Ashly LIMITER/COMPRESSORS CL-100 User manual

ASHLY
owner’s manual
CL-100 CL-5OE GL52E
LIMITER/COMPRESSORS
ASHLY AUDIO, INC.

TABLE OF CONTENTS
BRP OCU CH ON essenza A,
If You Are In A Hurry
Mechanical Installation........ sisaan
OAE |
Gain, Ratio, Attack Time, Release Time, Output Level,
Bypass Switch, Threshold/Gain Reduction Indicators
Initial Setup And Checkout .............
TREOLY -ccceeresareee
and Limiters Do, Compressor/Limiter Circuitry, The Ashly
VCA (Voltage Controlled Amplifier), Detectors
Applications ...........ssesnenzznenzznznnna PIERSI: 19
The Compressor/Limiter asa Protective Device, Alternatives
Voice-Over Compression (Ducking), Stereo Operation
Gain Reduction Meter Display, Output Level Meter Display,
In Case of Trouble ............ssenuzzznnnnnznennnnenanse 28

The
Ashlv
CL-SERIES
of
Peak
Compressor/Limiters
was
designed
in
response
to
the
need
for
a
universal
peak-sensitive
automatic
gain
control
(AGC)
device
with
audio
performance
comparable
to
that
of
a
professional
mixing
console.
It
took
over
five
years
of
research
to
realize
this
goal,
with
Ti
much
of
that
time
spent
in
developing
a
wide-bandwidth,
ultra-low-
distortion,
low-noise
VCA
(voltage
controlled
amplifier).
The
resulting
product
is
a
versatile
and
highly
listenable
compressor/limiter
suitable
for
use
in
professional
sound
reinforcement,
recording
and
broadcasting.
The
Ashly
CL-52E
Stereo
Peak
Compressor/Limiter
was
developed
to
meet
the
need
for
a
stereo
compressor/limiter
that
maintains
correct
stereo
imaging
at
all
times.
The
CL-100
is
the
half-rack
version
of
the
CL-50E.
The
greater
part
of
the
text
and
illustrations
in
this
manual
therefore
refers
to
the
CL-50E.
Special
features
of
the
CL-52E
are
explained
separately.
We
ask
that
you
please
read
this
instruction
manual
thoroughly
before
operation
so
that
you
may
realize
all
the
features
and
benefits
that
the
Ashly
Compressor/
Limiter
has
to
offer.
Q
IF
YOU
ARE
IN
A
HURRY
We
will
assume
that
you
are
familiar
with
compressor/limiters
in
general.
Please
keep
the
following
points
in
mind:
1.
For
safety,
your
CL-50E
or
CL-52E
should
be
connected
to
a
standard
ground
AC
outlet
supplying
120
volts,
60Hz.
The
CL-100
uses
standard
remote
RAP-6
power
pack.
This
should
also
be
connected
to
a
grounded
120
volt,
60Hz
outlet.
2.
The
inputs
and
outputs
of
Ashly
compressor
[limiters
can
be
used
as
either
balanced
or
unbalanced
depending
on
the
connector.
For
unbalanced
connections,
use
a
standard
mono
Phone
plug.
For
connections,
use
a
stereo
phone
plug
with
the
(+)
signal
on
the
tip,
the
(-)
signal
on
the
ring,
and
the
ground
on
the
sleeve.
3.
For
a
good
starting
point,
set
the
input
and
output
levels
at
“0”
and
T
the
ratio,
attack,
and
release
controls
at
mid-rotation.
Apply
signal
and
adjust
from
there
to
suit
your
application.
4. If you have problems, please refer to the more detailed sections and to
the troubleshooting guide in this manual. Q
MECHANICAL INSTALLATION
The CL-50E and CL-52E Compressor/Limiters mount in a standard 19
inch equipment rack. The mounting screw threads vary with different rack
manufacturers and you should refer to your rack instructions for proper
hardware. An oval head or flat head screw with a plastic countersink washer
is preferred to protect the finish of the compressor/limiter under the screw.
