The Ultimate Overdrive
The same concept can be used to create a drop-in module and mod any existing diode-based overdrive, as long as there’s room for two DIPs and all the diodes.
- Germanium: clips softly at 300-400mV. I used 1N34A, but bretty much any old germanium diode will work here and produce similar results.
- Silicon: 1N4148: clips a bit harder at 600-700mV, standard for vast majority of overdrive pedals. I put two of them in the diode array to be able to match stock Timmy setting with two diodes per side in addition to using a single diode. I used 1N4148, but most other silicon diodes can be used with the same performance.
- Rectifier: these are also essentially silicon diodes, so they perform similarly to 1N4148. I had some UF4007 that I like using for rectifiers, so I used these.
- Schottkey: some of them have even lower forward voltage drop than germanium diodes. I used BAT46 that clip at 250mV and also got some BAT41 as well so I can mix, those clip at 400mV.
- MOSFET: there are few different ways to wire a three legged transistor and use it as a diode. The one that’s different than a regular silicon diode uses drain and gate connected together, acting as a cathode and source acting as an anode (for P-channel MOSFETS it will be reversed). MOSFETs clips a bit softer than silicone diodes at higher voltages, usually between 1.5 and 3V. They need another diode in series to polarize them because it has a silicone intrinsic body diode pointing the other way. Unless we use another diode to polarize the FET, the silicon diode will clip the other phase of the signal. I intentionally left it up to the user to choose which of the remaining 6 diodes, if any, will be used together with the MOSFET. MOSFETs (often BS170 or 2N7000) are used by some “boutique” overdrive/dist pedals, like Fulltone OCD. Because MOSFET can clip the signal both ways, we can achieve asymmetrical clipping using only one MOSFET that clips both phases. I used BS250: P-channel MOSFET.
- LED: different colors have different forward voltages, but clipping characteristics are about the same. They have high headroom due to high forward voltage drop and they clip harder than other diode types. Infra-red can get as low as 1.5V, and ultra-violet as high as 4V. Other colors will fall in between. I suggest getting LEDs with as low voltage as possible, so that we can get some of that clipping at all.
Asymmetrical clipping is sometimes used as a way to add some “flavor” to the tone. What it means is that we use different types or different number of diodes for each phase, so signal gets clipped differently on positive and negative side of the waveform. However, I found that arrangements where ratio between the total forward voltages between the two sections is higher than 2 tend to sound ugly. That means that it’s perfectly fine to use 1.5V on one side and 2V on the other, but I would avoid combinations that have say a germanium diode (375mV) on one side and blue LED (3V) on the other. But I encourage you to try it and decide for yourself 🙂
- NJM4558D: high gain bipolar op-amp, used in Tubescreamer and countless similar overdrive pedals.
- JRC4559D: stock chip in Paul Cochrane’s Timmy and Tim pedals. Similar to 4558D, but with a tad more gain and faster slew rate.
- LM1458N: low power consumption op-amp. Reported to work really well in Timmy, bringing more clarity.
- TL072ACP: commonly used, low noise and low power consumption JFET op-amp.
- LF353N: wide bandwidth JFET input op-amp.
- OPA2134PA: hi-fi audio FET op-amp.
- NE5532P: bipolar input op-amp, higher current consumption than the rest of the heard.
So far I only really tried two of them. NJM4558D was my first choice and it works really nice, lots of drive and resembles Tubescreamer with that almost nasally sound midrange hump. Then I tried OPA2134PA which is wider band and more neutral sounding and really liked it, so it stayed there.
Parts and Construction
- Dale resistors (cool CMF brownies)
- Nichicon poly film caps
- Cornell-Dubilier 1uF poly film caps
- Panasonic FM electrolytic caps
- Mill-max wrap IC socket
- Neutrik jacks
- Alpha 3PDT switch
- Alpha 16mm pots
- 1/8″ thick home-made eyelet board with #35 keystone eyelets. Note that they are smaller than #45 typically used for tube amps.
- CM 8-pole DIP switches
- Teflon #20 AWG shielded wire for longer runs
- Thin #28 AWG ribbon wire for wiring controls
DIP switches are rarely exposed for users to mess with all the time, but I wanted to avoid having to open the pedal each time I wanted to play with the clipping section. This made construction a bit more complicated as it required having two rectangular holes in the enclosure and mounting DIP switches on the back of the circuit board. Having no specialized tools for rectangular holes, we drilled a bunch of small round holes and then filed the hole into shape using small flat files.
As you can see on the layout, 16 DIP pins are connected to only 9 eyelets. Clipping diodes zig-zag from one eyelet to the other and all but the outer two eyelets connect two neighboring pins of the switch. I wanted to cover as much ground as possible, so I hardwired 4 diodes and installed socket pins for the remaining three so I can experiment even more if needed. Every other diode uses sockets, so I only need to have up to one socket pin per eyelet.
Finally, a top tip for playing with DIP switches: use the tip of the pick to flick the mini switches.
Schematic and Layout
Click on an image to see more details.
Click on a thumbnail to play the video on YouTube.
Scoping the Output