How this tone control works

This is the tone control behind the Klon, and it is a different animal from a passive treble-cut stack. The first op-amp (OA1) is just a unity buffer: the signal comes in through a 10 Ω stopper and a 1 µF coupling cap, gets biased to vref through R2, and leaves the buffer low-impedance so the tone network sees a clean source. The second op-amp (OA2) does the real work. It is a plain inverting amplifier with R4 in and R6 as feedback, so at low frequencies, where C2 blocks, the gain is exactly −R6/R4 no matter where the knob sits. With the stock 100k/100k pair that is unity. The lows never move. That is the whole trick, and it is why this control feels "active" and never sucks level the way a passive stack does.

The tone pot is a bridge between the buffered input and OA2's output: one end reaches the input through R3, the other end reaches the output through R5, and the wiper feeds C2 into the virtual ground at OA2's inverting input. Turn the knob bright and the wiper leans toward the input side, so at high frequencies extra input current pours into the summing node through a much smaller resistance than R4: treble boost. Turn it dark and the wiper leans toward the output side, so C2 becomes extra feedback: treble cut. The result is a first-order shelf that pivots around the bass floor rather than only cutting. If that architecture sounds familiar, it should: this is a Baxandall-type active tone control, the classic hi-fi boost/cut circuit, stripped down to its treble half with a single knob. The Klon did not invent the idea, it picked the right one.

Three things fall out of the stock values. The control is asymmetric on purpose: R3 (1.8k) is smaller than R5 (4.7k), so there is far more boost on tap (about +17 dB) than cut (about −7 dB). Noon is not flat: the center position sits near +3 dB of treble, which is part of why this circuit reads as bright and cutting even with the knob centered. And the transition region parks around 500 Hz to 3 kHz, working the presence range instead of acting like a typical 700 Hz one-pole treble roll. Push R3 or R5 all the way to their 0 Ω jumpers and the ideal math says the end-stop shelf goes to ±infinity; a real TL082 runs out of gain long before that, but the lesson stands: those two resistors set how far the knob can take you.

Skewing R4 against R6 tilts the whole control: the bass floor moves to −R6/R4 and everything pivots around the new floor, which is why the Klon keeps them equal. C2 sets where in the spectrum the shelf acts: bigger reaches down into the mids, smaller confines it to the top. The pot value rescales everything at once, since it sits inside both R_a and R_b. Note the stage inverts (both gains carry a minus sign); on its own that phase flip is inaudible. The output leaves through C3 into R7, a 16 Hz high-pass, with R8 isolating whatever comes next. The dashed curve is the nearest-E12 build with the nearest standard pot.