Overview
My goal was to design a very versatile distortion unit for my friend Malcolm. When talking about making music, he told me he was a bit dissatisfied with his digital effect unit; it was just not easy enough to operate. As with the PSU, Mendelt helped me a lot with making the schematics, and we are definitely going to adapt the schematics for synthesizer use.
Sound tweaking options
Drive potentiometer (1st Opamp)
Clipping potentiometer adds clipping to the standard opamp overdrive
Clipping switch selects between soft and hard clipping
Big Muff tone section with potentiometer
Level potentiometer (2nd Opamp)
Technical details
Partlist
1 dual opamp (IC1)
2 electrolytic capacitor 100uF (C3, C8)
2 electrolytic capacitors 1uF (C9, C10)
2 film capacitors 100nF (104, C1, C6)
1 film capacitor 22nF (223, C4)
1 film capacitor 10nF (103, C5)
1 ceramic capacitor 470pF (C2)
1 ceramic capacitor 220pF (C7)
2 potmeters 100K LIN (R3, R7)
1 potmeter 100K LOG (R9)
1 potmeter 10K LIN (R5)
2 resistors 1M (R1, R2)
2 resistors 47K (R8,R12)
1 resistor 39K (R6)
3 resistors 10K (R10, R13, R14)
2 resistors 1K (R4, R11)
2 diodes (D1, D2)
2 6,3mm mono jack plugs (X1, X2)
1 DPDT switch (bypass, S1)
1 SPST switch (clipping, S2)
It is also possible to use a 3PDT switch and an LED to indicate the position of the bypass switch, instead of the listed DPDT switch, I used this and it is a nice addition.
A small 2-pole on/off toggle switch to turn off the battery can be a nice addition.
What the parts really do
X1 and X2 are the input and output jacks
S1 is the bypass switch
S2 switches between soft and hard clipping.
R1 acts as a click-preventing resistor. It cuts out all clicks and other unpleasent sounds from the signal.
C1 filters DC voltage from getting into your instrument.
R2 makes sure the potential at the input of IC1a is always equal to GND.
IC1 is the dual opamp.
R3 is the Drive potmeter, and controls the amount of feedback through the first opamp.
R4 and C2 form a 33KHz lowpass filter that cuts out all high frequency noise.
D1 and D2 are clipping diodes.
C3 protects the first opamp from shorting when hard clipping the signal.
R5 is the Clipping potmeter, and turns from clipping to standard overdrive.
R6, R8, C4 and C5 form lowpass and highpass filters.
R7 is the Tone potmeter and controls the frequency that the tonestack operates on.
R9 is the Volume potmeter and controls the amount of signal fed into the second opamp.
R10 and C6 form a lowpass filter for the amplification frequency of IC1b.
R10 and R11 also set the fixed gain for the second opamp to 11.
C7 prevents DC voltage from the second opamp from reaching the output jack.
R12 keeps the output at 0v when there's no signal going through the circuit.
R13, R14, C8 and C9 pull GND to half of the supplied voltage, to be able to use the bi-powered opamp with a unipolar supply like a 9v battery.
The circuit can be powered with anything ranging between 4 and 30 volts DC, but 9v is preferred to preserve the designed sound characteristics (lower voltage makes the opamps distort earlier, higher voltage provides more headroom).
Any dual opamp can be used, just watch the pinout. I used a TL082 which has a low current drain, excellent for battery operated devices such as a guitar stompbox.
The clipping diode section can be populated by a wide variety of diodes. Often used clipping diodes include Germanium diodes 1N34 and 1N270, Silicium diodes 1N914 and 1N4001 and LEDs. It is also possible to use JFETs or MOSFETs as clipping diodes.
For assymetrical clipping use different diodes on both sides, or stack 2 diodes on one side and only one on the other. I used 1N34 and 1N4001. If you use assymetrical clipping you should place an extra 100nF capacitor between R7 and R9. This is to prevent offset DC voltage from the assymetrical clipping to damage the second opamp.
http://home.quicknet.nl/qn/prive/alex.span/distortion/distortion.sch
http://www.midibox.org/forum/index.php?topic=9612
http://www.generalguitargadgets.com/richardo/distortion/ http://www.runoffgroove.com/articles.html