this article at http://www.kennedyaudio.com/cansolid.htm was the first spark. It claims that half & soft sequentially clipping of the signal gives a modulated duty cycle, producing even order harmonics. By doing simulations on Tina at (it is software, not my girlfriend
), I noticed that duty cycle can change up to 5%, but the gain have to be very high, pushing opamps to rails, adding rude opamp's harmonics. To overcome this, I used asymetric clipping on each opamp instead of half-clipping to prevent from reaching the rails.I thought to keep the circuitry of 2-3 oamps in series with low to moderate gain each, in order to avoid loading one opamp heavily (I really hate harmonics of overdriven opamps).
The second step was finding a way to slowly increase the gain to have enough range between slight-overdrive and heavy overdrive. A double (gang) potensiometer is not of my liking for some reasons. I used the gain-pot circuitry of Marshall stompboxes as a bedrock. A single pot that controls two opamps together.
Rolling-off the treble in each stage is very crucial to limit high-order harmonics.
TLC272 & TLC2262 were chosen because of low current consumption, low noise, high input impedance. Both are of CMOS technology, cheap enough. OP275 & OPA2604 sounded nice but demand more current.
The first two oamps mimic the soft, dynamic & asymetric clipping of two Class-A moderate gain tubes in series. The third low gain opamp mimics the hard symmetric clipping of the (current driven) power stage that takes over when is driven hard by the previous stages. Also, there is a some drastic filtering (33nF, 470nF) in the 3rd opamp to shape the spectrum.
The circuit gives a transparent clean sound at gain=0. Turning the gain pot to 3 o'clock gives a slighly compressed brown sound, like the 60's butique one. At 6 o'clock, gives low-crunchy chords and a light octave-up effect on solos. At 9 o'clock it is an heavy overdrive for 70's electric-blues soles, but notes are still dinstict. At full gain is a marshall-like heavy fat overdrive, with even harmonics make the sound thick & fat (but u can't play Slayer riffs
).Generally, high order harmonics appears at maximum gain only, when some harsh is desireable to play metal stuff. In low to midle gain, higher harmonics are suppressed and the sound is sweet, warm, brown. Even harmonics are audible giving this octaveup-like singing tone at midle overdrive and this fat/thick at full overdrive. The sound is old fashioned in every gain step.
Circuit is still on breadboard. It is not finished. I haven't tried tone controls yet. There is also one more opamp that must be used somewhere (maybe as output buffer after tone controls, maybe as votage divider for steady 4.5v). When it will be completed, a perfboard could be done.
