User manual

• Sweep Speed Fast mode: The first accent causes a strong positive output, but

subsequent accents produce a smaller output.

• Sweep Speed Normal mode: This is the same as the standard TB-303. The first

accent causes a positive output, but when the resonance pot is fully clockwise, this

sweeps upwards and some charge remains in a capacitor (C13) by the time the next

accent occurs. Consequently the second and subsequent accent pulses cause a higher

output than the first. This is one of the keys to the emotional nature of the TB-303 –

you poke it and it squeals a little . . . you poke it again and it squeals even more. The

machine gets worked up at these repeated proddings – and the charge is stored in

C13. Humans interpret such responses as “I can’t stand it!!” – the response of an

organism under stress and/or provocation.

• Sweep Speed Slow mode: Similar to normal mode, but with a much longer time-

constant. The output takes longer to rise, but it can rise twice as high. It also takes

longer to cool down. This can be perceived when the filter frequency is settling down

in the half second or so of non-accented notes which follow.

The Accent Sweep circuit, described above, is disabled when the Acc Sweep – Hi

Resonance switch is in its lower position. With the switch in the mid and upper positions,

the circuit is in operation.

The range of the Resonance control can be switched to double the usual feedback so as to

allow the filter to self-oscillate at mid and high frequencies. Since the Overdrive pot

enables the VCO signal to the filter to be turned down to zero, the filter can be oscillating by

itself and the VCO signal can be introduced manually by turning up the Overdrive pot. The

high resonance is selected by the mid and lower positions of the Acc Sweep – Hi

Resonance switch.

In the TB-303 the filter frequency was not affected by the pitch of the note the oscillator was

playing. The Filter Tracking pot enables the filter frequency to track the note being played.

The range is from 0, through normal tracking to over-tracking. This is imprecise, so it is not

possible to play accurately tuned notes using just the self oscillating filter. The centre note

for zero change in filter frequency is approximately C (2 volts) or D (2 2/12 volts) at the

bottom of the normal octave – an octave above the lowest C in the “Transpose down”

octave. Notes below this will cause the filter frequency to go down when the Filter Tracking

pot is turned up. The circuit is not very accurate – the exact note which is the “zero point”

depends on what else is driving the filter frequency: the Cut Off pot, the MEG signals

coming through the Env Mod pot, the Accented MEG signal coming through the Accent

Sweep circuit and the external filter input voltage. The higher the filter frequency as a result

of these inputs, the higher the “zero point” note for the Filter Tracking pot. At maximum,

the maximum Filter Tracking is about 2.7 kHz per volt (i.e. the filter goes up ~2.7 kHz when

the oscillator CV goes up an octave. At higher filter frequencies, this is not as significant as

at low frequencies.

One important use of Filter Tracking is for some low notes to cause the filter frequency to go

below the first harmonic of the oscillator signal – or at least to go below the lowest harmonic

of whatever signal is passing through the filter (since both the Oscillator and Audio In to

Filter signals drive the filter). This cuts the sound off entirely, or almost entirely, for these

notes.