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sweeps fast. If the proportion is small, the integrator sweeps slowly. R116 sets the minimum
speed. Don't be tempted to lower this value any more to get really slow sweeps. Input errors
within the integrator op-amp will take over and your LFO won't oscillate any more.
With C52 at 100nF the range of the LFO is about 0.05Hz (one cycle in 20 seconds) to 50Hz.
If you make C52 a 220nF capacitor the LFO will be proportionately slower and you will have
a range of 0.02Hz to 23Hz.
The square wave output is derived from the Schmitt trigger's output. D25 allows only positive
excursions through. R123 and C55 act as a simple low pass filter to round of the waveforms
edges a little bit. Very fast edges end up as CV breakthrough on the main audio output and are
pretty unpleasant.
The trigger output is simply generated by a transistor, Q12, that turns on when the output of
the Schmitt trigger goes high. The 'auto' switch switches the function off by shorting the base
to the emitter when not required.
The last thing to describe is the power supply. This is a standard 'three terminal regulator'
design straight out of the data book. R96 and C42 provide a decoupled version of +15V for
the logic circuitry. The logic circuitry can generate little spikes on the power supply, that
could get back into the audio if not decoupled properly. R9 and the 'on' LED provide power
supply indication. I have put it on the negative supply only to even up the power drains on
both rails.
The Filtrex's power supply can function either with a half wave rectifier for wall warts, or with
full wave for internal transformers. This will covered in more detail later on the document.
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