Sonic Crayon GAS!

[multi_s]

yeah basically. coupled with the fact that I don't understand the symptoms at all, let alone any possible solution. some weird electricity shit happening between SOME guitars, hollow earth and SOME amps, even with an isolated power supply.

i know i mentioned this before but i could probably help you fix it in less than an afternoon. for serious.

[cloudscapes]

i know i mentioned this before but i could probably help you fix it in less than an afternoon. for serious.

I need to rebreadboard the whole thing from pcb layout. I'm not looking forward to that.

I have to find an amp where the problem occurs too. I think that was tyler's amp at the diy thing we went to. and the problem definitely happened that time.

btw, have you played around with digital pots? know if the pot part is electronically isolated from the digital part?

[multi_s]

i know i mentioned this before but i could probably help you fix it in less than an afternoon. for serious.

I need to rebreadboard the whole thing from pcb layout. I'm not looking forward to that.

I have to find an amp where the problem occurs too. I think that was tyler's amp at the diy thing we went to. and the problem definitely happened that time.

btw, have you played around with digital pots? know if the pot part is electronically isolated from the digital part?

ya i dunno from the demo that day i had a few ideas on what you could check. even without a problematic amp/guitar combo and probably right on your pcb as you have it.

as far as i know digital pots are just a giant resistor ladder with a gate at each tap. the 'digital' part selects which tap goes to the output by opening one of the N analog gates. Just imagine say 256 1k ohm resistors in series with a small mcu that connects one of the interconnects to the output pin. In this sense its not technically electronically isolated. Like if you have a mosfet whose gate is fed by the 'digital' section, but whose source and drain channel the 'analog' signal.

You can see a nice diagram on the first page of this spec

http://ww1.microchip.com/downloads/en/D ... 21978c.pdf

and for example it says also on page 3 that Vmax into the pot terminals is limited to in between Vdd and Vss. So definitely not electronically isolated (at least this model).

Not sure if your asking cause your thinking of switching the VCA in hallow earth to a digital pot setup but trust me it is totally doable with the PWM/LED/LDR style you are (probably) already using.

[cloudscapes]

Yeah the digi pot was a last resort, really. I am indeed using an LDR and PWM. I'll see if I can translate the buffer/LDR to schematic tonight. I honestly have not looked at the circuit in over a year.

Progress! Well, ish. I can now reproduce the problem by hooking the pedal up to my preamp and monitor! From what I can tell, I get silence whenever the PWM/LDR is below 50%, and normal-ish signal above 50%. An audible pop is heard when it crosses 50. Meter says 30-ish mv on output (even with silence input) below 50, and the normal 0-ish mv above.

[multi_s]

ya its *probably* because you are hitting the rails of the opamp at the input. if you ahve the analog section schem i can see if/why thats possible. It sounds like somehow some DC offset is getting amplified when the LDR changes and the signal gets smashed into the rails.

heres part of the schem for a "random" trem i did for soemone on ILF using a PWM/LED/LDR setup. No DC offset goes to the final stage and you can calibrate the "zero" point with the trim. Values of a lot of the resistors will depend on your LDR but you can see the idea. IM sure many other schemes are possible/even more robust too.

[cloudscapes]

mine basically is this



there's more, but it's on the PWM's side of the circuit (2 extra LDRs for CV out) and seeing how the audio LDR seems to work fine, I didn't think it useful to include them. quad opamp is getting 9v. the extra opamp at the bottom is a buffer for adc sampling. the LDR I use has an off resistance of a few hundred k, and on resistance of 100 ohm.



it's an old buffer I think I borrowed off the tremulus lune, and then altered somewhat.

maybe I should be pulling ends of the LDR to ground a bit, kind of like what you're doing

[multi_s]

yes the problem may well be that you are letting the DC out of the first stage to enter the second stage. Since you are using single ended op amp style you have 1/2 Vcc + the audio signal at the output of the first stage. So what is amplified? 1/2 VCC + the audio of course. Now when the gain is say 2 or more the output attempts to be 2* ( 1/2 Vcc + audio) which is Vcc + 2x audio right? thats going to clip the audio to shit since the output is at best, with a rail to rail op amp, Vcc. Going much further you wont hear the audio at all. you have buried the signal beyond the output capability of the setup. whoops, maybe not it exactly but still check the rest...

