some help with this 4049 based preamp..

[rankot]

This is really interesting overdrive design. I've modelled it in LTspice and it works fine. Breadboard is next!

[anotherjim]

Fitting source resistors to the 4007 X3,M3 pair like that carries a risk, since those pins are the chip Vdd and Vss connections. You must be sure none of the other pairs channels can have voltages exceeding those source voltages by more than 0.3v WRT Vdd or Vss or else the substrate diodes will start to conduct and you have unforeseen interactions going on - which might sound good, I don't know - I've always avoided it.
The internal diodes go from channel to P or N substrate and the appropriate power pin - not strapped to source as shown, which is of a conventional discrete MOSFET symbol.

[DDD]

Playing with CD4007UB I've found the following method to avoid the unwilling conductivity of substrate diodes:
1. Source resistors at X3 and M3 should be at least two times less than resistors at X1,X2 and M1,M2 sources (3.6k and 8.2k respectively);
2. Source resistors at X3 and M3 (pins 14 and 7) should be shunted with the big enough capacitance - 10...100 uF depending on the lowest signal frequency.
    If so, there is no any interference between clipping and output (level compensation) stages.
*** By the by, the 8.2 kOhm source resistors in clipping stages seem to set optimal invertors' gain and current consumption (battery saving measure).

[DDD]

The picture shows schematic diagram of the output stage with 1MOhm volume control pot that is typically used with 3-knob tone controls.
This arrangement gives comfortable volume control curve.

[rankot]

Dmitry,

If you have this already built (or on breadboard), could you please try to add a feedback loop like this:



Simulation shows that with fixed resistor of 220k (which then sets minimum feedback) and 1Meg potentiometer, amount of overdrive could be nicely set from none to very much. I don't have parts at hand, so I can't test it yet.

[DDD]

Hi Rankot,
To my regrets, all of the gadgets have been sold out and I do not have possibility to check your idea.
At the same time I am sure that any feedback from the circuit output to clipping stage input will cause uncontrollable self-oscillations.
--------------------
By the by, you connected the output stage directly to the clipping stage, so the output stage will distort heavily. Hence you'll get sharp and "usual flat" distortion instead of "special MOSFET" sound.

[rankot]

I built this with feedback loop and interestingly - it sounds OK when using tone generator and small 2W amp for testing purposes, but when I connect it to real guitar and real amp, it starts to self-oscillate and the volume drops a lot. Without the feedback loop it works just fine!

[anotherjim]

With the feedback around the 3 sections, it could be a phase shift oscillator. The input RC networks on each section may need simplifying/rationalizing/omitting?

[rankot]

You're right, Jim, now I connected feedback only over the last stage and it works really nice! Thanks for a hint!

There's a schematic: