More useful distortion from a second stage that is preceded by a gain control

[Mike Sulzer]

I think the main reason is that they have a larger intrinsic series resistance (as can be seen from the slope of the characteristic).

That makes sense. The germanium diode is faster because of higher charge carrier mobility, and I was thinking that might give lower resistance as well, but there is more to it than that.

[Mike Sulzer]

Not a discussion on the incorporation of diodes, but possibly still a useful idea...

Something I do quite often when using a lower value capacitor to trim LF behind the earliest clipping stage is to use a dual ganged pot for the gain control to increase circuit impedance in the early, low capacitance stage as gain is decreased while simultaneously cutting gain in a more linear way on the following stage. The result is more LF when gain is lowered and a tighter sound when clipping. This and the careful use of bypass caps on the pot can get pretty close to a do-all circuit but it can be tricky.

That sounds like it might be useful for extending the "range" of a single channel amp, and so avoid needing both high and low gain channels. In the circuit here a dual ganged plot might be useful in turning the gain control into a distortion control. I think you might start by adding in some of the input signal to the output, as discussed above and then use the dual ganged pot to make it more useful at higher levels. Just an idea.

[Chuck H]

That sounds like it might be useful for extending the "range" of a single channel amp, and so avoid needing both high and low gain channels. In the circuit here a dual ganged plot might be useful in turning the gain control into a distortion control. I think you might start by adding in some of the input signal to the output, as discussed above and then use the dual ganged pot to make it more useful at higher levels. Just an idea.

Thank you. That's just what I was thinking. And yes, I'm a bit of a dinosaur with my singe channel designs I just think it's fun to have one channel that behaves intuitively whatever you're trying to use it for.

[Helmholtz]

That makes sense....

Years ago I measured the forward voltages of many different diodes (Si small signal, Si rectifier, Ge small signal, Schottky small signal, LEDs and even Mosfet body diodes) at 0.1mA and 1mA.
From this data the average differential resistance between 0.1mA and 1mA is around 100R for Si and around 300R for Ge diodes.
Means that with an increase of 1mA a Si diode has an increase in forward voltage of 0.1V, while a Ge diode shows 0.3V.
This rise with current should be viewed in relation to the forward voltage at 0.1mA, i.e. the forward voltage of a Si diode rises by only 25% while the forward voltage of a Ge diode roughly doubles at 1mA.

[Mike Sulzer]

Years ago I measured the forward voltages of may different diodes (Si small signal, Si rectifier, Ge small signal, Schottky small signal, LEDs and even Mosfet body diodes) at 0.1mA and 1mA.
From this data the average differential resistance between 0.1mA and 1mA is around 100R for Si and around 300R for Ge diodes.
Means that with an increase of 1mA a Si diode has an increase in forward voltage of 0.1V, while a Ge diode shows 0.3V.
This rise with current should be viewed in relation to the forward voltage at 0.1mA, i.e. the forward voltage of a Si diode rises by only 25% while the forward voltage of a Ge diode roughly doubles at 1mA.

That is good information; I'll look around and see what I can find.

[Helmholtz]

Of course the rounder signal top of a Ge diode can be added to a SI diode by wiring them in series. A similar effect can be achieved with a series resistor.
I scoped a lot of different diode combinations.