- Tune & Manual Pulse Width Control
- FM & PWM Attenutation Control
- Switch between Manual PW & PWM
- CV Inputs : V/Oct, PWM, FM
- Audio Outputs : Saw, Sqr, Tri, STS
The TB-O shares the same saw-core oscillator circuit as the Roland TB-303. It also generates the distinctive, not quite square using the same technique as the original, including making use of the same transistor types – afterall, the name is Transistor Bass…
The module can output four waveforms simultaneously, all controlled by the same single 1V/octave CV input.
As you would expect we have the Saw and Square as provided by the original 303. The TB-O adds pulse width control to the square wave which can be applied in two ways.
Using the PW knob you can manually dial in the PW you desire (flick the switch to the left). The original, almost 50% duty cycle can be found at around 10 o’clock on the PW knob.
Secondly, using a CV source for PWM with an attenuator knob to set the strength of your PWM signal (flick the switch to the right). The input is expected to be in the +/-5v range. Again the 303 waveforms are unique in their offset and range, being offset +5v with an amplitude of about 7v. The circuitry will compensate for this but only if the input is in the +/-5v window.
Both of these modulated waveforms are output from the square jack, but only one type of PW modulation can be applied at a time, based on the direction of the switch.
Due to the way the TB-303 wave-shapes the saw into the square, when you adjust the PW it changes both the offset and the amplitude of the waveform. Be careful at extreme PW settings both via the knob and CV control as the signal can get quite hot – extending outside the normal 10V p2p of Eurorack.
There are two other waveforms, a very quiet (as most are) triangle. Triangles are very pure, so not much harmonic fun here. The final waveform is and a sort of shark-tooth-saw-sine (STS) strange shape. Both are included as I stumbled on them when playing around with wave-shaping the saw.
The STS wave sounds similar to the Saw, but can also be modulated with the PW and PWM controls.
The module also includes some basic FM capability via another CV input. As with most FM, sine waves, and I found very high frequency sine waves or even squares can give a really nice gritty edge to the waveforms.
Tuning / Calibration Update : 29th July 2022
Depending on where you look, there is much debate about what frequency equates to what octave range, I had been using an old reference that suggested that C1 for example should be 32.7hz – and that is what I’ve been calibrating the pre-built modules to, and the build guide up until now has suggested.
You maybe able to adjust your TB-O by simply turning the TM4-TUNE trimmer on the rear counter-clockwise to increase the pitch – you should not need to change the TM5-WIDTH.
However, you may find you can’t quite get enough range to get to 65.41Hz (without moving the TUNE POT on the front away from its center position) – if this bugs you, as it did me, then you can either replace R102 with a 47K ohm resistor, or add a second 100K ohm resistor in parallel to R102. This should extend the range of the tune trimmer to allow you to set the correct value. See picture here of “quick” fix.