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often considered to be a balanced signal but in truth the receiving end of the balanced
connection cares not for the absolute signal levels on the hot and cold connections but only the
difference between them. And because all balanced input circuits do not have an infinite input
impedance the source impedance is very important to this differencing process. By keeping
both the hot and cold's source impedance as close in value as possible you maximise the ability
of the balanced input's circuitry to reject any unwanted interference on the audio signal.
The advantage over the impedance balanced circuit over the full balanced circuit is two fold:
Simplicity – you have less circuitry in the audio pathway to either go wrong or alter the signal
in some way. And compatibility – the impedance balanced circuit can be used for both
balanced systems and unbalanced systems with no possible problems with stability or short
circuits. Now there are EBOS (electronically balanced outputs stage) circuits around that will
automatically compensate for unbalanced connections but they do add complexity and may be
prone to instability under certain load conditions.
The disadvantage of solely impedance matching is a comparative loss of headroom. The full
balanced system has an additional 6dB of output signal to play with. It is the author's opinion
that this does not make a great deal of difference in the real world.
If the venerable 5534 is replaced by a more modern single op-amp you should remove C28 as
additional compensation will probably not be required. You may also be able to link out the
coupling capacitor, C57 and its twin, C29, if the output offset voltage is negligible. R50 and
R48 may also be removed in this case.
That completes the description of the audio path. Now let us take a look at the processing
circuitry of the Filtrex. Staying on page two of the schematics, the second set of circuits down
the page belongs to the full wave rectifier and threshold detector. This takes its input from
wither the pre-amp output or the external side chain input. The choice is determined by the
‘cont’ switch.
The switch's wiper leads straight into some amplification based around U14a. This is an
inverting amplifier with a gain of -3.3. The main pre-amp is expected to produce a maximum
output of around 5Vp-p in normal use, so U14a boosts this signal up to 15Vp-p. The
amplified signal is now full wave rectified by the circuitry based around U14b and associating
circuitry. Full wave rectification can be described by the mathematical ‘absolute’ function. In
other words, the output of the full wave rectifier (FWR) is always positive. If you present
+10V to the input, you will get +10V. But if you present it with -10V you will also get +10V.
Likewise, -5V turns into +5V, -3V into +3V. Now if you put an audio signal into this circuit,
you will get a series of positive bumps that correspond to the up and downs of the audio
signal.
U14d forms a special buffer circuit. This configuration, allows the op-amp to drive medium to
high capacitive loads without instability. We also saw it being used in the output amplifier of
the Filtrex. The output of the full wave rectifier is therefore protected by the odd load
presented by the next set of circuits.
Now, no real time system can recover envelope information without some disadvantage of
some sort. Some systems employ the
peak and droop
method. These are fast to respond to
sudden changes in loudness or envelope. They work by simply charging a capacitor as fast as
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