Author Topic: Weird problems with circuit.  (Read 986 times)

FingerBlisters

Re: Weird problems with circuit.
« Reply #20 on: January 24, 2021, 02:55:55 PM »
Glad you figured out the meter issue. I haven't used mine in awhile, and I hate to think what state the battery might be in. :icon_eek:

The issue with the resistance of the tonestack isn't the overall resistance, it's that the amount of series resistance is larger on the bass side than the treble side. Since input resistance is part of the gain equation of inverting opamp stages, that means the gain will always be different for the two sides of the pot. Bass will always be lower. The more I think about it, the more I think this is the source of your treble issue, rather than filtering in the gain stages.

The solution (if, y'know, I'm right about any of this :icon_razz:) is to make the input impedance of the last stage so high that the difference in input resistance is small by comparison. Maybe try an R9/R10/C10 combination of 470k/1M/50pf and see if it makes a big difference.

Well, I did as recommended and had some crazy squealing when the tone pot was turned past 65% or so. Changing the 680k all the way down to a 100k fixed the squeal and still kept enough gain to still be reasonably chunky.

There's still a lack of bass, it's like the whole frequency spectrum has been moved a few khz over to the treble side as even the lowest setting on the TONE knob is pretty bright, and then it gets into crazy cat-fight screech at higher settings.

Next step is to remove R13, then reroute a C13 10uf to before the buffer as oppose to after. That suggestion seemed to crop up a lot here and can't hurt to try.

FingerBlisters

Re: Weird problems with circuit.
« Reply #21 on: January 29, 2021, 03:23:34 AM »
electro smash quite often has mistakes and problems on their site. take what they post with a grain of salt.

cheers, Iain

Hey there, sorry to drag you back but abiut the suggestion to raise the resistors in the last op amp to 470k and 1m. Would that make the output impedance huge? I've got a 1k in series with the volume pot to keep it low but just wondering if there's gonna be a problem downstream with a 1m+ resistor in there?

iainpunk

Re: Weird problems with circuit.
« Reply #22 on: January 29, 2021, 10:50:11 AM »
electro smash quite often has mistakes and problems on their site. take what they post with a grain of salt.

cheers, Iain

Hey there, sorry to drag you back but abiut the suggestion to raise the resistors in the last op amp to 470k and 1m. Would that make the output impedance huge? I've got a 1k in series with the volume pot to keep it low but just wondering if there's gonna be a problem downstream with a 1m+ resistor in there?
those resistors, don't affect output impedance what so ever. they are in the feedback loop, not in the output signal path! in a complicated way, it slightly decreases the output impedance, but that's less than .1% in this case.

cheers, Iain
half man - half snail - 6 feet to scale - Snail man's - not frail - He's been - to jail - snail man is fuccing real
『snailpilled』

antonis

Re: Weird problems with circuit.
« Reply #23 on: January 29, 2021, 11:04:39 AM »
Also, making 10k -> 470k raises op-amp inverting input impedance (tonestack "sees" a much higher resistance - much lighter loading)

Beware of C10 value (47pF should be a good value for 1M feedback resistor..)
"I'm getting older while being taught all the time" Solon the Athenian..
"I don't mind  being taught all the time but I do mind a lot getting old" Antonis the Thessalonian..

iainpunk

Re: Weird problems with circuit.
« Reply #24 on: January 29, 2021, 11:27:48 AM »
Also, making 10k -> 470k raises op-amp inverting input impedance (tonestack "sees" a much higher resistance - much lighter loading)

Beware of C10 value (47pF should be a good value for 1M feedback resistor..)
he stated that there was way to much treble so maybe keeping it at 2n2 is alright, if more treble is desired, lower this capacitor. ,not that it makes such a huge difference, since the gain is only 2.13, and the treble cut shelves at 1

cheers, Iain
half man - half snail - 6 feet to scale - Snail man's - not frail - He's been - to jail - snail man is fuccing real
『snailpilled』

antonis

Re: Weird problems with circuit.
« Reply #25 on: January 29, 2021, 01:54:34 PM »
he stated that there was way to much treble so maybe keeping it at 2n2 is alright, if more treble is desired, lower this capacitor. ,not that it makes such a huge difference, since the gain is only 2.13, and the treble cut shelves at 1
cheers, Iain

So, there is no need for last stage greater than unity gain 'cause ALL guitar frequencies are shelved at 1 (1M/2.2nF LPF cut-off frequency at 72Hz..)  :icon_wink:
"I'm getting older while being taught all the time" Solon the Athenian..
"I don't mind  being taught all the time but I do mind a lot getting old" Antonis the Thessalonian..

FingerBlisters

Re: Weird problems with circuit.
« Reply #26 on: January 29, 2021, 01:58:02 PM »
Also, making 10k -> 470k raises op-amp inverting input impedance (tonestack "sees" a much higher resistance - much lighter loading)

Beware of C10 value (47pF should be a good value for 1M feedback resistor..)
he stated that there was way to much treble so maybe keeping it at 2n2 is alright, if more treble is desired, lower this capacitor. ,not that it makes such a huge difference, since the gain is only 2.13, and the treble cut shelves at 1

cheers, Iain

Thanks for the replies. I reworked the circuit to use a 1m/2.2m combo since I had them handy and also reworked the tonestack around a b25k pot which seems to be great in terms of sweep. Overall it's become the pedal I was hoping it would be. Using a 50pf cap on the last stage btw, around 1.5khz.

Thank you so much for your help man, you opened my eyes to a bunch of things I wasn't looking at.

iainpunk

Re: Weird problems with circuit.
« Reply #27 on: January 29, 2021, 08:22:05 PM »
he stated that there was way to much treble so maybe keeping it at 2n2 is alright, if more treble is desired, lower this capacitor. ,not that it makes such a huge difference, since the gain is only 2.13, and the treble cut shelves at 1
cheers, Iain

So, there is no need for last stage greater than unity gain 'cause ALL guitar frequencies are shelved at 1 (1M/2.2nF LPF cut-off frequency at 72Hz..)  :icon_wink:
so i entered 100k in to the calculator instead of 1M, i thought 724Hz is a nice cutoff point, but you are right, it should be at least 10x smaller.

two furhter questions tho:
why do you have the gain stage after the tone stack as an inverting gain stage? why not non-inverting?

are you working on a breadboard?
i love bread-boarding, coming up and experimenting with circuits, honestly more than building pedals, and seem to have most circuits on my breadboard way to long, 3 of my 4 breadboards have been occupied by the same circuits for a few months, and the other one had the previous circuit ripped of last tuesday, i had it on there since April or May.

what values do you have in the big muff tone stack?

cheers, Iain
« Last Edit: January 29, 2021, 08:24:48 PM by iainpunk »
half man - half snail - 6 feet to scale - Snail man's - not frail - He's been - to jail - snail man is fuccing real
『snailpilled』