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< MuffRat > TL072 easy DIY Distortion pedal project

Distortion Circuits and Schematics

< MuffRat > TL072 easy DIY Distortion pedal project

Postby maz » April 19th, 2014, 10:57 am

Quick Overview:

    - Type: Distortion
    - Level of complexity: 2/3
    - Nb of parts: 34
    - Nb of amplification stages: 3
    - IC: TL072 dual OP Amp
    - Input: NPN Transistor 2N3904
    - Clipping: Soft / Hard (3mm red led, 1n914 diodes, 2N7000 mosfets)
    - Nb of Potentiometers: 3 - (Distortion - Tone - Volume)
    - Schematic tested: Breadboard
    - PCB Layout: Yes
    - PNP transfer: Yes
    - PCB Size: 24 X 14 holes
    - PCB tested: no
    - Veroboard Layout: Yes
    - Veroboard Size: 24 X 16 holes
    - Veroboard tested: no
    - Soundtracks: Yes

The MuffRat Distortion is a simple distorted effect generated within 3 stages :

- 2N3904 NPN transistor input Common Emitter stage tweaked for medium gain, inspired from the Big Muff PI.
- TL072 OP amp IC:
-> Side 1 for a highly distorted stage.
-> Side 2 to bring back the inverted signal and also to add some gain before the tone control.

The soft clipping section is positionned at the second stage while the hard clipping section is just after the third stage. Several clipping options can be done :

With no LED at the first clipping section (soft clipping) it reminds the fat saturation of the old cranked tube amps, but if you add (2) 3mm red leds there, now the pedal sounds different with a much more "cut" and "modern" feel. The leds can be swapped for 2 standard silicon 1N914 (or 1N4148) diodes, or 2 of each seried and hooked symmetrically, like this :


By doing so, the forward voltage of each seried diode will sum. A standard 3 mm red led has a forward voltage of about 1.8V, a silicon diode is roughly 0.6V so by connecting 2 of them we get a forward voltage of 1.2V and it's just in between the led and silicon. A 3PDT switch at the soft clipping section may also be used to have both the leds and silicon diodes, at the center position both of them will be disabled and it's just like you put nothing there. As you play with the volume knob on the guitar, in addition with the, lets call "Vintage/Modern" switch mentionned earlier, you can reach many kind of distorted sounds, the circuit react very well to the guitar volume.

Q2 & Q3 may also be mounted using another 3PDT toggle switch, given more clipping options, ie: one side with 3mm red led pair and the other side with the mosfets. Using a 3PDT switch at the hard clipping section gives the advantage to disable both of them again just like you put nothing there, resulting in a more overdriven tone with more volume. I don't like the clipping resulting from 2 Germanium diodes, although by using only one, paired with a mosfet or a led I was able to get some interesting tone.

Different ICs having the same pinout may also be used, so think about using sockets to experiment.

The transistor (2N3904) at the input buffer can also be swapped for any NPN (having the same pinout), just make sure to adjust the 100K/510K resistors and get as closest as possible to 4.5V at the transistor collector pin. If you don't want to do the maths, just get a transistor in the 150-250 hFE range and you should be good to go, but it's up to you, sometimes a little miss biased condition might be good. By the way, I really hate the little holes on the cheapo testers, now it's much better modified using an IC socket :



:clip: Clip 1 (5.82Mb) - MP3 : Various music compilation at different setting.
Recording: Strat Guitar -> MuffRat -> Beltone AP-A tube amp (clean at low volume, 8 inches speaker) -> SM57 shure mic -> mixer (Flat EQ) -> soundcard -> software.

Schematic < Tested on breadboard > :


:pdf: Download the Complete pdf project file for direct PnP transfer, layout and schematic.

:txt: Download Bill of Materials in txt format.

MuffRat clipping options example :


The MuffRat built on Veroboard :



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Re: < MuffRat > TL072 easy DIY Distortion pedal project

Postby bugs » April 24th, 2014, 6:34 pm

man this is so tempting
Posts: 10
Joined: April 15th, 2014, 12:56 pm

Re: < MuffRat > TL072 easy DIY Distortion pedal project

Postby hors_sujet » April 24th, 2014, 6:38 pm

Ill make a try for sure :mrgreen:
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