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

[laurie]

More info can be found here:
https://www.learningelectronics.net/VA3 ... iodes.html

I'll show why the softer curve makes a difference in tone over the weekend.

Oh, and in case you are interested I'm using KiCad for the schematics and PCB layout.

[laurie]

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[laurie]

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Think of it like this...

Silicon is like a door slamming shut with all sorts of reverberations.
Germanium is like having a soft-closer on the door.

[laurie]

Even though the input buffer doesn't clip, using a germanium transistor makes that stage "softer". Because germanium transistors are much lower gain that silicon transistors, using a germanium transistor also lowers the input impedance of the pedal so that there is more interaction with the volume control on the guitar - giving another element of control over the tone.

FYI - the gain of the germanium transistor is about 60. The gain of the input transistor on the old DingoTone pedals is about 1,000.

[laurie]

Next, let's talk about "drive".

The input signal from a guitar is a couple of hundred millivolts. To do anything useful that must be boosted much higher. Preferably to about 8 volts peak-to-peak (the most you can get with a 9V supply).

So we need a voltage amplifier.

The easiest way to do this is using a common-emitter amplifier. Something like this:
https://www.electronics-tutorials.ws/am ... amp_5.html

There are a few tweaks required to the standard configuration:
1) It needs to be with a PNP transistor, not NPN as shown.
2) There is a problem... it is assumed that the transistor has LOTS of gain. But the Russian ones do not.

Converting to PNP is easy. The schematic I'll post next shows how.

Not having enough gain in the transistor can be overcome by using two transistors - one driving into the other. Called a "Darlington Pair":
https://www.electronics-tutorials.ws/tr ... istor.html

[laurie]

And this is the actual circuit. Description to follow in the next lot of posts.
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[laurie]

Here are the actual signals in the pedal. All signals have the same scale.
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[sclitheroe]

Because germanium transistors are much lower gain that silicon transistors, using a germanium transistor also lowers the input impedance of the pedal so that there is more interaction with the volume control on the guitar - giving another element of control over the tone.

FYI - the gain of the germanium transistor is about 60. The gain of the input transistor on the old DingoTone pedals is about 1,000.

** If you don't want your train of posts interrupted, and have us wait till later to comment, have the mods remove this post, that's fine.

This lower input impedance is great - it addresses my one possible complaint about the BSD (I might have one other nitpick, but I digress) - it's not as sensitive to guitar volume as perhaps it could be.

[laurie]

Because germanium transistors are much lower gain that silicon transistors, using a germanium transistor also lowers the input impedance of the pedal so that there is more interaction with the volume control on the guitar - giving another element of control over the tone.

FYI - the gain of the germanium transistor is about 60. The gain of the input transistor on the old DingoTone pedals is about 1,000.

** If you don't want your train of posts interrupted, and have us wait till later to comment, have the mods remove this post, that's fine.

This lower input impedance is great - it addresses my one possible complaint about the BSD (I might have one other nitpick, but I digress) - it's not as sensitive to guitar volume as perhaps it could be.

Comments are cool. We all own this space, so go for it.

What's the other nitpick with the BSD?

[sclitheroe]

Because germanium transistors are much lower gain that silicon transistors, using a germanium transistor also lowers the input impedance of the pedal so that there is more interaction with the volume control on the guitar - giving another element of control over the tone.

FYI - the gain of the germanium transistor is about 60. The gain of the input transistor on the old DingoTone pedals is about 1,000.

** If you don't want your train of posts interrupted, and have us wait till later to comment, have the mods remove this post, that's fine.

This lower input impedance is great - it addresses my one possible complaint about the BSD (I might have one other nitpick, but I digress) - it's not as sensitive to guitar volume as perhaps it could be.

Comments are cool. We all own this space, so go for it.

What's the other nitpick with the BSD?

I'm probably quite wrong in my thinking, but I sometimes can't help but wonder what the EQ stage would have been like in front of the clipping instead of behind it - mostly with an eye to being able to tighten up the bass just a bit when desired, while preserving the overall gain structure. Sometimes it's just a bit too loose on the bottom end, although most of the time that is also it's appeal.

Edit: my rationale is the the BSD is unique in having an active EQ - so with EQ in front of the clippers, you could decide how much clipping occurs for a given frequency range by driving more or less bass or treble signal into the clippers.

Makes me wonder if the dry blend is actually what I've been looking for