Thursday, 26 October 2017

Buchla Easel - Portabellabz bob expander.

These are my build notes for the Bucha easel BOB by Portabellabz.
You can find the docs here .
It's quite a neat little unit. And it's expandable.

The two ICs are a TL071 op amp and a CD 4093. The TL071 is I think used to invert the EG.... the second left output jack.  The 4093 is I think used for converting gates to triggers ???... for the pulser trigger out ???

The trimmer sets the voltage level of the pulse trigger out. ... If using eurorack the voltage level of the pulse out can be reduced to 5V

Papz has left a section on the right blank to add your own circuit.
I'm putting in a square wave LFO/audio oscillator which you can find in the build notes.
I first tested this on some bread board.

Used just one 741 op amp, one capacitor (220nf) & 7 resistors.
The uber basic circuit is here
It's about as simple a circuit as you can get. An op-amp with positive and negative feedbacks.
The inverting input is connected to a RC (resistor capacitor) network. The non-inverting input is connected to a voltage divider.
It's classified as a astable multivibrator which has two states, neither of which are stable. It is constantly switching between these two states with the time spent in each state controlled by the charging or discharging of the capacitor through a resistor. The op-amp works as an analogue comparator... comparing the voltages at its two inputs.
The full schematic is here:

 Yup. It works both as a modulation oscillator and a audio oscillator.

Now to build it into the board.

So the the connections from left to right are:
- sequencer pulse in
- envelope inverted out / envelope trigger out (switchable)
- pulser pulse in
- pulser trig out (the internal short pulse occuring at the end of its cycle, not the ramp on yellow bananas)
- MO modulation switch CV
- LPG1 mode switch CV
- LPG2 mode switch CV
- LPG2 signal routing switch CV
- extra oscillator - switchable between LFO & Audio.

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