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Build Notes (cont.)
Since the EQ Gain knob boosts and cuts, with the center position being zero, I prefer using a center-detent
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pot here, and I've linked to one from Small Bear. This is purely cosmetic, and the downside is that they are
only available in solder-lug and so you have to run the wires to the pads. Any 10kB pot will work fine here.
The 100kC dual pot is pretty easy to find since it's the same value used for the classic Uni-Vibe's speed
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control. However, Build Your Own Clone is the only place that sells it in PCB-mount—I've linked to them above.
I put in a request with Small Bear Electronics to stock the right-angle version, but it probably won't be available
until mid-2015. The solder-lug version will work just fine for now, it's just a lot of extra wiring.
Modifications & Experimentation
Input capacitor: The stock circuit is very well-balanced, but if you're doing any experimentation with the
clipping section then you may want to reduce the input cap value to tighten up the bass a little. Try 22n for C1.
Q (frequency width) adjustment: C6 and C7 set the Q (width) of the parametric frequency band. By changing
the values of these capacitors in relation to each other, you can create either a narrower or a wider Q than the
stock circuit. For narrower Q, try C6 = 2n2 and C7 = 33n. For wider Q, try C6 = 6n8 and C7 = 100n.
Gain/EQ adjustment: By changing C11 and R24, you can adjust the overall gain of the circuit as well as the
EQ. In the stock circuit, these values give the same corner frequency roll-off as a Tube Screamer (723 Hz). You
can increase the gain by reducing the value of R24—just increase the value of C11 by the same proportion to
keep the same EQ. If you use more common Tube Screamer values, you may need a different drive pot value
to compensate: C11 = 47n, R24 = 4k7, and a 500kA drive pot.
Brightness: R25 and C12 form a low-pass filter with a corner frequency of 1061 Hz. You can raise C12 to 18n
or 22n to dampen the high-end, or you could lower it to 12n or 10n to increase the brightness.
Minimum gain: Drop R23 down to 4k7 or even 1k to reduce the minimum gain of the circuit. I'd recommend
this even if you're going for an otherwise-stock build.
Volume: The stock circuit uses a 10kB volume pot. You'll get a better taper adjustment by switching to a more
common 100kA pot as seen in most Tube Screamer variants.
Op amp experimentation: I designed the op-amp layout so that one primarily controls the parametric EQ
section and one primarily handles the feedback clipping. This way, if you want to try a higher-fidelity op amp for
the EQ, you can still get the 4558 "mojo" in the clipping section. Try a TL072 or an OPA2134 for IC1.
Additional Part Notes
• Capacitors are shown in nanofarads (n or nF) where appropriate. 1000n = 1uF. Many online suppliers do
not use nanofarads, so you'll often have to look for 0.047uF instead of 47n, 0.0056uF instead of 5n6, etc.
• The PCB layout assumes the use of film capacitors with 5mm lead spacing for all values 1nF through
470nF. I prefer
EPCOS box film
• Potentiometers are Alpha 16mm right-angle PCB mount.
• Switches are Taiway (Small Bear) or Mountain Switch (Mouser) brand with solder lugs. I prefer the short-
toggle variety, but that's just a matter of aesthetics.
• I recommend using
the board and prevent shorts. If you don't use these, use some electrical tape or cardboard to act as
insulation. The right-angle pots will make direct contact with the solder pads otherwise.
PEARL OD-05 OVERDRIVE / FRACTAL OVERDRIVE
or
Panasonic
these dust covers / insulators
ECQ-B/V-series.
from Small Bear to insulate the back of the pots from
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