Harmonic Percolator noise and LPF?

[goldenmonkeycolor]

I recently have spent some serious time on a harmonic percolator clone using the vero layout over at tagboard effects.  I did include the switch to take the clipping diodes in and out of the circuit and it sounds killer!  as a little side note here this circuit does actually respond to component composition hugely I really recommend the tantalum caps and silver mica and all that crap.  Anyways, it sounds killer with the diodes in or out, but when they are in the circuit I get this sort of glitchy staticy noise.  it's not like typical fuzz pedal noise but it's weird like i said kinda glitchy static'y and it only really happens between noon and 3 o clock on the harmonics pot.  near maxxed out and it doesnt make the noise (though when i do sustain notes I hear this kind of weird glitchy undertone, but its really quiet. 

Is this part of the circuit?  or do I just have a bad transistor or some other part (i did use a 2n3565 silicon and a 2n404a germanium from smallbear- ive tried multiple 2n3565's too)  i've tried replacing the diodes just because its only doing it when the diodes are in circuit.

I found some dude that's selling harmonic percolator clones with a low pass filter in order to remove some of the noise in there.  i tried throwing in a low pass filter using the calculator from this site http://www.muzique.com/schem/filter.htm   i started at 10,000hz low pass filter and that was superrrrr muffled.  i went up to 20,000hz and it was still muffled! using a .68uf cap and a 12 ohm resistor to get a 20,000hz low pass filter and it's super muffled!  I thought 20,000hz was the human ear hearing limit.  is there something wrong with this low pass filter or should i try a different method?  (it did remove the noise btw!  but ruined the sound of the pedal!)

if this is part of the circuit i'd like to reduce the noise some by cutting off high frequencies a bit using a LPF- got any ideas?

[GibsonGM]

Don't know the circuit or anything, but it seems weird you're getting noise only with the diodes engaged...I'll take a risk at kind of a stupid guess....could that actually be switching noise as they turn on and off?  I've never 'heard' it myself, but I've read that it exists.
This is more of a question than anything given with knowledge behind it, lol

[JFace]

Check your potentiometer...I've had similar problems that were solved when the pot was replaced.

Regarding the muffled filter...make sure the output impedance of the stage preceding your filter is at least 10 times smaller than the filter, and make sure the stage after the filter is at least 10 times higher. For instance, an op amp output could drive a filter in the 10k ohm range, and then have an op amp buffer afterwards. This ensures the rolloff is where you want it. If it's not a steep enough roll off, try an active filter:
http://sound.westhost.com/articles/active-filters.htm

[midwayfair]

i started at 10,000hz low pass filter

The simple answer is "no you didn't" -- you no doubt neglected to take into consideration the actual output impedance of Q2 when you were calculating your frequencies. I'm not entirely certain what it is (I don't know how to deal with Q1 being in the way for the path to ground for Q2's emitter), but it's pretty large (in part because of the high collector resistor value), certainly well over, say, 12K, and if it were ONLY 10K, your 680nF cap would put you just below a 20Hz, which would indeed be super muffled.

Grab a gew much smaller caps. And then use your ears, not a calculator. You can always figure out the cutoff frequency later, after you've found a good value to use.

[duck_arse]

i went up to 20,000hz and it was still muffled! using a .68uf cap and a 12 ohm resistor to get a 20,000hz low pass filter and it's super muffled! 

the "12 ohm" is the problem. the numbers work out right, but they will start to real world work better if you scale the resistor up by about 1000X.

[midwayfair]

i went up to 20,000hz and it was still muffled! using a .68uf cap and a 12 ohm resistor to get a 20,000hz low pass filter and it's super muffled! 

the "12 ohm" is the problem. the numbers work out right, but they will start to real world work better if you scale the resistor up by about 1000X.

Even then, the biggest problem is that the resistance isn't actually 12R, because a common emitter amplifier doesn't have an output impedance of 0 (nothing does!), and the output impedance is so proportionally huge that the 12R might as well not be there ... but yeah, any value smaller than a few K for the resistor, or a similarly small value of capacitor (like under 470pF) tends to be finicky to work with, and in dealing with filters above a few thousand Hz, even very tiny changes within the tolerance of the parts can radically alter the filter frequencies. Iiiiiit's a problem that I'm sure is much more of a problem for circuits that can't get away with the brute force of a guitar pedal.

[goldenmonkeycolor]

well changing the pot helped with the random noise between different parts of the pots rotation.  but i get this weird like undertone distortion underneath the note as it decays... it's not a terrible problem, but id like it to be gone.  is this typical of a harmonic percolator? 

regarding the filter.  if i scale up the resistor to like a 2.8k with a .0047 for a 12000hz low pass will i be able to just place it after or before the effect in the box passively?  will i still need to worry about buffering?  should i just try an active crossover?

i dont think the noise that i could filter off with a low pass filter is as big of a problem now with the new pot.  but i'm still curious about the filter and if this distortion is just typical behavior of the circuit or not/

[midwayfair]

if i scale up the resistor to like a 2.8k with a .0047 for a 12000hz low pass

Again, you have WAAAAY more than 2.8K impedance at the output of Q2 already. You're calculating your frequencies as if Q2 has 0 output impedance. There's no such amplifier. Read up on determining the output impedance of a common emitter transistor amplifier. At an absolute minimum it's going to be the collector resistor of Q2 in parallel with the emitter resistor of Q1 (and some other factors, I think, it's been a while since I tried to understand this stuff).

[tombaker]

FWIW I had a lot of radio station noise in my circuit so I put a small value capacitor shunted to ground straight after the input pot.
I can't recall the value but i used a circuit sim to find something that worked, but that will give you a variable filter because of the pot. The more you turn up the input/harmonics, the more high frequencies get cut.
I used the george giblet vero layout and it did the trick although I can't say my pedal sounds like the albini version but I like it nonetheless.

[Rob Strand]

Just for the record the output impedance at the collector, with the 50k "balance" pot present, is about 18kohm to 19kohm.
For a 10kHz low pass you will need about 820pF.

The transistor biasing can change the sound.    With touchy circuits the same part values don't end-up with the exact same sound.   You see many DIY circuits of the Pecolator where people have tweak the resistors to suit their taste and/or to suit the transistors they used on the day.  You could play around with the values of the 20k resistor (R4 on Giblet schematic) and/or 750k collector-base resistor (R1) to see if the behaviour you mentioned reduces or disappears.

[goldenmonkeycolor]

thanks for the help!  i'm gonna do a little more research on this topic and go ahead and play around with some values.  ill let you know when i find the results

[goldenmonkeycolor]

i really think i just have something wrong with the circuit but im still unsure- i made a demo of the noise if anyone would be so kind to listen to it and tell me if thats just normal operation- id really appreciate it!

im having trouble getting the link a youtube video function here- so here's a link!

http://youtu.be/toSAs2j2drA