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C18 provide AC coupling of the outputs to remove the slight DC offset from the top
differential amplifier.
The resonance in a moog ladder filter is controlled by the level of audio feedback applied to
the bottom pair of transistors from the output of the differential amplifier at top rung of the
ladder. The level of is typically controlled by the resonance pot, wired as a variable resistor
and a trimming resistor, TRIM. It is traditional to use a 50K reverse log pot for the resonance
control in the classic moog ladder circuit. When set to a value of 50K this pretty much allows
the feedback path to be ignored, and no resonance can be heard. The 'reverse log law' is
needed so that as you turn the pot the resonance increases smoothly. An ordinary linear law
pot would do nothing for most of its travel, and then all the resonance would be introduced in
the last quarter of a turn. Not very 'smooth' or musical. Unfortunately, 50K reverse log pots
are difficult to find and quite expensive.
In the Filtrex-II, we use an 50K linear pot, but we wire it as a potential divider and not as a
variable resistor. The drawback of using a pot on its own like this is that it has a variable
output impedance and the ladder is therefore unbalanced, the degree of which depends on
where the resonance control is set. We get around this by using a buffer on the pot's output.
U3, a unity gain voltage follower, or buffer, 'sniffs' the voltage at the wiper of the resonance
pot and provides a copy at its own output that has a constant impedance.
The resonance pot is a dual gang type too. One gang controls the feedback loop as we have
seen, and the other sets the gain of the input. Normally the passband gain of a moog ladder
decreases as you turn the resonance up. In other words the volume drops as you increase the
resonance. This can be quite a problem for a post processor like the Filtrex. So in the Filtrex,
the input level is automatically turned up as resonance increases. Thus the overall effect is of a
constant volume at all values of resonance. I thought this was quite clever of me to invent a
way of doing this without increasing noise levels. However, I found out later that the very
same principle was used in the Roland SH-2000 as long ago as 1974.
The other half of U3 acts again as a voltage follower on the wiper of the gain control part of
the resonance pot. This ensures that the ladder's input is fed from a constant resistance no
matter where the resonance pot is positioned.
C11 provides the appropriate decoupling and is sufficiently large so as to allow the filter to
oscillate above 200Hz or so.
Four inputs control the filter cut-off frequency via an exponential convertor based around U9.
The filter frequency can be directly controlled with the FREQ pot via R63. R65 sets the
sensitivity of the envelope processor's output. While R67 does the same job for the LFO. The
CV input provides a nominal 1V/octave response, and would normally be accessed via a jack
socket on the rear panel.
The exponential convertor is temperature compensated. R61 is the positive temperature
coefficient resistor providing an approximate cancellation of the exponential convertor's
inherent temperature coefficient.
Note the two pin header, STV, and R60. These allow two Filtrex-II modules to be ganged
together to form stereo processing. There is more about this later in the document, but we can
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