The FX Files: Fulltone OCD

An op-amp based overdrive in the family tree of the Tube Screamer, the OCD is widely reckoned to be one of the most open-sounding drives on the market

When Mike Fuller of Fulltone originally devised the OCD, he wanted an open-sounding distortion pedal with decent headroom to more faithfully emulate a driven tube amp. This desire for extra headroom is why the OCD can be run from anything from 9-18V, with the pedal delivering a noticeably different, less compressed sound when run at the higher voltage.

Although this has never been confirmed by Mike Fuller himself, the OCD takes its driving inspiration from a now littleknown pedal partly descended from the Ibanez Tube Screamer. As can be discerned from looking at the schematics side-by-side, the original OCD is essentially a Voodoo Labs Overdrive, itself similar to a cascaded MXR Distortion+. Although the first version of the OCD featured an expoxied or ‘gooped’ board, later iterations would prove popular platforms for modding by the DIY community.

There have been seven distinct versions of the OCD, identifiable by the PCBS and layout on the inside, alongside visual cues like the size of the logo on the front panel. The changes on the versions between 1.1 and 1.7 are relatively subtle, and are mainly component value tweaks to slightly alter the EQ reponse. The most interesting change is on version 1.3 - increasing the drive pot to 1M for more sustain, and the addition around 1.4 of a germanium clipping diode.

The leap that was made with version 2 brought even more dramatic change. The three key differences are a new output buffer, a discrete 2N5457 JFET transistor input stage, which raises the input impedance significantly - as you’d find on an Echoplex or Space Echo preamp - and the option to use a new type of switching, called enhanced bypass, instead of true bypass.

mike fuller originally wanted an open-sounding distortion pedal