The CL-100 is housed in an industry standard half-rack enclosure which
provides a variety of mounting options. It can be used free-standing, in a half-
rack-sized enclosure, mounted in a vertical 19 inch adaptor with up to eight
other half-rack products, or side by side with another half-rack product in a
standard 19-inch rack. Mounting accessories are available from Ashly to
accomplish this.
Your Ashly compressor/limiter is housed in a rugged steel case and will
tolerate moderate abuse. However, for road systems which may be dropped
or otherwise subjected to extreme forces, we recommend some rear support
for the chassis to prevent bending the front panel when these forces occur. Q
WIRING
The CL-50E and CL-52E Compressor/Limiter should be connected to a
3-wire grounded outlet supplying 120 Volts, 50-60 Hz. Power consumption is
12 watts. The CL-100 uses a standard RAP-6 remote AC power supply.
The INPUT is a 10k ohm active balanced type on a standard stereo
phone plug that will accept levels of up to +20 dBV. The (+) or in-phase
connection is on the tip and the (-) or out-of-phase connection is on the ring.
To use the input as a common unbalanced type, simply use a mono phone
plug in the usual way.
The OUTPUT connection is a standard 1/4” phone jack and mates with a
standard phone plug such as a Switchcraft 280. Output impedance is 200

ohms,
and
full
headroom
is
realized
with
any
load
of
600
ohms
or
greater.
For
rack-mounted
unbalanced
audio
systems
the
output
ground
may
be
separated
from
the
case
ground
by
using
a
stereo
phone
plug
for
the
output
a
connection.
The
output ground
is
then
wired
to
the
ring
of
the
stereo
plugs
rather
than
the
sleeve.
In
this
manner,
ground
loops
in
the
rack
may
be
eliminated.
This
output
can
be
fed
to
a
balanced
input
by
wiring
the
(+)
input
to
the
tip,
the
(-)
input
to
the
ring,
and
the
shield
to
ground.
Stereo Tie
This
patch
point
is
used
to
tie
together
two
or
more
CL-50E’s
(or
CL-100's)
for
stereo
or
multi-channel
operation.
To
connect,
place
a
patch
cord
between
the
STEREO
TIE
connections
of
both
units.
If
signal
ground
isolation
is
being
used,
be
sure
to
use
a
stereo
patch
cord.
NOTE:
If
more
than
two
units
are
to be
tied
together,
use
a
common
“Y”
type
adaptor
to
split
the
signal
as
necessary.
The
Stereo
Tie
point
actually
represents
the
DC
output
of
the
detector circuit.
When
tied
in
parallel
for
stereo
operation,
the
VCA's
of
the
two
units
will
both
respond
to
the
most
negative
control
voltage
present
at
the
Stereo
Tie
point,
thus
insuring
that
the
stereo
image
never
shifts
left
or
right.
The
Stereo
Tie
points
of
CL-50E’s
and
CL-100's
may
be
directly
interconnected.
Detector Patch
The
CL-50E
and
CL-100
Compressor/Limiters
have
a
DETECTOR
PATCH
point
which
can
be
used
in
conjunction
with
an
equalizer
to
produce
discussed
later.
Alternatives
For
Compressor/Limiter
Sound
System
Installations
To
install
the
CL
unit
in
a
sound
system
using
a
passive
crossover,
insert
it
between
your
mixing
console
output
and
the
power
amplifier
input.
For
systems
using
electronic
crossovers,
there
are
two
ways
to
use
an
Ashly
compressor/limiter.
If
it
is
inserted
between
the
console
output
and
the
crossover
input,
it
will
act
on
the
entire
audio
frequency
spectrum.
+
Alternately,
the
compressor/limiter
may
be
inserted
between
an
output
of
the
electronic crossover and the input of a power amp, in which case it will only
affect a specific band of frequencies. 0
CONTROLS AND INDICATORS
Gain
The gain control is used to match the level of the signal source to the
fixed 0 dBV threshold of your Compressor/Limiter. It should be adjusted so
the yellow threshold indicator illuminates only when the signal reaches the
maximum level desired. Then, above this level, the Compressor/Limiter will
tend to minimize further increases in level.
