Ashly DPX-100 Operating Manual page 20

Operating Manual - DPX-100 Graphic Equalizer - Compressor/Limiter
Voltage Controlled Amplifiers
Early VCA's were based on vacuum tubes with a "remote cutoff" characteristic. The
tube would simply change its gain in response to a changing bias voltage. Tubes
developed for this purpose did an excellent job, in fact they could exceed the noise and
distortion performance of today's best solid state VCA's. Unfortunately, they also had some
serious disadvantages peculiar to tubes - change of gain and matching as aging took
place, heat, microphonics, high cost, and the need for both high-voltage and filament
power supplies.
Over the years the need for good, low-cost, solid state VCA brought about many
innovative approaches. A good example is the electro-optical attenuator where a photocell
is used as one leg of a potentiometer. Since the photocell behaves as a true resistor,
distortion and noise are very low. Unfortunately, the response time of photocells is slow
and unpredictable so their use in a fast peak-limiter is really not feasible. Also, the
matching between units is very poor so that stereo tracking is not possible without
tedious hand-matching of photocells.
Another approach uses a field-effect transistor (FET) as a variable resistor. Here, at
least, the response time is fast (in the nanosecond range), but matching between units is
still poor, requiring hand matching for stereo. An additional problem is that a FET will only
act as a pure resistor with very small signals applied so it is necessary to attenuate an input
signal before the gain control FET and then amplify it again. Of course this results in less
than ideal noise performance and imposes a frustrating trade-off: less noise = more
distortion.
A number of VCA's based on the exponential voltage-current characteristic of a
bipolar junction transistor have been used. One of the most common is called
a"transconductance amplifier". Using the inherent matching obtained by integrated
circuit technology, these devices have very predictable control characteristics. Tracking
within 1 dB over a 40 dB range is common. Not only do the control characteristics match
well from unit to unit, but they can easily be made exponential (logarithmic) so that even
increments of control voltage produce even increments of gain change in decibels. The
response time is also very fast.
The problem with simple transconductance amplifiers is that, like FET VCA's, they
can handle only very small signals so the noise performance is poor. A number of
linearizing circuits have been devised to minimize this problem, but even the best
transconductance amplifiers have an equivalent input noise of about -80 dBv, which
compares poorly to straight linear amplifiers.
The best analog compromise to date is the "class AB current ratio multiplier." Early
implementation of this circuit used two matched pairs of transistors, one pair of NPN's and
one pair of PNP's. The problem here is that excellent matched integrated NPN pairs were
available, but integrated PNP's were not. The PNP's had to be hand-tested and matched.
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