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Circuit Description
Like many complex analogue circuits the Filtrex-II issue 2 circuit can be split up in to little
bits. The first bit we will look at is the pre-amplifier stage on page one of the schematic. The
input signal comes in through a small network of passive parts: L1 acts as a filter for radio
frequency interference. C5 and R2 act as a high pass filter removing any DC from the input
signal. R3 and C26 act as a low pass filter removing any unwanted high frequency components
from the input. R3 also protects U1 from possible over voltage.
The pre-amp is built around U1. I have specified the low noise audio op-amp, the OPA2134.
The pre-amplifier is a two stage design. The first stage is a non-inverting amplifier whose
voltage gain can be varied from 1 to 12. C1 keeps the gain for DC and very low frequency
signals at near to one. This prevents any offsets within U1 from being amplified unnecessarily
C3 provides a little bit of high frequency roll-off to keep the amplifier stable.
The second stage of the pre-amp is an inverting amplifier. The GAIN pot is used in a slightly
offbeat way. It is in both the feedback and the input resistors. This way we can control the
gain over a wide range from -0.4 to -10.1. The minus in these numbers shows the inverting
properties of the amplifier.
The voltage gain of the two pre-amplifier stages in tandem can be varied from -0.4 to -122. A
gain of 0.4 means that the output of the pre-amp is only 40% of the input level. While a gain
of 122 means that the output level is 122 times bigger than the input. In audio circles this
would normally defined in dB. This pre-amplifier will give you a gain from -8dB to +42dB.
Because the pre-amplifier is made from a inverting and non-inverting stage, the overall
behaviour is inverting. This means the output is completely out of phase with the input. This is
not a problem since the inversion is corrected later on the Filtrex.
Q7 and associating circuitry drive the peak LED. This is designed to light up just as the ladder
filter starts to show heavy distortion.
The main audio path continues on to the filter ladder itself via one half of the resonance pot.
However, the pre-amp also provides the signal for the 'dry/wet' circuit and the envelope
processor. We will deal with these two later.
The filter is based around the traditional ladder as designed originally by Dr. Moog. I have
used THAT300 matched NPN array for the top and bottom pairs in the ladder. This minimises
control current breakthrough to almost zero. Current breakthrough manifests itself as a copy
of the modulating signals on the output. Generally, this is not a good thing. 'BAL' biases the
base of the left hand bottom pair (U2, pin 9), via R21, by a small amount to even out any
differences within the ladder. This minimises breakthrough still further.
Two of the rungs of the ladder are 'sniffed' by a differential amplifier. Each of these is
identical, based around the classic three op-amp implementation. They are all DC coupled, and
rely on 'close' matching to remove any DC offset. A differential amplifier is a device that
makes larger the voltage difference between two points. In our case, the voltage across the
top and bottom filter capacitors. The gain of the differential amplifiers is set higher than
normal ladder filters to improve signal to noise ratio in the following mixer stage. C17 and
7
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