Ratio
This control determines the ratio of change in output level to changes in
input level for all signals above threshold. The numbers printed around the
Ratio control are calibrated in dB and indicate the dB increase in input above
threshold required to produce a 1 dB increase in output. This can be
expressed conveniently as a ratio. If the output remains constant no matter
how high the input level, we have an infinite (cc) input/output ratio. It should
be remembered that the Ratio control has no effect on signals which are
below threshold, since the CL-SERIES products do not alter signals at below-
threshold levels.
There is a common but incorrect notion that limiting always implies the
use of an infinite ratio. Although there are times when an infinite ratio is
desirable, there will be situations where infinite, or “hard”, limiting action is
neither appropriate nor necessary. In fact, it should be noted that an infinite
ratio setting is likely to cause noticeable side effects in the sound, and may
not be usable on programs where subtle control is desired.
Attack Time
The response of the compressor/limiter to signal levels above threshold
is further defined by the ATTACK TIME control. Attack time is the amount
of time that the unit takes to attenuate the output level after threshold has

been reached. For verv fast transients, such as hand claps, snare drums, or
other percussive sounds, a very fast attack time is usually desirable so that
the limiter can respond in time to control the peak level. On other types of
program material, a slower attack time may be preferred. A too-fast attack
may, on some material, “square off” the top of a waveform, producing a
distorted sound. The Ashly CL Series provides continuously variable attack
times from 200 microseconds to 20 milliseconds.
Release Time
Another parameter which affects compressor/limiter performance is
Release time, or the time required for the limiter to restore system gain to
normal after the input signal has fallen below threshold level. Again, proper
Release time will depend on the type of program material being processed
and the way in which the limiter is being used.
When subtle limiting is desired, slow release times are often chosen to
avoid a condition referred to as “pumping” or “breathing”. This occurs when
overall gain is modulated up and down by repeated peaks which are followed
by quieter intervals. If the release time is set too fast, then the overall level
will jump up and down, producing an objectionable and unsettling effect.
Note that, in some cases, an individual track or channel which seems to be
pumping may sound acceptable when heard in context of a complete mix.
A unique feature of the Ashly Compressor/Limiters is the incorporation
of
a
double
release-time
constant.
When
a
conventional
compressor/limiter
is
adjusted for slow release times, transients such as mic “pops” and record
scratches
may
cause
a
severe
reduction
in
gain
followed
by
a
slow
fade-up,
making
the
action
of
the
limiter
very
obvious.
With
the
double
time
constant,
release
from
gain
reduction
after
a
brief
transient
is
always
fast,
with
a
slower release after a sustained overdrive.
Output Level
Because
compression
and
limiting
are
both gain
reduction
processes,
the
output
of
a
compression/limiter
is
frequently
at
a
lower
level
than
the
nominal
system
level.
To
make
up
for
this
loss,
an
OUTPUT
level
control
is
provided to restore up to 18 dB of system gain. NOTE: When the unit is in
the BYPASS mode the OUTPUT control still functions.
Bypass Switch
This switch enables you to quickly switch the CL compressor/limiter in
or out of the audio chain, making comparisons between processed and
unprocessed signals easy. When the switch is in the OUT position, all
limiting and compression controls and functions are bypassed, with the
exception of the output control, which continues to function as a
straightforward level control. For all normal compress and limit functions,
this switch should be depressed to the IN position.
Threshold/Gain Reduction Indicators
Five LED’s on the CL-50E and CL-100 front panels provide a convenient
visual indication of the amount of gain reduction that is taking place at any
time. As soon as the 0dBV threshold level is reached, the yellow LED
illuminates. Depending on how far the input level rises above threshold, and
the settings of the RATIO, ATTACK, and RELEASE controls, successive red
LED’s will illuminate, indicating increasing gain reduction.
Gain reduction is a useful way of expressing compressor/limiter action.
