User manual

24
Idiosyncrasies
The Devil Fish is a denizen of the catacombs of Idiosyncrasy City. Here are some of its
peccadilloes.
With version 2.1 and earlier, there used to be a certain amount of clicking at the start of
notes and sometimes at the end of notes – even when no sound was being produced, on
un-accented notes when the Soft Attack pot was fully clockwise and the Decay pot was
fully anti-clockwise. Version 2.1A largely fixed these clicks, but due to a mistake I made
with the choice of a diode, clicks may appear with some machines at high temperatures.
In Version 2.1B I further refined the circuit to minimise these clicks under all
circumstances.
The TB-303 filter is a diode-capacitor ladder filter – comparable to those in many Moog
synthesizers. The filter is unchanged in the Devil Fish, although its audio input can be
driven harder and its frequency control can be driven over a wider range by a large
number of signals. Electronically its behaviour far from the perfect model of a filter. I
still don’t know exactly why it sounds the way it does, but it has a fantastic vocal/nasal
twang that gives it a really musical spirit. The filter will only self oscillate at mid and
high frequencies. Surgical intervention, aimed at improving its gain at lower frequencies,
made it sound more like a normal filter, so I abandoned this line of investigation.
With high levels of VCO and/or external signal into the filter, some clipping can occur –
depending on the filter frequency and resonance.
With high levels of signal coming out of the VCA and with the Volume pot set at or near
maximum, some clipping can occur in the output amplifier. This typically only happens
on the loudest (for instance the accented) notes, and can be quite a useful effect.
The polarity of the main output signal is opposite that of the Filter Audio Out signal. If
you mix the two into mono, then depending on the VCA gain, the two resulting
contributions to the final mix may be equal but opposite signals and so will cancel out.
A useful, convenient and drastic effect is to partially insert a 1/4 inch (6.5mm) mono or
stereo plug into the old headphone output socket. This form of Filter FM is described
above in the Overdrive, Filter Resonance and Audio Filter FM section above.
Reasons for no audible output
The filter can have such a low frequency that it passes no audio at all. For instance, with
notes below the low C (2.0 volts) of the standard range (i.e. in the octave accessible with
the transpose down function: 1.0 to 2.0 volts) the Filter Tracking pot will lower the filter
frequency.
The Cutoff Pot can be turned anti-clockwise to the point where the filter passes no audio
at the frequencies the VCO is producing.
The external Filter CV input can lower the filter frequency, including when 0 volts is
applied. Any voltage below about 3.3 volts will lower the filter frequency.
If you hear no sound when the Gate is On (as indicated by the LED in the ‘e’ of "Devil
Fish") and the Overdrive pot is not at zero, then try turning up the Cutoff pot, turning