Neutron, next question...microphonic ceramic capacitors?

[half_smith]

My neutron actually works pretty well, but I have noticed that some of the ceramic caps I used are very microphonic. I wonder if I'm having problems because of them.

The effect sweeps well and the whine/hiss is gone after adding the "chokes" and jumpering pins 1,8 on my 1044

There's this exaggerated chortle, almost like feedback (that's why I suspect the microphonic caps) that seems too wet, too sensitive to be how the pedal is supposed to be.

Any clues?

I plan to replace the ceramics with film caps (R5, R6, R7, R8), already swapped in a 4.7uf Tantalum for C1 (was electrolytic) and it was microphonic too!

[Kevin Mitchell]

Something to consider.
The circuit was designed to work with many instruments - including guitar, bass, synths and so on. Much of the range of settings will seem unusable as different instruments produce a different range of frequencies & at a different levels.

The most simple thing to adjust to get an appropriate sweep response is typically the level adjustment on the instrument itself - from my experience with most envelope filters at least.

[amptramp]

If you are wondering about microphonic capacitors, try playing into headphones so there is no feedback and compare that to playing into a speaker at the same volume setting.  If the sound is different, the capacitors are microphonic.  You can also place a loud sound source next to the unit as see what the output looks like on a scope or on a headphone amp.

Or just replace all the ceramic caps with film and see what that does.

[idy]

"chortle" sounds like "gargle", (there is another name...) which is instability as the envelope decays, which inter-modulates with the audio. Neuron/Mutron are touchy, what you have is a clone (by good people) of a circuit that went through some changes and was based on proprietary vactrols...And the designer has complained about re-issues and etc. etc.

I would suggest reading more about Mutron clones and their problems.
Try different LDRs...(are you using vactrols or "roll your own" LED/LDR combos? Just because a doc says use CLMxxx or VTLxxx or NSLxxx does not mean you will be happy with that.

The thing has "peak" which is feedback and "gain" which is sensitivity. These controls are very sensitive and not all setting work with all instruments. The originals were more stable/consistent, at least seem so on demos. The clones need fine tuning of LDR... this can make it act like a different beast.

[half_smith]

Idy, yes I understand the potential issues you listed.
I'm using a Hamamatsu P873-13 (a dual LED package used in early Mutrons and QTron), it's the "better" of the 4 options in RG Keen's document.
I put a 4-turn 50k trimpot on Rx and done my best to bias it by ear.

When I say Chortle, I distinguish from the "gargle" that I expect from an envelope. The bad sound I'm hearing is more of a whistle or ringing harmonic -


I've replaced the ceramics with film, it seems maybe less touchy now.
I've found that regardless of the bias tweeking, there's a small Goldilocks zone on the Gain pot where the envelope just opens, any higher and the thing will blow your ears off.

[idy]

Gain is 1M reverse log or "C" taper?
(Or Audio taper wired backwards...)

[half_smith]

Yes 1M reverse log

[half_smith]

Hey gang, I'm back trying to clean up my Neutron. It's "usable" but I feel like this feedback/harmonic hum thing is not correct and I would love to crack the case on this.

Things I've done in search of the cause of the "bad":

REPLACED a number of ceramic caps that I was suspicious of microphonics,
BRIDGED a part of the PCB that seemed to make noise when tapped,
ADDED Inductors into the circuit (option from the original Keen design pdf),
SWAPPED out the Hamamatsu P873-13 (from a USA EBAY seller) for a pair of NSL32s (not much difference for future reference, a slightly different signature in the trail off)
PROBED through the circuit and it's not clear where the issue begins (because this circuit loops back on itself so many times)

Am I just over-reacting to this pedal's NORMAL resting "c0cked wah" face?

HERE'S A 1MIN VIDEO SHOWING THE ISSUE
https://www.youtube.com/watch?v=S1bwx8AM-5A

Any help much appreciated, thanks!

[idy]

Have you tried adjusting the trimmer inside?

I don't know exactly which board you are using so I am not sure what yours does. The RG Keen schematic I don't see a trimmer, but that looks like your board. The Mad Bean clone (naughty fish or nautilus) has a trimmer that dims the LED. That would help. It just sounds like the LED is going too bright too fast. If that isn't your trimmer I would try to add one...Keen calls it Rx, you must have chosen its value. Perhaps too low....

That low resonant hum, I don't know what it is. It implies other issues.

[half_smith]

I have a 10k trim pot at Rx, I have it around the middle of the usable range, I can try on the low end.

That low resonant hum, I don't know what it is. It implies other issues.

Please elaborate, this is what I'm asking about.

[idy]

My guess is too much feedback on the filter.
Does turning the peak control down get rid of it?
Does changing the direction from up to down change the drone?
What about the hi/lo switch?
What value did you use for Peak pot? Keen says original was 150k, the clones I have made used 250k, the meatsphere is only 100k and in parallel with a resistor. A smaller value means you always have more resonance, you can't turn it down. So it would be safest to go up to 250k. Maybe snip that wire as an experiment.
Then there is double checking the value of all resistors in the filter section....
Also possible is wires radiating; you would check if moving those loose wires (like to the Peak pot) changes the sound.

The trimmer I would think to try it at maximum resistance. You want to dim the LED a bit. You once had a 50k multi-turn there. That is more than you need. I know Keen said 4.7k, but that is a real old-school value for CLR on an LED and modern low current/high brightness LEDs still shine like crazy with 10k, so unless maxing the trimmer out totally kills the filter, I would increase it to 20k or even 50k. I know you are using optos.... I lot of us "roll our own" and find they work better than the ready-mades.

The other way to bring that "itchy trigger" under control is to turn the input ("Gain", but really an input trimmer) down. Does it trigger more normally then?

[idy]

I looked at the GGG project. What kind of charge pump are you using?
With TC1044SCPA, MAX1044CPA or ICL7660SCPA you jumper pins 1 and 8, with the 1054 you don't. If you don't have SCPA or CPA the bridging ("boost" the frequency) won't help.

I find a lot of pedals with charge pumps whine when I use a cheap power supply instead of a trusty "one-spot." That is heterodyning, with the charge pump and the power supply creating and audible difference tone. If this is the problem turning knobs and flipping switches will not affect the frequency of the whine. If you don't have a great power supply you can wire up a battery just to test. Does that remove the whine?

[half_smith]

It's a 1054 (yes the 7660 was whinning), using the IC with higher freq cut that out completely.

I've been using a One-Spot thru a Furman for testing, but same as when on the cheap Pedal Power board supply.

Turning the PEAK pot doesn't make the hum go away, but at extremes the effect becomes null.

What value did you use for Peak pot? Keen says original was 150k, the clones I have made used 250k, the meatsphere is only 100k and in parallel with a resistor. A smaller value means you always have more resonance, you can't turn it down. So it would be safest to go up to 250k.

I used 150k, I will try 250

I will try the Rx trim pot at the upper end of the usable range.

[half_smith]

here's another quick video (40sec) showing how sensitive the PCB is to my finger, it sounds like the same kind of woofy feedback hum that I am trying to remedy,
anyone have a spark of knowing when you see this?

https://youtu.be/FA_HKXBWI3o