Specifications

12
Operating Manual - CLX-52 and CLX-51 Compressor/Limiter
Tracking within 1dB over a 40dB 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 volt-
age produce even increments of gain change in deci-
bels. 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 de-
vised to minimize this problem, but even the best
transconductance amplifiers have an equivalent input
noise of about -80dBv, which compares poorly to
straight linear amplifiers.
The best analog compromise to date is the
“class AB current ratio multiplier.” Early implemen-
tation of this circuit used two matched pairs of transis-
tors, 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. Careful
trimming was necessary for low distortion and even
minor temperature changes made re-trimming neces-
sary because of differing characteristics between the
two types.
The Ashly VCA
The Ashly VCA is an integrated current ratio
multiplier circuit. It has low noise (-90dBv), low dis-
tortion (.05%), excellent response time and tracking
and does not suffer from thermal drift. The noise and
distortion are at state-of-the-art levels and the circuit
is consistent in mass production with minimal trim-
ming and no hand-selection of transistors.
Detectors
It would seem that, of the two components in
a compressor/limiter, the VCA is the more critical since
the audio passes through it and the detector only pro-
vides it with a control voltage. Experience showed us
that both are crucial to the overall sound and that, if
anything, the detector’s performance is the harder to
judge by conventional measuring techniques. While
the VCA is doing its job if it has low noise and distor-
tion, the detector must constantly adjust the gain of
the audio path in a manner which keeps the level un-
der control while sounding acceptable to the listener.
This constantly changing gain is a Dynamic action,
while conventional audio measurements like noise and
distortion checks are Static (at a constant level). We
became painfully aware of this problem with some of
usually set as fast as possible, consistent with distortion-
free performance. Release time would also be relatively
short, so that the output signal would be restored to nor-
mal as quickly as possible after the transient.
Compression is frequently used to keep overall
signal level within a specific dynamic range, and for this
application, slower attack and release times are usually
chosen. This approach is analogous to our manual gain
riding example, where our operator is fading the music up
and down to keep it fairly constant, but is doing it slowly
enough so that the listener is unaware that the gain is be-
ing altered.
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, micro-
phonics, 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 pho-
tocell 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 photo-
cells 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 be-
tween units is still poor, requiring hand matching for ste-
reo. An additional problem is that a FET will only act as
a pure resistor with very small signals applied so it is nec-
essary 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 frustrat-
ing trade-off: less noise = more distortion.
A number of VCA’s based on the exponential volt-
age-current characteristic of a bipolar junction transistor
have been used. One of the most common is called a
“transconductance amplifier”. Using the inherent match-
ing obtained by integrated circuit technology, these de-
vices have very predictable control characteristics.