The Isotope Amp Overdrive is an adaptation of the “Tube Voice” channel of the Intersound Instrument Voicing Preamplifier, a rack preamp from the late 1970s that was favored by Steve Albini and the Grateful Dead among others.
The IVP’s Tube Sound channel was unique in its method of overdriving the signal. There are no diodes to be found, but rather, the signal is clipped by overloading a miniature transformer. Add in a two-band tone control and you’ve got a really flexible circuit that doesn’t look or sound quite like anything else.
The Isotope Amp Overdrive is a greatly reduced version of the IVP, extracting just the Tube Voice channel which makes up approximately 25% of the whole preamp circuit, which also includes a clean channel and 4-band parametric equalizer. It’s been adapted to run off of +/-9V instead of +/-15V like the original. However, all the character of the drive channel is retained and it makes a great standalone drive pedal if you want something a little different!
One thing to mention: the Treble knob may oscillate at the very high end of rotation. We tried several things to mitigate this during development, including better power filtering and tweaking the values of the treble control, even a new PCB layout, but nothing really fixed it. However, since it only impacts the last 5-10% of the rotation which is really outside the useful range of the control, we opted to release it as-is. Several people have built it and not experienced the oscillation, so it may be rig-dependent.