Chuck D. Bones
Circuit Wizard
Cooder had asked me if there was a way to build the VFE Dragon without having to use the C10K/C100K dual pots. I have been tossing that same thought around in the back of my mind for some time and I knew it was possible. The only tricky bit might be doing it with a reasonable parts count. Here is the Dragon schematic. Think of it as a Timmy with steeper high-pass and low-pass filters. Right after the input buffer is a 2-stage passive high-pass filter. The trick Peter Rutter uses is he scales the impedances of two stages 10:1 so that the filter has a sharp corner at the -3dB freq. He does the same thing with the low-pass filter after the distortion stage. There's no free lunch and the price we pay to do it this way is we need a dual pot with two different resistances. It's a custom part and difficult to come by. One of the alternatives is to build an active 2nd-order filter. I went that route. It only took one more opamp and a few more R's and C's to do it.
If we want filters that have HUGE range and are very well-behaved like the ones in the Dragon, we still need dual pots, but at least the two halves can be matched. I did a little experimenting to see if I could make some acceptable compromises in the filter design so that I could use single-gang pots. This was the result. It works well. I'm still tweaking a few component values, but it's very close to done. I kept the input buffer (U1B), but upped the gain from 1x to 5x. If you're running hot pickups you might want to reduce R5. U1B, C2-C4, R6-R8 and VR1 form an active 2nd-order high-pass filter. It's tunable from 40Hz to 1.2KHz and has 12dB/octave slope for most of the range. The midrange frequencies are attenuated at more like 6 to 8 dB/octave. I wanted to have different clipping options, so I added a 6-position rotary switch. So far I've only run the breadboard with the blue LED / 1N4003 combination and with no clipping diodes. The COMP control is anything but subtle. I has a major influence on the compression and harmonic content. U3A, C8-C10, R11-R13 and VR4 form an active 2nd-order low-pass filter. It's tunable from 600Hz to 7.3KHz and has 12dB/octave slope across the entire range. I tried an NE5534 and a CA3130 for U2. Both work well. The NE5534 clips asymmetrically when driven to the rails. The CA3130 is much more symmetric. Many different single opamps will work, just pay attention to whether a compensation capacitor (C6) is needed and which pins it uses. With the clippers out of the circuit and the GAIN kept low, this pedal will play clean. By adjusting the controls you can get just the right amount of dirt, from a wee-bit to a metric shit-ton. Added bonus: this circuit can be run on 18V for additional headroom and volume.
If we want filters that have HUGE range and are very well-behaved like the ones in the Dragon, we still need dual pots, but at least the two halves can be matched. I did a little experimenting to see if I could make some acceptable compromises in the filter design so that I could use single-gang pots. This was the result. It works well. I'm still tweaking a few component values, but it's very close to done. I kept the input buffer (U1B), but upped the gain from 1x to 5x. If you're running hot pickups you might want to reduce R5. U1B, C2-C4, R6-R8 and VR1 form an active 2nd-order high-pass filter. It's tunable from 40Hz to 1.2KHz and has 12dB/octave slope for most of the range. The midrange frequencies are attenuated at more like 6 to 8 dB/octave. I wanted to have different clipping options, so I added a 6-position rotary switch. So far I've only run the breadboard with the blue LED / 1N4003 combination and with no clipping diodes. The COMP control is anything but subtle. I has a major influence on the compression and harmonic content. U3A, C8-C10, R11-R13 and VR4 form an active 2nd-order low-pass filter. It's tunable from 600Hz to 7.3KHz and has 12dB/octave slope across the entire range. I tried an NE5534 and a CA3130 for U2. Both work well. The NE5534 clips asymmetrically when driven to the rails. The CA3130 is much more symmetric. Many different single opamps will work, just pay attention to whether a compensation capacitor (C6) is needed and which pins it uses. With the clippers out of the circuit and the GAIN kept low, this pedal will play clean. By adjusting the controls you can get just the right amount of dirt, from a wee-bit to a metric shit-ton. Added bonus: this circuit can be run on 18V for additional headroom and volume.
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