The 32 OD Pure Germanium Overdrive pedal - design and build in words and pictures

[laurie]

No worries!

The schematic now has two blocks - the input buffer and the voltage amplifier.
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Schem1.jpg (98.97 KiB) Viewed 4469 times

[laurie]

The overdrive section. It looks really simple - only 4 components. But there is a LOT going on here.
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ODstage1.jpg (49.35 KiB) Viewed 4463 times

[laurie]

It is a "clipper" overdrive. But the clipping here is VERY different to the clipping in, say, a DS-1.

This circuit does not give distortion or fuzz, it gives tube-like overdrive.

The "clipping diodes" are implemented with the base-emitter junction of the germanium transistors. If you know transistor symbols, you will see the base and collector are tied together as the cathode of a "diode", and the emitter is the anode of the "diode".

Why this instead of actual diodes?
1) Wiring a germanium transistor like this makes a "spongy" diode ... it doesn't have sharp turn-on and turn-off characteristics (remember the diode graphs earlier).
2) I have the vintage MP16B transistors laying around "for free" and I wanted to use them for something useful.

Using a transistor like this is risky - it can exceed one of the design specifications of the transistor, the base-emitter reverse voltage. Applying a reverse voltage between the base and emitter will kill the transistor if it is too high. And using transistors as a clipping diode like the circuit above will apply at least 4.5V in the "wrong direction".

A common silicon transistor that I've used a lot is the 2N5088. The 2N5088 has a base-emitter voltage of limit of 3V. So using a 2N5088 like this would eventually kill it.

The MP16B, however, is a military specification transistor and has a base-emitter voltage limit of 15V on paper. So it should be OK to be used like this.

How accurate are the specs for the MP16B, written in Cyrillic in the 1970s or 1980s? Could the MP16B actually be more fragile than that? To prove it will be ok I've had the prototype running continuously on the bench for over a week powered by 12V and it hasn't missed a beat. I'll leave it running for a month before I'll be happy, but it looks ok.

[laurie]

OK. So the transistors are behaving like diodes with soft turn and turn off characteristics.

C31 and R31 are also critical to the circuit and the tone. C31 is an AC coupling capacitor, but it has another more important function.

R31 has two functions - it limits the current flowing into Q31 and Q32 when they are conducting like forward biased diodes, plus, R31 and C3 have a time-constant for charging and discharging C31. The time-constant calculation for C31 also includes the output impedance of the previous stage.

The time-constant is a measure of the time it takes to charge and discharge the capacitor - it takes time to move electrons onto the plates in the capacitor and move them off again as the signal voltage changes.

There is a lot of math involved to model this... the diodes "slamming" (softly) open and closed, and C31 charging and discharging.

Once you get close the right values, auditioning them is the best way forward - tweak the values until it sounds good. By pure chance a couple of nice round values sounded good to me - 1uF and 1k Ohms.

So what does this actually do? It clips the sine wave but with VERY rounded corners - just like an overdriven tube amp does.

[laurie]

clipping1.jpg (69.99 KiB) Viewed 4461 times

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The signal is inverted because the voltage amplifier is an inverting amp.

The yellow output trace has the shape of really nice, pure, saturated tube-like overdrive.

[laurie]

Now we have 3 of the blocks - input buffer, voltage amplifier, and overdrive.
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Schem2.jpg (90.33 KiB) Viewed 4460 times

[laurie]

Next is the EQ section. I really like Baxandall EQ... it just sounds good to my ear. Explained here:
https://www.ti.com/lit/an/sloa042/sloa042.pdf

It is an industry "standard" and I have regularly used it in the past.

An active Baxandall EQ is usually implemented with an opamp. I always use the JRC4558D because it is quite musical.

Here is the EQ for the 32 OD pedal:
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Baxandall1.jpg (72.07 KiB) Viewed 4456 times

[laurie]

C41 is an AC coupling capacitor the same as C21.

C46 is a radio frequency interference limiter, similarly to C22.

The opamp requires a stable 4.5V reference and this is provided by a simple resistor and capacitor divider. The resistors divide the voltage and the capacitors keep it stable (at least to some extent):
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4V5_1.jpg (35.86 KiB) Viewed 4452 times

[laurie]

So now we have four of the blocks:
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Schem3.jpg (133.32 KiB) Viewed 4452 times