[sdiy] distortion pedal for a modular system

[Rutger Vlek]

Hi!

I'm also a big fan of overdrives and distortions. When you port a design
from guitar to modular you'll discover how different the bandwidth of a
synth is. A standard guitar distortion easily cuts all low end and some top
end. It may be what you want, but personally I prefer changing input
coupling capacitors to extend a distortion for use with low frequency. The
is often done for bass guitar effects.

A good start for a modular distortion would be an opamp with a plain diode
clipper. You can google for various versions. Alternatively you can make a
nice soft overdrive with zeners back to back in the feedback path of the
opamp. Drive control is simply an input level control.


Rutger

Op di 1 mei 2018 07:31 schreef stepan kayukov <stepan.kayukov at gmail.com>:

> Hi Walker,
>
> Thank you for the advice. It definitely sounds different than it would on
> a guitar, but I still like it. The real problem arises from hearing things
> at the output of the distortion, even when nothing is at the input.
>
> The R7/C1 distortion and the way the JFET is coupled created a kind of
> gating effect that I was fond of, but if you think it might be the reason
> for the overall bleeding, I should try it the more conventional way.
>
> Both of your suggestions are very helpful. I will try them immediately.
> Since I am new to this list, I do not know if I should post afterwards.
> Should I give an update?
>
>
>
> On Mon, Apr 30, 2018 at 8:35 PM, Walker Shurlds <walkershurlds at gmail.com>
> wrote:
>
>> Hey Stepan,
>> Pretty cool idea!
>> One thing: (supposedly) part of the fuzz face sound is that due to the
>> circuit’s low input impedance and the relatively high output impedance of
>> guitar pickups, the distortion has a kind of “sagging” sound. This will not
>> happen when you use either a guitar with active pickups, or the output of
>> most synth modules. This might be part of your noise issues.
>>
>> The part with the JFET looks wrong to me, but I’m not certain. Shouldn’t
>> the capacitor be in series with the JFET’s source, rather than across the
>> source and drain? The goal here is for DC current through R7 to stabilize
>> Q2, but to let a variable amount of AC current though, so that the amount
>> of degeneration applied to the signal varies. As it is right now, the DC
>> current varies but the AC gain is always on max. I haven’t ever used JFETs
>> in a fuzz that way, personally, but I’m sure someone else will chime in.
>>
>> I would suggest biasing IC1A to ground and coupling the output from Q2,
>> rather than having IC1A buffer the voltage from Q2’s collector *before*
>> it’s coupled onto IC1B.
>>
>> HTH,
>> Walker
>>
>> On Apr 30, 2018, 6:53 PM -0500, stepan kayukov <stepan.kayukov at gmail.com>,
>> wrote:
>>
>> hello -
>>
>> i am new to "synths", i have mainly built guitar pedals in the past. i am
>> currently learning EAGLE, so please excuse my drawing.
>>
>> my goal is to bring a distortion pedal design into a +/- 12v system.
>> i executed the attached design, but it is extremely noisy and i hear a
>> great deal of bleedthrough, even though i have added many bypass
>> capacitors.
>> it is a "fuzz face" pedal going to a buffer, then going to a coupled
>> inverting amplifier.  the input signal enters the collector of the first
>> transistor after being put through a resistor divider, from 12v to 2v, and
>> then passes through another coupling capacitor.
>> i also attempted to use a JFET around R7/C1, as a means of getting "CV"
>> for the distortion control.  this could be improved also probably.
>>
>> https://ibb.co/jSnXOn
>>
>> can anyone help me understand how to isolate this circuit from the rest
>> of my synths?
>>
>> thank you,
>> stepan
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