Try to put a cap (say .1 to 1uf) in-between the output of the first stage and the LDR. This will decouple the DC from the second stage.

You can verify the problem by leaving it as it is and putting a scope (DC coupled) on the output pin of the second stage before the 1uf cap. With no input signal vary the PWM slowly. If you see a DC level changing then there you have it.

A good value of the decoupling cap can be found by measuring Rmin of the LDR. IT will form a high pass filter so you want to make sure the cap is big enough to pass the bass, but not the LFO if possible. Test with the above values though to see if it solves your problem to begin with.

THe principles of design betwenn mine and yours are a bit different. Yours is relying on the gain of the op amp is basically Rfb/R- and so you modulate R-. Mine i used a sort of wheatstone bridge where the difference between the 2 legs is fed to the opamp and amplified a fixed amount. Both will work. You dont have to change much, or how the LDR is connected or whatever, just try blocking the DC from the first stage output from the second stage input.

[phantasmagorovich]

Just make out already, you two!


(kidding, it's kind of awesome what you guys are doing here. looks so pro.)

[Eric!]

And I thought I had Montreal Assembly GAS before.........


.............

[cloudscapes]

with a scope (or meter) on the output of the first stage, it doesn't vary. it stays pretty much at 4.6v all the time.

unfortunately 0.1uf between the first stage opamp and the LDR didn't help either.

check it:
http://www.tonepad.com/getFileInfo.asp?id=84

the buffer is basically from this (they done have a cap between 1st and 2nd stage either), and this is a project that actually works. it's pretty weird that mine doesn't. I'm now thinking maybe it's because I'm using a different LDR? mine is 60-100 ohm full bright, theirs is (according to part datasheet) 500 ohm. also I think my dark resistance is lower than theirs.

my LDR is specced to go down as low as 15m (after several seconds, though). since the pop happens on the dark end of the VCF, maybe it's going too low. I'm gonna try and put 1m across it and see if that helps.

edit: ugh, that didnt help either.

edit 2: but 100k did help immensely! now it only pops when the LFO is slow enough. but then I lose depth. silence is totally silent.

[cloudscapes]

revised:



added 0.1uf before the ldr and a 1m across it.

the problem seemingly goes away completely if I touch the *other* side of the LDR (not the side with the cap) with my finger. so, what's in a finger. capacitance, and a very large resistor to ground. I tied that end of the LDR to ground with 4m. no real change.

I wish I understood this.

[multi_s]

with a scope (or meter) on the output of the first stage, it doesn't vary. it stays pretty much at 4.6v all the time.

Do you really mean first stage? i meant to check at the out of the second stage, a multimeter is fine if the LFO is slow enough. See if the offset is dancing with the LFO. Even if the cap did not help this would be useful to know.

my LDR is specced to go down as low as 15m (after several seconds, though). since the pop happens on the dark end of the VCF, maybe it's going too low.

To be honest that is the opposite of what i would expect so eeeesh. To confirm, when the LED is dark, the LDR has a Larger resistance and you have less gain right?

edit 2: but 100k did help immensely! now it only pops when the LFO is slow enough. but then I lose depth. silence is totally silent.

This is maybe because when the LFO is fast the LDR does not have enough time to settle at extreme values (as you noted it takes a while to get to those really large resistances)? Is it possible or have you actually measured what LDR values you get? Maybe pop out the LDR and measure with a real slow LFO?

[multi_s]

also if you are working on it now im at my house just having a coffee. maybe it would be faster if i just swung by? ill be around most of the afternoon.

[cloudscapes]

it's fixed! thanks (again) for the offer, though!

I hunted around for other LDR tremolo schematics and found another tremulus variant.
http://commonsound.org/tremulus/pcb2.4.1/schematic.pdf

just looking at the buffer, except for actual values of parts, it's almost exactly like my buffer. except they have a 330pf across the gain pot in the second stage. I chucked one in at the same spot and lo and behold it works. I don't really know why it works, but it does. fuck.

[multi_s]

sweet sweet times. (: