giving a TS-808 "bump" to a different type of circuit

[mordechai]

I really like the clarity and grind of the ROG Professor Tweed...but I'd also like to modify it to have the midrange bump that we get with the classic TS-808 circuit.  I understand that this is the result of a .047uf cap in series with a 4.7k resistor (right?).  Would this work in the Professor Tweed circuit, and if so, where would I place it?

[Gus]

Take a look at this circuit.  Another one that has been in schematics for years.
http://www.diystompboxes.com/pedals/gusOverdrive.gif
What can you do with one transistor
Input is bootstrapped for higher input resistance
Gain is controlled about X4.7 until R5 and C2 start to increase the gain to just under X9.4
R7 and C5 roll off the top(this simple circuit was designed to be used with device having an input resistance of >=1meg

Circuit buffers and boosts with an EQ curve.  For fun calculate the mid boost curve.

Could place it before the Professor Tweed

[mordechai]

That's an interesting circui and I can see how it works; I'll give it a shot on its own.  What I am wondering, though, in specific, is how the .047uf cap/4.7k resistor combination of the ts-808 would be incorporated in to a transistor based rather than an IC based circuit, since the clipping in the Professor Tweed doesn't rely on diodes in the feedback loop of an IC.  In the ts-808 schematic, I can see where/how the .047uf cap/4.7k resistor get wired in, but I'm trying to understand -- at least in theory -- where this would happen in a Professor Tweed circuit or any transistor based device...like the Colorsound Overdriver, for example.

This is more of an academic/theoretical curiosity for me...

[CynicalMan]

The 0.47u cap just makes the distortion stage act as a high-pass filter. The treble cut mostly comes from the 1k resistor and .22u cap after the distortion stage. So, for transistors you can just increase the value of the coupling cap before them until you're cutting bass with -3dB at around 700Hz. Then, put a low-pass filter on the output with -3dB at around 900Hz. Remember that the output impedance of a transistor stage is higher than that of an op amp, so you have to keep that in mind when calculating for the low-pass filter.

[BadIdeas]

If you are referring to the resistor and capacitor in the feedback loop of the clipping circuit, they keep the distortion out of the lower range to minimize inter-modular distortion. The low-pass filter after the clipping circuit brings the high frequencies back down to the same level as the lower frequencies. The bump likely has more to do with the output buffer stage. Check out "Technology of the Tubescreamer" on geofex.com.

[CynicalMan]

The low pass filter does bring down highs, but it doesn't make the eq flat. Also, the tone stage (when set at half) and the output buffer have very little effect on tone.
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