We have seen that the increase in output level of a compressor/limiter is less
than the increase in input level by some amount. Using a limit ratio, output
level will remain nearly constant as input levels increase above threshold.
With a gentle ratio, say 2:1, input signals above threshold will be “gain-
reduced” at the output by exactly 1/2. Thus, gain reduction can be expressed
as input level increase divided by output level increase. For example, a +12
dBV input signal that is 3:1 compressed will produce 4 dB of change at the
output, and 8 dB of gain reduction has occurred (12 dB input minus 4 dB
output = 8 dB gain reduction). The -6 dB red LED on the CL-50E or CL-100
front panel would be seen to light up because we are past 6dB reduction, but
not yet to 10dB. Q

INITIAL SETUP AND CHECKOUT
Exact
control
of
system
headroom
requires
defining
the
clipping
point
of
the
amplifiers
in
relation
to
the
nominal
zero
VU
of
the
mixing
console.
A
signal
generator
and
oscilloscope
would
be
helpful
in
determining
this
point.
Since
many
people
do
not
have
the
facilities
or
time
to
set
up
their
systems
in
this
way,
an
alternate
approach
is suggested.
First,
realize
that
the
sound
system
may
have
several
different
“0
dB”
levels,
especially
if
equipment
of
different
manufacturers
are
being
mixed.
0
VU
on
the
mixer's
meters
may
or
may
not
correspond
to
a
0
dBV
level
of
.778
Volts.
Also,
note
that
input
sensitivities
of
power
amplifiers
also
vary.
1. Begin by turning the crossover level controls down.
2. Switch the unit’s LIMIT switch to the OUT position.
3. Set both the GAIN and OUTPUT level controls to 0 dB.
4
L. Set the RATIO control to 10 and adjust the ATTACK and RELEASE
controls to a relatively fast setting.
5.
Plug
a
good
quality
music
source
into
the
mixing
console
and
adjust
the console so that its meter peaks at around 0 VU.
6.
Look
at
the
LED
display
on
the
CL-50E
and
adjust
the
GAIN
control
so
that
the
yellow
THRESHOLD
LED
lights
only
on
peaks.
Note
the
new
position
of
the
GAIN
control
and
adjust
the
OUTPUT
control
inversely.
That
is,
if
you
increased
the
Gain
control
to
+10
dB
in
order
to
light
the
Threshold
LED,
then
decrease
the
Output
control
to
-10
dB.
Or,
if
you
had
to
decrease
the
Gain
control
to
light
the
Threshold
LED,
then
increase
the
Output
by an
equal
amount.
You
have
now
gotten
the
console
and
the
compressor
[limiter
into
relative
agreement
over
what
is
a
maximum
permissible
signal
level;
as
long
as
the
console’s
output
meter
stays
out
of
the
red,
little
or
no
limiting
will
occur.
At
or
above
console
0
VU,
limiting
will
take
place.
8.
Turn
the
input
volume
control
of
your
electronic
crossover
up
to
its
nominal
zero
point
(for
Ashly
crossovers
this
is
#7).
Turn
the
OUTPUT
level
controls
of
the
crossover
up
until
you
achieve
a
good
musical
balance
of
low
and
high
frequencies,
and
continue
to
increase
the
output
volume
controls
until
the
sound
system
just
begins
to
sound
a
little
distorted.
Since
audible
distortion
may
already
be
around
5%,
its
a
good
idea
to
back
off
the
output
volume
controls
just
a
touch
after
you
first
begin
to
hear
any
distortion.
All
elements
of
the
sound
system
N
are now in agreement. Console OVU = Limiter Threshold = onset of
clipping in your power amplifiers. If you wish to allow extra
headroom in the system between console OVU and the onset of
clipping, turn down the CL-50E’s OUTPUT volume control by 5 dB,
10 GB, or whatever safety margin you desire. (You can turn the music
down now.)
9. Start with a 1 ms. ATTACK time, a .5 sec. RELEASE time, and a
RATIO of 10. Adjust these parameters according to the type of
program material, and firmness of control desired.
