How
to use this module
Connect
the CV input to a voltage source such as a keyboard,
envelope generator or sequencer. Connect the output
to a VCA or amplifier. Feed the signal to be filtered
into the high-pass, band-pass or low-pass input. If
the same signal is used in all inputs, the result
is reminiscent of a phaser. The real fun starts when
you feed different signals into each input, then you
get a frequency based "interpolating scanner",
where panning between different sound sources is possible,
though also subject to the frequency at which they
are running.
Each
of the filter type inputs has its own level control.
The ALL input is also affected by these level pots
as it mixed with the individual inputs prior to the
level controls. If using only a single input, it may
be better to feed the signal into the ALL input, and
adjust the level pots to select LP, BP or HP, rather
than changing the patch cord between the specific
input jacks.
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A
little on how it works
The
circuit uses a standard, non-inverting amplifier configuration.
The three modes (HL, BP, LP) are obtained by injecting
the signal into three different points of the circuit.
An increase in the gain of the amplifier increases
the filter's Q. The Q remains almost constant as the
filter is swept across the audio spectrum. In the
circuit, diode strings are used as voltage controlled
resistors.
The
differential-amplifier transistors apply the bias
voltage to the parallel diode string RC networks in
opposing phase. The opposing phases cancel the control
voltage so that none appears at the output. The final
pair of transistors form a non-inverting amplifier.
The variable resistor adjusts the gain of this amplifier,
and thus its Q.
The
final stage is a simple gain stage.
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