Angry Charles - 100k Treble Potentiometer Issue

[Caldo71]

Hi Gang,

So I just finished an Angry Charles build and it's generally great-sounding and really quiet, but I am having one weird issue: with the 100k treble pot, once I turn it up past about 90% of a full rotation, it basically "mutes" the whole circuit—everything just drops out and the red LEDs stop lighting up as well.

My first thought (possibly a newb prediction) was that it was a mechanical issue on the part of the potentiometer itself, so I immediately swapped out to another brand-new B100K pot, being very careful with iron temp and all that....same exact results.

Does this behavior signal any obvious "red flags" with the experienced builders here? Is the pot somehow not "playing nice" with another component it has a vital relationship with? Maybe something I mis-soldered or mis-spec'ed? Or I should try a different value on something? I was exceedingly careful with both making sure I had all the right resistors and triple-checking their color codes before soldering in, plus I feel like I did a really clean soldering job with no cold solders etc...but who knows.

For reference, I'm using these slightly smaller pots from Mouser with the 13mm diameter back housing...

https://www.mouser.com/ProductDetail/313-1210F-100K

the other one of the same value that's used for the Drive control seems to work just great.

I'm housing this pedal in a 2RU rack enclosure along with other pedals with no DPDT switch (always-on) and lots of wire leads to the jacks and pots etc, so it's REALLY hard for me to capture a clean photo of my build...but attached to this post is a quick Photoshop composite of two photos I shot so I can try and get you guys SOME idea of my components used.

 

[fig]

curious...what is in the signal chain before and after this?

 

[Caldo71]

curious...what is in the signal chain before and after this?

Good question...well, the before/after signal chain is actually NOTHING, technically-speaking:

As-mentioned I have this housed in an 2RU enclosure with other pedals (probably why you're asking), but they are all completely electronically independent from each other (aside from ground of course) with their very own separate inputs/outputs on the back panel.

I control which pedals are in/out my signal chain at any given moment externally using a Voodoo Labs HEX loop controller and a midi pedal. This is also why I have the DPDT pads wired directly to the in/out jacks so that the pedals are "always on".

So for testing purpooses with this new build I literally do not even have cables plugged into the in/out jacks for the other pedals in the enclosure...it's just completely alone for all intents and purposes currently.

 

[Chuck D. Bones]

Looks like you have all of the pots mounted outboard. Can you show us the whole picture? Length and proximity of the wires is a concern because stray coupling could cause the circuit to oscillate, maybe at frequencies only a dog could hear. It's also possible that there is a miswire or bad connection. Does the TREBLE pot act normally from 0 to 90%? Do a thorough visual inspection and most likely, you will find the problem.

 

[Caldo71]

Looks like you have all of the pots mounted outboard. Can you show us the whole picture? Length and proximity of the wires is a concern because stray coupling could cause the circuit to oscillate, maybe at frequencies only a dog could hear. It's also possible that there is a miswire or bad connection. Does the TREBLE pot act normally from 0 to 90%? Do a thorough visual inspection and most likely, you will find the problem.

Chuck can you explain the term "stray coupling" to me? Not sure I get what that means.

As to your other question, yes the treble seems to increase for the first 90% although it seems less of an aggresive increase in treble than I'd expect. But yes you do hear the treble gradually increase and there re no other weird audio artifacts happening over the course of its range.

I'll try and snap a decent picture of the length of wiring between the pot and the PCB, as well as inspect it myself, to see what's what. Stay tuned.

 

[Chuck D. Bones]

Stray coupling is any unwanted coupling between two (or more) parts of the circuit. Two conductors in proximity, such as wires, circuit board traces, etc. form a capacitor. In a high-gain circuit, it only takes a few pF to turn an amplifier into an oscillator. Two conductors running parallel will act like two windings in a transformer. The inductive coupling is not very much, but it doesn't take much. Shielded wire helps, as long as it's well grounded. Best way to minimize stray coupling is to route wires going to different pots away from each other and everything else. Since the enclosure is conductive and grounded, laying wires against the walls of the enclosure helps with shielding.

 

[Caldo71]

Thank you for that clear and damned interesting explanation. Haven't had a chance to get in there to inspect things and take better pics yet, but will do so very soon.

 

[Caldo71]

Stray coupling is any unwanted coupling between two (or more) parts of the circuit. Two conductors in proximity, such as wires, circuit board traces, etc. form a capacitor. In a high-gain circuit, it only takes a few pF to turn an amplifier into an oscillator. Two conductors running parallel will act like two windings in a transformer. The inductive coupling is not very much, but it doesn't take much. Shielded wire helps, as long as it's well grounded. Best way to minimize stray coupling is to route wires going to different pots away from each other and everything else. Since the enclosure is conductive and grounded, laying wires against the walls of the enclosure helps with shielding.

@Chuck D. Bones here finally are some pics of my Angry Charles where I pulled it out of my 2RU enclosure and pulled all the cables out of the way so you can see the components, as well as the soldering work on the backside. Also some closeup pics of where the cables for the B100 treble pot leave the PCB and connect to the pot.

I didn’t want to say anything before I posted these, but I’m not sure this “stray coupling” theory is the culprit...I feel like I did really clean work on this?

Also some “long shots” so you can see it sitting in its enclosure alongside some other builds. Dwarven Hammer on the upper deck to its left, some other ones coming soon to the empty spots.