10. Activate the compressor | limiter by depressing the LIMIT IN/OUT
switch. Now, loud bursts of energy above 0 VU will cause little or no
clipping in the system. U
THEORY
The Need For Gain Control
The human ear excels in its ability to detect an extremely wide range of
loudness levels, from the quietest whisper to the roar of a jumbo jet. When
we attempt to reproduce this dynamic range, by means of amplifiers, tape
recorders, records, or radio transmitters, we run into one of the fundamental
limitations of these electronic media: limited dynamic range. Amplifier
dynamic range is quite good, and is adequate for most musical program
material. However, some types of audio equipment, such as cassette tape
recorders, have a very narrow useful dynamic range.
What is it that compromises the dynamic range of this equipment? The
useful operating region of a piece of audio equipment is squeezed in between
noise and distortion. As program level decreases, it approaches what is
known as the “noise floor”, and if the volume of the program material goes
lower still, it is engulfed by the noise. The noise floor, or minimum constant
noise level, will consist of hiss, hum, transistor noise, record scratches, tape
hiss, buzz and whatever noises are inherent in the medium. When the
program level is considerably higher than the noise floor, our hearing masks
the noise, and it is not a problem. However, when listening to very quiet
sections of a program for example, a pause between movements of a string
quartet the noise can become very bothersome.

Comparison of the dvnamic range of several common audio svstems.
At the other end of the loudness spectrum, the limitation on dynamic
range is usually distortion, either in the form of amplifier overload or tape
saturation. In most transistorized equipment, the transition from clean,
undistorted operation to severe distortion is very abrupt. Therefore, it is
common practice to operate a piece of equipment at a level that is somewhat
below the distortion point, leaving a margin of safety for unexpected,
transient volume peaks in the music. This safety margin is known as
headroom, and may range from 10 to 25 dB. Lowering our standard
operating level to leave ourselves some headroom helps prevent distortion,
but at the same time it moves our average program level closer to the noise
floor, thereby compromising signal-to-noise performance. It becomes
apparent that to get the most out of an audio system, you have to keep your
standard operating level as high as possible without risking distortion.
CLIPPING
nt Distortion Occurs Above This Level
The useful dynamic range of any piece of audio equipment
15 lmited by noise ard distortion. ey
Gain Riding
One
solution
to
the
noise
vs.
distortion
trade-off
is
to
keep
your
hand
on
the
level
control
and
manually
adjust
gain
to
suit
the
program.
Indeed,
there
are
times
when this
approach
is
entirely
satisfactory.
However,
in
most
types
of
music
there
are
instantaneous,
short
duration
volume
peaks,
or
transients,
which
would
be
difficult
to
anticipate
and
impossible
to
respond
to
with
manual
gain
riding;
you
simply
could
not
bring
the
level
down
fast
enough.
In
many
situations,
this
can
present
real
problems.
For
example,
in
recording,
an
extra
burst
of
enthusiasm
from
a
lead
singer
might
overload
the
capabilities
of
your
recording
tape,
causing
ragged
distortion
and
necessitating
another
take.
In
sound
reinforcement,
a
sudden
burst
of
energy
through
the
system
can
blow
fuses
or
even
damage
loudspeakers.
In
addition
to
the
problem
of
response
time with
manual
gain
riding,
it
also
requires
your
constant
attention,
which
takes
you
away
from
more
important
jobs.
The
need
for
a
fast-acting,
reliable,
automatic
gain
control
is
answered
by
limiters
and
compressors.
What Compressors and Limiters Do
Limiting
and
Compression
are
closely
related
effects,
their
differences
sometimes
being
very
subtle.
Since
limiting
is
generally
an
easier
concept
to
deal
with
than
compression,
the
following
sections
will
talk
about
limiting
first,
followed
by
a
discussion
of
compression.
The
use
of
the
front
panel
controls
is
discussed
in
terms
of
limiting
also.
Bear
in
mind
that
these
controls
will
have
the
same
function
when
the
CL-SERIES
unit
is
used
as
a
compressor.
Limiti
In
any
musical program
there
are
constant
changes
in
loudness.
