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

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

Post by laurie » Wed Mar 31, 2021 12:35 am

NO WARRANTIES OF ANY SORT ABOUT THIS DESIGN ARE MADE.
NO SUPPORT IS AVAILABLE.
IF YOU CHOOSE TO USE THE DESIGN OR ANY OF THE CONCEPTS YOU DO SO AT YOUR OWN RISK.
THIS DESIGN IS COPYRIGHT AND MAY NOT BE USED FOR COMMERCIAL GAIN.

==========================

G'day!

So, another OD pedal. And the world doesn't need yet another OD pedal. So why do it?

I have an innate desire to not see electronics "go to waste". That's why I buy and repair any old dead pedals or other music gear that I find - to rescue them and get them back into circulation.

Stay with me - I'm going somewhere with this story.

When I built the first fuzz pedals 10 years ago I found that Russian surplus germaniums sometimes had the right gain for Q1 in a fuzz (about hfe = 75, low leakage), and they sound pretty good. In any batch of 1,000 PNP Germanium transistors between 20 and 50 transistors had the right gain.

So I'd buy the damn things in lots of 1,000 and test every one. And be left with about 950 that weren't any use for a fuzz.

The first time I did this I threw the 950 transistors away. And that just felt like a total waste. Most of them weren't bad, they just didn't have high enough gain for the fuzz circuit.

When I bought the batch of 1,000 a couple of years ago to build the Fuzz Architect pedals I didn't throw out the 950 unsuitable transistors. I couldn't bring myself to waste them.

The 950 transistors sort into buckets like this:
- About 300 have a gain between 60 and 75 and good leakage numbers. Not enough for a fuzz, but not bad.
- About 200 are bad. Failed under test (no gain, super low gain, runaway gain, super high leakage) - they can be thrown away
- About 450 have low gain, but also low leakage and are generally OK, but useless as amplifiers.

The question became "what can I do with them"?

Back in the DingoTone days one of my favorite designs was the BSD. I like germanium. But the BSD uses a silicon transistor as the input buffer and an opamp as the gain stage. The germanium is in a pair of NOS diodes used for very light clipping/shaping.

So I started thinking about an all-germanium overdrive. And that thinking has taken about 2 years of fiddling... I wanted to do something simple, but toneful, and preferably something that isn't "common".

The first real foray came in June 2019 with this thread: viewtopic.php?f=13&t=315&hilit=geode

But the approach I was trying to take didn't produce the pedal I wanted. Or, indeed, any pedal. It simply didn't work because I had the engineering wrong. So things just sat until I had a bit of a brain-fart a couple of months ago.

And this time it worked. A pure germanium overdrive, using five (5) of the germanium transistors per pedal.
Last edited by laurie on Sun Apr 11, 2021 12:32 pm, edited 8 times in total.

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Re: The "32 OD" pedal - design and build in words and pictures

Post by laurie » Wed Mar 31, 2021 1:19 am

These are the requirements I laid down for myself:
1) The OD section must be all germanium, and the input buffer must be germanium
2) The germanium must be implemented with the out-of-spec transistors from the fuzz build
3) The signal path must be simple - minimum stuff in the way of the tone
4) It must be negative ground, and able to be powered from a standard PSA adapter
5) Fit in a "BB" size box or smaller (BB is the size used for the Fuzz Architect)
6) Not be battery-hungry
7) Use readily available parts
8) Be a true overdrive, not a fuzz or distortion with the gain dialed back
9) Somehow have added versatility (this ended up being the "add clean")
Last edited by laurie on Sat Apr 03, 2021 11:44 am, edited 1 time in total.

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Re: The "32 OD" pedal - design and build in words and pictures

Post by fernieite » Wed Mar 31, 2021 10:55 am

Right on man! Glad you worked out a great new design. I might have to get one sometime; I dont currently have any overdrives.(or any extra funds at the moment) :cry:

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Re: The "32 OD" pedal - design and build in words and pictures

Post by laurie » Wed Mar 31, 2021 12:01 pm

There are lots of ways to approach a pedal design. Some pedals are a "big amorphous chunk" of electronics - all functionality lumped in together. The fuzzface is like that - input, output and fuzz all handled by two transistors serving multiple roles... Q1 is both the input plus the first half of the fuzz gain. Q2 is the second half of the fuzz gain plus output drive.

Other pedals have discrete "blocks" of functionality that do different things. Boss pedals are generally like that.

