I am building an Wooly Mammoth audio circuit, and I am really interested in finding how and why it works. You can check out and simulate the whole thing here.

I must say **I** haven't been able to get any valuable result from simulation.

It provides a fuzz distortion effect. In fact, it's based in the well-known Fuzz Face. You may notice Q1's collector and Q2's base common node, as well as the feedback net that connects Q1's base and Q2's emitter. In the article I link to, RG Keen makes an extensive analysis and calls that configuration *voltage feedback biasing*. Unfortunately, I haven't been able to trace any other data about this.

I have isolated the DC circuit for you:

When trying to solve it, I get stuck at proposing and checking hypothesis: I don't really know whether one or both transistors are in saturation region. I assume that, being this kind of effect, at least one of them is. But I find no way to prove it.

One of the Mammoth's addition to the Fuzz face circuit is the *Pinch* potentiometer. It seems to change the bias point, while users say it works as some sort of noise gate. I have not been able to see that by simulating.

So I would take a good piece of advice on how to solve the DC circuit (is there a possible analytical approach?) to understand the principles of that bias configuration. Also, I would appreciate some help regarding simulation, as I'm sure there *must* be something useful to get.

Thank you very much.

## Best Answer

The DC analysis isn't that difficult.

EDIT: I see a far simpler analysis:

\$I_{B1} = \dfrac{V_{C1} - V_{BE2} - V_{BE1}}{R_2 + P_2}\$

\$V_{C1} = 9V - I_{C1} \cdot R_3 = 9V - \beta I_{B1} \cdot R_3\$

\$ \rightarrow I_{B1} = \dfrac{9V - V_{BE2} - V_{BE1}}{R_2 + P_2 + \beta R_3}\$

Now:

\$I_{E2} \approx \dfrac{V_{C1} - V_{BE2}}{R1}\$

\$V_{C2} = 9V - I_{C2} \cdot R_4 \$

Assume a value for the base-emitter voltages and P2 ,solve for the collector currents and check for active/saturation operation.