Idea #05Idea #05
This is a very simple idea that can have multiple uses, and permits to pack a lot of behaviour on a single control.
You have two DC coupled opamp stages. The pot with the wiper at the output of stage 1 acts as two ganged resistors, one controlling the gain of the first stage by means of the feedback resistance, and the other controlling gain of the second stage by means of the input resistance.
The two stages increase and decrease gain together. With the values shown you go from a total gain of 1 (1 at stage 1, and almost 1 at stage 2) to a total gain of 1100 (11 at stage 1 and 100 at stage 2). Click on the image to enlarge.Of course the circuit as shown is almost useless. Now let's use this in a more interesting way.
The first stage is a simple diode clipper, with a modest gain of 100. The second stage is a high pass filter - notice the 2n2 capacitor (C4) in parallel with the part of the pot that goes to the second stage. The amount of gain and filtering is controlled in opposite ways by potentiometer P1. Finally, there is a fixed RC filter at the output that provides high-cut at the corner frequency of 723Hz.
At full gain, all of P1 resistance goes to the feedback path of stage 1, so C4 has no effect. The second stage acts then as an allpass with gain 1 and can be ignored. So, the first stage has full gain, wich makes the diodes clip, then comes the allpass filter stage, then the fixed low pass filter that removes all the harshness of the clipping.
At min gain, all of P1 resistance goes to the inverting pin of stage 2, in parallel with C4. This acts as a high pass with a corner frequency of 723hz, wich in turn compensates with the fixed filter at the output for a somewhat flat response. So, with no gain (and no clipping) at the first stage, you get an almost flat eq at the same time.
Basically the trick is to have a fixed low pass filter, and cancel it with an oposite filter at a degree that depends on the gain aplied. This allows to have low gain settings that don't sound muffled, and high gain settings that don't sound harsh, with a single control.
This circuit is only an snippet anyway, and must be tweaked to get a usable pedal out of it. It would need for sure a gain recovery stage at the end, and maybe more clipping (how about sticking the good old minibooster there?).
The cancelling of the filters is not perfect because one of them is a shelving high pass, and the other a standard 6db/oct high cut.
Miguel Canel, Buenos Aires, Argentina
bioroids.miguelNOSPAM@yahoo.com.ar (Quitar NOSPAM) http://www.bioroids.com.ar