Chuck D. Bones
Circuit Wizard
This design was whipped up by HamishR, based on the BJFe Model G. Here's what he sent me. The main difference from the Model G is the clipping diodes. He used Schottky instead of LEDs for the soft clippers (D1 & D2); 1N4001 instead of JFETs for the hard clippers (D3 & D4). He deleted the C and NATURE knobs, replacing them with BASS and TREBLE. The JFET 2nd stage was replaced by and opamp. This is a low-to-medium gain amp simulator. Very mild compression. Notes and chords ring true and there is a smooth edge-of-breakup tone. I guess I'd call it an OD because It makes enough volume to overdrive whatever follows it, pedal or amp. Bottom line: it sounds great.
I wanted a little more range on the BASS & TREBLE knobs, so I tweaked a few component values. I tried a few different TREBLE circuits, but none worked as well as the shelving circuit shown above. Next, I got the idea to unbalance the soft clippers to obtain a little bit of 2nd harmonic. At high signal levels, the hard clippers (D3 & D4) dominate and we get symmetric clipping, mostly 3rd & 5th harmonics. As the signal decays, the soft clippers take over and make mostly 2nd & 3rd harmonics. The note decay is very natural. Finally, I saw a way to ditch C8 and R10. This is where I ended up.
Knobs (L-R): LEVEL - TREBLE - BASS - DRIVE
I had one more trick up my sleeve. Ordinarily, I don't like returning signal to Vref. but I thought I'd give it a try here. It saves one part: C4. It works. The asymmetric drive to D3 & D4 pushes Vref positive a few tens of mV when playing hard. It changes the harmonic structure a little bit, but honestly, I can't tell the difference. I added a toggle switch to the breadboard that allows me to return D3 & D4 either to GND thru C4 or to Vref. I even tried changing R13 & R14 to 47K in order to increase the DC shift on Vref. Still, no apparent difference. In the end, v0.1.1 and v0.1.2 work the same and sound the same. V0.1.2 has one less cap.
I wanted a little more range on the BASS & TREBLE knobs, so I tweaked a few component values. I tried a few different TREBLE circuits, but none worked as well as the shelving circuit shown above. Next, I got the idea to unbalance the soft clippers to obtain a little bit of 2nd harmonic. At high signal levels, the hard clippers (D3 & D4) dominate and we get symmetric clipping, mostly 3rd & 5th harmonics. As the signal decays, the soft clippers take over and make mostly 2nd & 3rd harmonics. The note decay is very natural. Finally, I saw a way to ditch C8 and R10. This is where I ended up.
Knobs (L-R): LEVEL - TREBLE - BASS - DRIVE
I had one more trick up my sleeve. Ordinarily, I don't like returning signal to Vref. but I thought I'd give it a try here. It saves one part: C4. It works. The asymmetric drive to D3 & D4 pushes Vref positive a few tens of mV when playing hard. It changes the harmonic structure a little bit, but honestly, I can't tell the difference. I added a toggle switch to the breadboard that allows me to return D3 & D4 either to GND thru C4 or to Vref. I even tried changing R13 & R14 to 47K in order to increase the DC shift on Vref. Still, no apparent difference. In the end, v0.1.1 and v0.1.2 work the same and sound the same. V0.1.2 has one less cap.
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. Looking forward to this one!