Business Card Build Guide:
Introduction:
This is a fairly simple circuit, with a nice, hot output thanks to the high clipping threshold; a very broad tone control that can go from meaty low end to snarling high end; and a very novel bias circuit that enables overtone generation via wavefolding.
If you’ve ever successfully built a pedal before, this will be a breeze. If not, this may not be the best place to start. Not because this is a particularly difficult build, but rather because covering every step of the process would be quite exhaustive and bog down what I intend to be a fairly breezy run-through. There are many other good places to start if you’re interested - the aptly named BYOC Confidence Booster being one!
Preamp
The circuit begins with a non-inverting amplifier stage, configured as a soft clipper - the two pairs of diodes set the clipping threshold relatively high at 1.4v. C5 is a 470pF (220pF or 330pF will do in a pinch) ceramic capacitor that rounds off the sharper edges of the clipped waveform, helping lend it a nice compressed sound when the pedal is just edging into overdrive territory.
The exact level of gain is dictated by the relationship of R5, R4 & the drive pot. The Drive pot has lugs 1 & 2 shorted together, turning it into a basic variable resistor that decreases in value as you turn it clockwise.
The gain of this stage is given by 1 + [R5/(R4 + DRIVE)]. The Drive Control has an ‘C’, or ‘antilog’ taper - this is important in order to get a smooth-sounding sweep.
C3 adds emphasis to the mids.
Tone
The tone network consists of two filters in parallel, fed into an active panner/mixer circuit. It’s a bit unconventional, but it gives an impressively wide range of tones.
As you turn the tone pot, the virtual ground (Vcc/2) gets closer to one signal path or the other. It’s a clever trick that allows one pot to essentially act as two attenuation pots at the same time.
If it’s all the way one way, it grounds out one of the signals. If it’s in the centre, the two signals have an equal amount of impedance between themselves and virtual ground. So long as both have roughly similar volumes to begin with, this lets you smoothly pan between them.
There’s a much better overview of the basic premise by the legendary R.G. Keen here.
Bias
The PCB says A2k-B5k, but I will insist that you use B5k. This control may feel reversed. It’s essentially inactive when fully clockwise.
This sets the virtual ground level at the non-inverting terminal of the second op amp stage. Assuming you’re using a 9v supply voltage, the incoming signal is biased to 4.5v. A bit of background theory: essentially, the op amp is always trying to make the voltage levels at the two input terminals match. So if + is held at 4.5v, and - is biased at 4.5v with a waveform on top, the op amp outputs an inverted version of the negative terminal’s waveform in an attempt to push the input back down. The larger the resistance between the output & input, the larger the output needs to be to push the input back to its bias point.
What the bias control does is intentionally set the non-inverting terminal’s bias to a value a few millivolts off from the bias that the incoming signal on the inverting terminal is using. This causes the output to overcorrect and shift, yielding some really wild results.
It only works in a very small range before mismatching the bias levels simply yields dull sounds, but this circuit is set up to allow variation between fairly normal sounds at fully clockwise, and some crazy overtones and shattered glass sounds at zero. The sound of this changes a lot with different tone settings.
Output
The output is simply an NPN transistor amplifier that flips the phase of the signal back around so that the input and output are in phase. This will also overdrive at higher drive settings which can add some interesting fuzzy high end content in. It can be fun to use a higher gain transistor as well for extra snazz, like a 2N5088 or BC549 (install this one backwards due to the reversed pinout).
Notes:
For even higher headroom, the circuit can safely be powered with 18v without any extra considerations beyond ensuring the components you use are rated for 25v or more.
Almost any dual op amp should work here. TL072s & TL062s are my favourites for this, but feel free to experiment. You may encounter some interesting wrinkles using others like JRC4558s, NE5532s or LM1458s, or they may sound identical. All should work without issue though.
The 47uF & 100uF capacitors will of course need to be electrolytics. I prefer the Nichicon Fine Gold electrolytic capacitors to the exclusion of all others, but follow your heart, or failing that, follow your pocketbook.
Footswitch Wiring
The footswitch wiring is shown here. The inner 4 wires are shown in the order they connect to the PCB.
The Input jack should of course have its sleeve connection grounded. If your jacks make good contact with the metal of your enclosure, you can likely get away without using a grounding wire on the sleeve connection of your output jack, as that will be made via the conductivity of the enclosure material. If you use insulating jacks, you will certainly need to ground both.