It
is
the
jobofa
limiter
to
detect
when
the
volume
has
exceeded
a
predetermined
level,
and
to
then
turn down
the
volume.
When
the
incoming
signal
returns
to
its
original
safe
level,
the
limiter
should
respond
by
restoring
the
gain
to
normal.
Thus,
when
the
level
is
within
a
specified
“safe”
range,
the
limiter
has
no
effect.
When
an
occasional
peak
occurs,
the
limiter
responds.
This
situation
is
completely
analogous
to
manual
gain
riding,
except
that
it
occurs
faster
and
more
consistently.

To determine which peaks are acceptable and which are too loud, the
limiter establishes a reference level known as the threshold, and then
responds to those peaks which exceed this point. In the Ashly CL-SERIES, ~
this threshold is fixed at OdBV. When the input level reaches 0dBV, the
limiter will begin to react.
If the source feeding the limiter is increased in overall level, then more of
the peaks will now be above OdBV; those peaks which were previously just
below threshold level will now be at or above threshold, in addition to the
volume peaks which had already been above threshold. Thus, varying the
input level to the limiter is inversely proportional to changing the threshold
level. On the Ashly CL-SERIES, this effect is easily accomplished by
changing the input GAIN control. Increasing the GAIN control is equivalent
to lowering the threshold point. A gain control range of plus or minus 30 dB
is provided. What the limiter does once a peak above threshold is detected is
determined by the RATIO, ATTACK TIME, and RELEASE TIME controls.
Compression
There is a good deal of disagreement and confusion in the audio industry
over the definitions of limiting and compression, and the terms are
frequently and incorrectly swapped back and forth. The fact is, compression
and limiting are closely related effects, and the differences between them are
largely a matter of degree. Indeed, it could be said that the difference
between compression and limiting is, more often than not, a matter of what
you intend the device to accomplish.
Returning to our original definition of limiting, we stated that gain
reduction occurred when input signals rose above a predetermined threshold
level of 0 dBV, and that the effective threshold could be varied by changing
the GAIN control.
A very significant difference in dynamic range is achieved simply by
changing the relationship between nominal signal level and threshold, as a
result of varying the GAIN control. The most interesting effect to be noted,
however, is seen by comparing the original Input signal with the Output
signal.
The
quietest
portions
of
the
original
signal
will
be
effectively
+
increased
in
volume
while
the
volume
of
the
loudest
portions
of
the
original
l
signal will be decreased. In effect, both ends of the dynamic spectrum will be
pushed toward the “middle”. This is quite different from simple limiting,
where only loud peaks are subjected to gain reduction. More than
anything else, it is this double-ended effect which distinguishes compression
Remember, in all of these examples, the only control which was
changed was the GAIN control. The RATIO, ATTACK TIME, and
RELEASE TIME were assumed to be identical in all cases. The only factor
which turned limiting into compression was a change in the relationship
between input level and threshold. Compression is further differentiated
from limiting by careful selection of Attack and Release times.
When limiting is employed to protect an audio system against
transient volume peaks and possible overload, Attack time is 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 normal 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 being altered.
It should now be clear that the Ashly CL-50E, CL-100, and CL52 can
function as either limiters or compressors, due to the wide range of control
parameters which are available to the user.
Compressor/Limiter Circuitry
Volt Controlled ifiers
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 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.

The Ashiv CL-50E, Compressor/Limiter
GAIN
Adjusts the input
sensitivitv so gain
control action starts at
the proper signal level.
393
e :
s
E oux e
- in
p +30
(dB)
J
ATTACK
Adjusts the speed at
which the gain is
reduced in response to a
signal above the
threshold level.
IN/OUT
This switch enables or
defeats the action of the
compressor/limiter.
RATIO
Adjusts the “rigidity” of
the ceiling placed on the
signal leval.
J
RELEASE
Adjusts the speed at
which the gain is
increased as the signal
falls below the
threshold level.
GAIN
REDUCTION
This indicator displays
the gain control action of
the compressor/limiter.
Gain Reduction (dB)___
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