Ashly LIMITER/COMPRESSORS CL-100 소유자 매뉴얼 - 페이지 9

{카테고리_이름} Ashly LIMITER/COMPRESSORS CL-100에 대한 소유자 매뉴얼을 온라인으로 검색하거나 PDF를 다운로드하세요. Ashly LIMITER/COMPRESSORS CL-100 18 페이지. Limiter/compressors
Ashly LIMITER/COMPRESSORS CL-100에 대해서도 마찬가지입니다: 소유자 매뉴얼 (18 페이지)

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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.