Circuit #02Circuit #02This is a nice sounding and simple overdrive effect. It uses two FET gain stages and two complementary russian made germanium transistors as clipping diodes. Because of this I named the pedal Yuri, after Yuri Gagarin, the first man on space. There is a last FET stage that provides level recovery, but also some extra distortion as well.
Also features a novel (?) gain control at the first stage which works by simultaneous negative feedback and voltage dividing. When reducing the gain, the negative feedback resistance and the resistance in series with the input are increased. I found that the gain of this stage can be calculated as the ratio of this two resistances (the two sides of the pot), very much like the gain of an inverting opamp stage (I need to further experiment with that). Using this gain control (instead of the usual volume control after a stage running at full gain) helps to reduce noise at low gain levels.
It has a good interaction with the guitar pickup, in particular at high gain settings, partly thanks to its low input impedance (50K at full gain, 100K at min gain). A possible place for mods are the input cap C1 and R2, which together with the pickup inductance form a resonant filter. C1 would control mostly the frequency and R2 mostly the amplitude of this peak.
You can substitute with your favourite FET, probably requiring to change the colector resistors (which should really be 10K trimpots to accomodate differences in the transistors). Also, you can substitute the clipping transistors with diodes, leds, mosfets or banana peels (not sure about the last one) as in any other diode clipping pedal.
Several settings using Gibson SG and Fender Blues Deluxe amp (Streaming MP3)Here's the schematic: Click on the image to enlarge
As always, any suggestions or comments are welcomed. And if you build it, let me know how it goes!
Regards!
Miguel
Miguel Canel, Buenos Aires, Argentina
bioroids.miguelNOSPAM@yahoo.com.ar (Quitar NOSPAM) http://www.bioroids.com.ar