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Re: FF: More Treble Wanted!

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8/23/1999 4:35 PM

Re: FF: More Treble Wanted!

Yep, you're right. That's what I get for being in a hurry. The 100K feedback resistor also appears to be in parallel with the input, but smaller by the voltage gain of the stage.

A first order approximation can be done this way: the stage gain is pretty close to Rc/Re', or about 33K/1K using very loose guesses at Re', or about 33. The 100K then looks like a 3K resistor paralleling the input impedance noted above.

The stage as it sits can be modelled either way, as a current to voltage or voltage gain stage depending on how and where you draw the lines around the amplifier in modelling. If you use the thevenin model, the pickup looks like a voltage source in series with the pickup impedance and driving the equivalent shunt impedance to the amplifier to ground, and the overall gain is Rf/Zineq for the whole stage. The Rc/Re' is the internal stage gain before external feedback is considered, the open loop gain you'd get by splitting the Rf into two halves and grounding AC feedback with a capacitor, or by running the distortion control to max. The distortion control directly controls the stage gain between open loop gain and Rf/Zin.

The 3tran fuzz complicates things a bit by effectively setting Zin to negligible. This makes the thing act like it's being driven by a voltage source, and to my ear at least hardens up the clipping. If you want to calculate out the effective gain with an input buffer, you have to compute the input current with the effective input impedance of the stage (Re'||Rfeq)and then compute that current across Rf, and modify it by the feedback "stolen" by the distortion control.

8/23/1999 7:10 PM
GFR	Well if the drive knob is all the way up (max. distortion) then the signal that would be fed back is instead shunt to ground by the cap, and you've got feedback only for DC but not for AC. So that input impedance calculation is OK if you consider the drive at "10". At lower drive settings the impedance do get lower because of the NFB.

8/22/1999 10:08 PM
Dave Chun	Try modding the circuit to that of the Vox Tonebender - the one that's virtually identical to the Fuzz Face, not any other two or three transistor version. I worked on an original and it's got treble in spades, but use a 500k volume pot (the schem that's floating around shows a 50k-100k volume pot.)

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