Germanium transistor simple amplifier

[Halban]

Hello dear community!

I love distortion/overdrive audio effects for my synthesizers.

I began to design my own distortion. The first stage is a very simple transistor amplifier (gain to -inf to about *4).

This is the circuit I've breadboarded:



I think it may be an usual circuit, but maybe I'm wrong? In any case, it works pretty well and produce a warm to really distorted sound, depending on the gain potentiometer and the audio input voltage (can be between -1V 1V to -10V +10V). I've tested multiple type of germanium transistors and this one works better than the others.

But on some potentiometer spots, and with some kind of audio input source, it may sounds bad, produces a gated sound and the overall behaviors sounds "buggy".

Do you think something can be improved?
For example, should I add a resistor between C10 and the opamp U1?

Thanks a lot for your help!
Cheers,

[Kleinstein]

The amplifier is a quiet normal common emitter amplifier stage. The OP-amp part defintely needs a resistor from after C10 towards ground to get a defined DC level.
The OP-amp also wants decoupling capacitors at the supplies

[Roehrenonkel]

Hi Halban,
 
...and you might want to put a cap betwen the pot-wiper and base.
Else you short the base-bias to ground ("at some pot-spots").

Good luck

[Ian.M]

Yes, a capacitor between the pot wiper and the base of Q1 is needed to avoid DC through the pot (which causes noise) and so the pot doesn't shift the transistor bias point.   You'll probably also need to increase R1 without the pot pulling the base down, or make it variable bias, with R1 fed from the wiper of an additional 1Meg pot between Q1 collector and ground.

[Halban]

Thanks a lot everybody for your reply!

1- I've added a R2=82K before the opamp (-3dB cutoff frequency : 20Hz)

2- I've added a C1 on the base of the PNP

3- About @Ian.M advice, I think I do not get enough analog electronics background to fully understand your idea about the bias, sorry Do you suggest to not pull P1 to ground, leave it open so P1 will act as a simple variable resistor? And to add a new pot to finetune the bias point?

[CaptDon]

Actually, part of the magic of selectable distortion was the pot draining different amounts of base bias to ground (but sadly effecting gain at the same time. Including the capacitor between the base and gain pot is a correct thing to do. The added bias pot now allows a selectable amount of distortion but as before will still mess with the gain. As you selectively swamp more base bias to ground to move the 'crunchy knee of distortion' you also have to add more input signal by turning up the input gain pot. We built this exact same circuit in the 70's without the opamp follower. I sold a bunch of them.

[Halban]

Hi!

I finally tested it and the add of C1 and R2 definitely improves the distortion, and sounds no more "buggy".

Thanks again for your help!

I've also tested the bias trim pot P2, but it was not changing the overdrive sound so much.