Microsynth squarewave fuzz

[the3secondrule]

Ages ago, I built a fuzz from the Squarewave section of the EHX microsynth (here: http://www.madbeanpedals.com/forum/index.php?topic=15892.0)

I was interested to revisit this fuzz after Lars' adventures with 3080's here: http://www.madbeanpedals.com/forum/index.php?topic=19039.15

The squarewave always seemed like a lot of parts for a fuzz, so I've stripped it right back. Single OTA and 2 dual opamps (previously it used 4 dual Opamps and a Dual OTA). Got it down to fitting in a 1590b, previously it was a tight fit in a BB.

I've adapted the squarewave fuzz for single supply, and removed the whole octave up / squarewave modulator section of the microsynth. I've implemented Lars' pot from V+ to the ampbias pin of the 3080.

I'm still a little iffy on the conversion from +/-9v to single supply, would appreciate if someone would give it the eye?

[Scruffie]

I've implemented Lars' pot from V+ to the ampbias pin of the 3080.

Your OTA will last all of the 10 seconds it takes to turn the pot down, that pin can see no more than 2mA of current or the chip will die.

You don't need R10, IC4 pin 3 should go to V.Ref.

[the3secondrule]

I've implemented Lars' pot from V+ to the ampbias pin of the 3080.

Your OTA will last all of the 10 seconds it takes to turn the pot down, that pin can see no more than 2mA of current or the chip will die.

You don't need R10, IC4 pin 3 should go to V.Ref.

I did wonder about the OTA. Right. More research needed.

Thanks Scruffie!

[the3secondrule]

Schem updated!

[Scruffie]

Have you done the current calculations for that 1k resistor you've added? You should give your self some leeway on that 2mA limit as well as that's terminal and also assume supply may not be exactly 9V, most give around 9.6V so call it 10V at 1.8mA to be safe.

I'm not sure if pin 3 of of IC6 should go to gnd, most likely V.Ref, you need to breadboard, I suggest with an LM13700 in its place as 3080 aren't so cheap these days.

[the3secondrule]

I did the calculations, but I wasn't paying very close attention obviously.
actually looks like I'll need a 5k1 resistor to pin 5

I will have to bust out the breadboard and try my luck, my breadboarding has not been too successful to date, but i'll just break it down into sections... I just wanted to get things in the ballpark first.

[lars]

One thing to remember on my accidental experiment, is that I dropped an OTA into an ESR graphic fuzz circuit. The power supply is split, so my V+ is only half the voltage of the 9v battery, plus I have an 820R resistor going from V+ to pin 1 of the 10k pot to limit the minimum sweep (at around 680R the sound completely cuts out). According to my meter, I read -4v on my pin 5 amplifier bias when the 10k trim is adjusted to where I think it sounds best.
Oddly enough if I take the same measurement from the wiper, which only has a jumper between it and pin 5, the voltage reads +4.4v. So there are some weird things going on that I can't explain, which is why I've called it an accidental experiment. Most likely if you try to duplicate it with another circuit, you might fry some chips, so proceed with caution as scruffie suggested.

[Scruffie]

I did the calculations, but I wasn't paying very close attention obviously.
actually looks like I'll need a 5k1 resistor to pin 5

I will have to bust out the breadboard and try my luck, my breadboarding has not been too successful to date, but i'll just break it down into sections... I just wanted to get things in the ballpark first.

Thought 1k looked a bit low

I just realised you added R9, remove it and R2 should go to V.Ref.

IC4 pin 5 might like its own bias resistor and DC blocking caps though.

Along with pin 3 of IC6 it's possible C11 & R19 should go to V.Ref too...

I was put off breadboarding for a long time too but once you get it, it's invaluable, as you say break it down and i'm sure you'll get there.

[the3secondrule]

Thanks Scruffie, I really appreciate your help with this. If I can get it operational I'll send you a board you too, Lars

I'll update the schem with the suggested changes this evening

[the3secondrule]

One thing to remember on my accidental experiment, is that I dropped an OTA into an ESR graphic fuzz circuit. The power supply is split, so my V+ is only half the voltage of the 9v battery, plus I have an 820R resistor going from V+ to pin 1 of the 10k pot to limit the minimum sweep (at around 680R the sound completely cuts out). According to my meter, I read -4v on my pin 5 amplifier bias when the 10k trim is adjusted to where I think it sounds best.
Oddly enough if I take the same measurement from the wiper, which only has a jumper between it and pin 5, the voltage reads +4.4v. So there are some weird things going on that I can't explain, which is why I've called it an accidental experiment. Most likely if you try to duplicate it with another circuit, you might fry some chips, so proceed with caution as scruffie suggested.

Thanks man. My original version of this operated fine off +/-9v supply with a lm13600 or 13700 (can't remember which right now). But I shall breadboard it before I go risking my 3080's

Funnily enough I'm also working on an ESR graphic fuzz running off +/-9v rails, to hopefully get around the weird power setup, without losing the essential fruitiness. It's a great sounding fuzz but it would be nice to be able to daisy chain it...

[the3secondrule]

updated schem with revisions. I'm not going to get near a breadboard this week, so I might as well get the schem as right as possible for now.