Ashly LIMITER/COMPRESSORS CL-50E Owner's Manual - Page 9
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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 vears, the need for a 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 repsonse 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
tradeoff: 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 -80dBV, which compares poorly to
straight linear amplifiers.
The best compromise to date is the "class AB current ratio multiplier."
Early implementations 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 are available, but integrated
PNP's are not. The PNP's must be hand-tested and matched. Careful
trimming is necessary for low distortion and even minor temperature
changes make re-trimming necessary because of differing characteristics
between the two types.
The Ashly VCA (Voltage Controlled Amplifier)
The Ashly VCA is an integrated current ratio multiplier circuit. It has
low noise (-90 dBV), low distortion (.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 trimming 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 provides it with a control voltage. Experience showed us that both are
crucial to the overall sound and that, if anything, the detectors performance
is the harder to judge by conventional measurement techniques. While the
VCA is doing its job if it has low noise and distortion, the detector must
constantly adjust the gain of the audio path in a manner which keeps the
level under control while sounding acceptable to the listener. This constantly
changing gain is a DYNAMIC action, and conventional audio measurements
like noise and distortion checks are STATIC (at a constant level). We became
painfully aware of this problem with some of our earlier limiter prototypes
which measured fine and sounded terrible. This led us to use a purely
subjective approach in the design of the detector-we did a lot of listening to
determine what sounded good and what didn't.