What I mean by blocks of functionality is:
.
Blocks.jpg
Blocks.jpg (27.75 KiB) Viewed 5684 times
This allows the designer to focus on smaller, more manageable blocks, then connect them together later. This is the way I've always designed pedals.

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Re: The "32 OD" pedal - design and build in words and pictures

Post by laurie » Wed Mar 31, 2021 12:09 pm

In the simplest view, an overdrive needs these blocks:
.
Blocks2.jpg
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Re: The "32 OD" pedal - design and build in words and pictures

Post by laurie » Wed Mar 31, 2021 12:26 pm

Why is an "input buffer" a good idea?

When connecting electronic systems together it is important that the output of one system is "matched" to the input of the one it is connected to.

There are generally two ways of "matching"... maximum signal (voltage) transfer, or maximum power transfer.

Maximum power transfer is used in power amplifier output stages - you want the most power delivered to the speaker. For maximum power transfer the output impedance of source should match the input impedance of the sink. When designed this way, the source is "loaded down" - maximum amperage (hence power) is extracted from the source.

For a guitar plugged into a pedal, you don't actually want the guitar pickups "loaded down". A loaded down guitar loses tone for a number of reasons that can be Googled. Plus you want maximum signal transfer. The higher the voltage applied to a pedal, the better the signal-to-noise ratio of the pedal. There are subtleties to this, but that's generally how it works.

Using an input buffer in a pedal presents a very high impedance load to the guitar (or the previous pedal) ensuring the thing feeding the pedal isn't loaded down and that maximum signal is transferred. I'm pretty sure I've never seen a Boss pedal without an input buffer. In the CE-2 for example, the input buffer is designed around Q1, a 2SC732TM small signal NPN silicon transistor. This buffer circuit gives the pedal a stated input impedance of 470,000 Ohms (470k) which is much higher than the guitar output which is perhaps 10,000 Ohms.

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Re: The "32 OD" pedal - design and build in words and pictures

Post by laurie » Wed Mar 31, 2021 12:35 pm

Transfer1.jpg
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Re: The "32 OD" pedal - design and build in words and pictures

Post by laurie » Thu Apr 01, 2021 2:03 am

One of the standard ways to get a very high impedance for a pedal input is to use a "common collector" or "voltage follower" amplifier.

The common collector configuration gives high input impedance which we need for connecting a guitar, and low output impedance which is useful to drive the subsequent blocks in the pedal.

See here for explanation:
https://en.wikipedia.org/wiki/Common_co ... e%20buffer.

Of particular importance is that this type of amplifier can be made using a single PNP transistor ... like the surplus ones I have. See here:
https://en.wikipedia.org/wiki/Common_co ... llower.svg

This is an input buffer for a pedal, using a PNP transistor, using negative ground. This is the actual schematic for the first block of the circuit for the 32 OD:
.
Input1.jpg
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Last edited by laurie on Thu Apr 01, 2021 12:45 pm, edited 3 times in total.

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Re: The "32 OD" pedal - design and build in words and pictures

Post by laurie » Thu Apr 01, 2021 2:17 am

The MP16B is one of the surplus Russian transistors. It has been selected to have a gain of 60 or greater and low leakage.

R12 is selected to allow current to flow between pins 1 and 3 of Q11 so that the voltage at pin 1 is approximately 4.5V with no input signal. This is the "bias point". This bias point is far less sensitive than the bias point of a fuzz pedal and it can be anywhere from about 2V to 7V and everything will be fine.

This configuration has no gain - or more correctly a gain of "one". The output voltage is the same as the input voltage.

There are many mathematical models for this type of amplifier, however, a simplified calculation for the input impedance is R12 in parallel with (hfe * R13).

For this circuit the input impedance is therefore roughly 300,000 Ohms (300k). High enough so that there is only slight interaction with the guitar pickups.

The output impedance of this stage is about 10k Ohms (R13). R11 protects the transistor from excessive current through pin 2.

So why use one of the vintage PNP transistors here when a new manufacture silicon transistor could raise the input impedance to over 1,000,000 Ohms and have essentially no interaction with the guitar pickups (like in the HZD pedal)? Well that's a story for tomorrow... about why germanium sounds good when used for dirt.

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Re: The "32 OD" pedal - design and build in words and pictures

Post by laurie » Thu Apr 01, 2021 12:35 pm

If we look at diodes, we see this, not to scale, exaggerated:
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Curves1.jpg
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.
Curves2.jpg
Curves2.jpg (23.04 KiB) Viewed 5617 times
Last edited by laurie on Thu Apr 01, 2021 12:37 pm, edited 1 time in total.

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