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previous: Gus R.G. the input Z is even lower I th... -- 8/23/1999 2:03 PM

Re: FF: More Treble Wanted!

8/23/1999 4:35 PM
R.G.	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 4:35 PM

R.G.

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.

Replies:

GFR Well if the drive knob is all the w... -- 8/23/1999 7:10 PM