Thursday, 1 November 2012

Secret Sauce

Something for you all (especially Miro!) to do with your low gain germaniums that you thought were going in the bin.  Info from mictester about his circuit:

I've got back to the work bench recently, and found that I had a lot of germanium diodes and transistors. Most of the transistors were very low gain rubbish, but all have that soft knee where they begin to conduct when used as diodes. They might not be particularly good transistors, but they're great for some things....

This Christmas, as usual, I'm going to ship a few pedals to a few friends as presents. This year's project is a slightly unusual one because it has germanium in it.... Usually, I hate germanium devices, but in this case, they give the pedal a sound all its own. It doesn't sound much like anything else out there, but you've already heard this on a few records!

Possible modifications include the addition of tone controls at the output end, changing the feedback resistors around the op-amps for more or less gain, rolling off the output by increasing the capacitor in parallel with the feedback the final op-amp..... As usual, the circuit has a high input impedance and low output impedance, but has a solid copper bypass because that's what seems to be wanted again at the moment.

UPDATED 3rd Nov 2012


  1. Brilliant idea to use them as diodes..

  2. you can tag this one verified.i didn't have any germanium transistors on hand, so i built it with 1N34A's. at first it didn't really impress me, but the more i play with it, the more i like it. has a nice bluesy vibe to it. i wonder if it would sound much different with transistors as diodes. hopefully someone will build it with some transistors and post the results. thanks again mark. keep em' coming!

    1. Top man, thanks for that

    2. I built this from Haralds layout (cos' I had a bit of board exactly the right size!) and tried AC125's. The sag is quite noticeable and slowish like a really knackered valve amp! The distortion is reasonable and quite compressed but the unit got unstable at high gain settings and the output was below unity. I swapped the AC125's for OA90's and I like it better. I expect OA91 etc would be similar. The output is unity although the gain is a bit less than with the trannies and the sag is less comical! I still couln'tt use full gain as it gets a bit unstable although less so but I doubt many would want that much compression and sag. I increased the feedback resistor in the second op amp stage to 750K (from 560K - column 14 on IvIarks layout I believe) and the output is now above unity. I may bump it up to 1 meg. This circuit is worth a go particularly if you have some Russian germies which is what Mictester based his on. They tend to be less "soggy" than crappy old ACnnn trannies! The other mod I did was to dump the 1K drive pot and increase 1K output pot to 10K. I put a 470K pot plus 33k resistor in place of OP1's 560K feedback resistor to use as a gain control and upped the 1K linking resistor to 3K. These mods made it quieter and more stable and gave a bit more usable drive.

  3. Cool idea! Never thouht of this..