Meet the Embryö Pink, a simple fuzz/high gain booster

[iainpunk]

so i wanted to build myself a pedal that did kind of a BOSS HM-2 kind of sound but more fuzzy and with some crossover distortion, mainly to brighten my somewhat dark/muddy amp, but i wanted to keep it simple and use circuit snippets i know to sound ""bad"".
i came up with the following schematic:



it has basically two knobs, a volume and a "mids" knob
the mids knob is a 28dB boost around 1073Hz, which sounds quite ginding when pushing it into an overdriven amp, when the mids knob is dialed back, the pedal sounds a bit choppy and really gnarly and fuzzy.

i didn't choose to use a gain knob because guitars have volume knobs for that purpose (hahaha )
ill try to share some sound clips when i find out how to record with my laptop (im an idiot when it comes to computers)

stay hydrated
-iain

[bool]

You may want to add a large-ish resistor across the first pair of leds, like a 1meg or so, to keep the opamp biased (from the muamp front). This would form a lowpass filter with the 1n cap so you may need to adjust the bass corner freq at the input to avoid the bass mud to spill-over to the opamp stage.

[iainpunk]

i know, but the LED´s i used are really old and have quite a large leakage current, compared to other led´s so that isnt much of an issue

[pinkjimiphoton]

looks cool bro!! love the name, too

got any clips yet? love to hear it!!!

[jafo]

No doubt a really stupid question (and sorry for the thread necromancy), but what does the first pair of diodes do? Rectification?

[FiveseveN]

Crossover distortion: https://www.diystompboxes.com/smfforum/index.php?topic=117513.0
And a bunch more references: https://www.diystompboxes.com/smfforum/index.php?topic=54854.msg420459#msg420459

[Vivek]

I entered the heart of the circuit into LTSPICE, with some assumptions which I hope are valid



Then I ran my infamous and totally unscientific AC TRANSFER FUNCTION routine. Basically I run 1Khz sine waves with -2Vp till +2Vp, with 400 steps in the middle. Between -2Vp and ), I plot Vtrough. Between 0 and +2Vp, I plot Vpeak of the output

Here's what I got :



Who am I to know anything about all this but I feel :
A) It's a voltage divider
B) With diodes as resistor legs
C) and the diodes dynamically change their effective resistance based on voltage across them
D) there is quite a lot of attenuation happening here, needing serious boost before and after this diode network
E) Initially, there is a fairly linear stage for inputs of 0vp to 0.7Vp. Then there is a smooth rise in gain. This circuit does not compress larger signals, it expands them.

The output impedance will vary a lot, being quite high for low signals when the diodes are not conducting

So when is crossover distortion not crossover distortion ? If the crossover region is large and the transition to next regime is smooth, is that still crossover distortion ?

Or has that crossover region got to be a very small part of output range to qualify as "crossover distortion" ?


PS : I expect that the Opamp also rail saturates, adding to the mayhem.

Do let me know if I am on the right track here.

[John Lyons]

I breadboarded this and while I get a good mid boost
I don't get that much of a volume boost.
The Mu Amp type first stage is only giving me unity gain.
I used 1N4148s for the diodes. What am I missing?

[antonis]

I used 1N4148s for the diodes. What am I missing?

You're missing about 1.2V (1.8 - 0.6) for each diode..

[John Lyons]

^ Oh boy...
So I guess this is suppossed to be unitiy gain then(?)

EDIT:
Replacing the first stage with Jack Orman's "Mini Booster"
fixed it and now it's good and gainy.

[Vivek]

Gainy even for medium to low signals ?

My SPICE analysis around the diodes said there is a big linear part in the middle, and distortion only starts with high signals


Last I studied it, Orman' s uAmp had gain of 78 or so, ie any signal over 60mvp will hit rails. Is that then the main cause of the distortion ?


TEST IF LED/DIODES ADD TO THE DISTORTION ?
replace both LEDs with one 560K resistor and replace both diodes with one 47K resistor, feed it medium strength signals, check if there is a difference in gaininess due to diodes.

[John Lyons]

With Silicon diodes there is clipping with the diodes to ground
as well as some tone shaping from the 0.1uf at full gain with the
mini booster as the first stage. The LEDs are subtle if at all at high gain.
With Silicon diodes back to back in series there is a little more clipping
but it's still subtle. Need to try some germanium diodes in series...

[Vivek]

Normal shunt to ground with current limiting resistor works as a voltage divider, with the resistance of the diode shunt to ground reducing effective resistance as it starts to conduct.

In this circuit, both legs are diodes, both vary their effective resistance along with signal level, leading to that transfer curve that I posted.