

Updated PCB prototypes are here, no more oscillation at any setting, yay!

What's that BB King song? The Squeal is Gone!

 

[Joben Magooch]

I just have to give some shout outs here. I know very little about circuit design, it's mostly paint-by-numbers for me, but am always looking to learn more...I don't have this particular pedal, but just wanna say it was cool to see this discussion go down, Chuck walking thru the circuit and finding a workaround/solution, and then Robert making a revision and offering it out for free to anyone with the old PCB. Cool gesture to see and I feel like I learned a little bit, to boot.

 

[fish1955]

It's done, waiting for PCBs to arrive to verify.

Anyone who has this issue, or has the PCB and hasn't assembled it yet can shoot me a message when the new revision is available and I'll make sure you get one, no charge.

Hi Robert, do you have an update on the timing for the revised build document for the Corduroy fuzz. Thanks

 

[xefned]

Hi Robert, do you have an update on the timing for the revised build document for the Corduroy fuzz. Thanks

I have both revisions. It appears that no parts changed since the previous revision.

 

[fish1955]

I have both revisions. It appears that no parts changed since the previous revision.

Thanks!