søndag 16. april 2017

Juno 60 pre-filter level control

The Juno 60 mixes several waveforms before the filter, and it has a clever way of controlling the sub oscillator volume. As the sub oscillator output is a square wave, the wave is considered a binary on/off signal. It is connected to the base of a transistor, which when turned on and off either sinks its collector to ground or leaves it as it is.

To control the wave volume, the collector input is connected to the panel potentiometer (through a series of buffers/mux/demuxes, but that doesn't really matter, the principle is the same). The wave's amplitude (or "on value") will then equal the level of the panel potentiometer, though negative as the potentiometer value is inverted by IC17 on panel board A. The voltage is passed through an analog switch, IC16, which in turn is controlled by the Sub oscillator on/off switch, to completely disable the sub oscillator (incidentally, this feature has been removed from the Juno 106, here the potentiometer voltage is connected directly to the transistor collector).

Interestingly, the same arrangement exists for the pulse wave, only this time the analog switch connects the transistor to -15V instead of to a variable voltage. If one would like a controllable pulse wave volume, it would likely be easy to inject the control signal here.

As for the saw wave, it also has a switch and a transistor connected to it. However, as the saw wave is not binary, it cannot be controlled the same way. Instead, the wave is connected to the collector, and the transistor base is connected to the analog switch on panel board A. The wave is then sunk to ground if the transistor is on. In other words, the saw wave has no pre-filter volume control, only an on/off switch.

The noise input has its own volume control on panel board A (all voices use the same noise generator).

Oh, and by the way - all waveforms have a range of 0 to maximum -15V (the datasheet says the saw wave is 12V p.p, I have not checked the others but they have to be about the same to be mixable - the sub oscillator level for example originates as a 0 to +5V voltage but is inverted and amplified by IC17, and the pulse wave level is always 0 to -15V). They are fed through a 10uF non-polar capacitor (C5) just before the filter, which probably centers them around 0V.



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