AN OVERVIEW OF COMPRESSOR /
LIMITERS AND THEIR GUTS

Gain
Control Devices, Detectors, Side Chains & Audio "make-up"
Amplifiers

ã 1999 by Eddie Ciletti and David Hill with comments by Paul Wolff at API

SEE VU PLAY

Every compressor / limiter review reminds me that the subject of dynamics
processing is very four-dimensional and not-so-well understood. Conversations
with a manufacturer-designer can often be revealing, as was recently the case
with Greg Gualtieri of Pendulum Audio. His physicist background literally
shed some light on the subject of optical limiters, specifically how he
overcame the idiosyncrasies of the devices used in his OCL-2.

I tend to be heavy-handed when testing compressor / limiters, slamming the
meters just to see how "bad" things can sound when abused. (To me,
the better boxes don’t sound that bad under aggressive settings.) My
approach did not make the Pendulum 6386, Variable-Mu "sing" until
taking into consideration the VU meter’s slow response time. By adjusting
for less than a full dB of (displayed) Gain Reduction, the "6386"
came alive, making me realize that older products with VU meters (Fairchild,
UREI, Teletronics or Neve) might also benefit from a kinder and gentler
approach. (Translation: more is going on inside than meets the eye outside.)

PW:I do the same thing. Being
"heavy handed" is an easy way to see what the features do at
their extremes, then you can easily imagine
what they would sound like at a lower level. I also make comparisons by
summing "dry" and "processed" sounds together, but
out-of-phase. Once you identify the "differences," you can
then go back and listen for those things within the sound. It has
really increased my ability to perceive the "little" things,
making me feel like I really am good…

For any device with a mechanical VU meter — from
analog tape machines to signal processors and pre-amps — the
meter’s response time must be a consideration, especially when
processing "transient rich material." While my goal is to assist in
the process of user-education, my own process of evaluation was
"adjusted" simply by the good fortune of finding people who are
patient enough to answer questions.

I re-learned that it is important to listen — the eyes, a stubborn
brain and an ego can easily get in the way of sonic perception — yet it
is equally important for products to accurately display the work that is
being done. (My other soapbox is ease of parameter access, especially with
digital gear).

PW:Ego? What
!@#$ing ego? Are you accusing me of having a large, over-blown
ego? Well !@!$ you. I don’t have a $#@ing
ego. Oh>>>sorry>>>errrrr back to work…

With all of that in mind, I welcomed an e-mail that came
from Crane Song’s David Hill (former designer of Summit gear) and now creator of the STC-8
and Trakker (dynamics manipulators). As a designer-manufacturer, David is not
alone in feeling that educated consumers make better customers and so with
his permission, his "detailed correspondence" was used as the
foundation for this column.

OOPS!

An investigation into our vintage audio heritage is helpful both for the
"happy accidents" that occurred (and endured) as well as gaining a
greater knowledge of what made these great boxes tick. This article is an
overview, with another planned to cover "just" the supporting amplifier
technology and its affect on the "sound." If you’re new to
compressor - limiters and just wanna get a leg up, skip to the paragraph
labeled "Questions."

DEARLY BELOVED

Surely the Teletronix LA-2A is the most famous and beloved Optical
Compressor / Limiter. A close second is its transistorized successor, the
LA-3A. The heart of both units is the T4 "optical attenuator,"
consisting of an Electro-Luminescent (EL) panel as light source and a light
dependent resistor (a.k.a. photo-resistor or photocell) as optical receiver
(as shown in Photo-1a and Photo-1b, respectively).

Electroluminescent
"EL" Panels come in all shapes and sizes.  Originally
intended to back-light aircraft instrumentation panels, these high
impedance devices are particularly well-suited to operation at 400Hz
(power).

Variations

on a Photo-Resistive theme.

EL Panels are sold commercially as "Nite-Lites" under several
brand names.

The "other" Gain Control topologies include a vacuum tube /
circuit called "Variable-Mu," the Field Effect Transistor (FET) and
the Voltage Controlled Amplifier (VCA). (See Table One for a quick
comparison.) All will be discussed in this article, "Optical"
first, because it’s easiest to understand.

TOPOLOGY

** Approximate
Range

of Gain Reduction **

Variable-Mu

12~25 dB

Optical

25~30 dB

pwm-FET

30 dB

FET

40~50 dB

VCA

100 dB

Table One: Topology Gain Reduction
"Window"

**
Note: The "approximate" range of Gain Reduction is typical
for what is practical for each topology.**

PW:Notice that all feedback type compressors have a max
reduction of about 30 dB. That is
because you can never get a feedback-based device to compress with a
ratio of more than 5:1. If you take the FB circuit and turn it into an
FF circuit — optimize it for 20:1 ratio, then switch it back to FB
mode — it will never get over 5:1 because of the gain riding FB
does.

PW: The units
that go over 5:1 typically have gain in the control voltage. If you were
to take this control design and turn a 20:1 FB circuit into an FF
circuit, the ratio would be over inf:1 and would be doing negative
reduction. (The dbx 160X had this feature.) I discovered this while designing the 225. As far as
I know, I am the only person to make it — FB and FF — a
selectable option.

PW: As an
experiment, I added gain to achieve a 20:1 ratio, but because of the
speed, it was on the edge of becoming unstable. The sonically strange
thing is that a 5:1 ratio in FB mode sounds more compressed than 5:1
should. That’s because you can’t really accurately correlate
ratios on FB compressors because they act differently with tones as
compared to program material. It’s a "dynamic" thang!

SPEED OF LIGHT

A photocell, responding to light, decreases its resistance as the amount
of light increases. The time it takes for this resistance change to occur
varies with optical device design and typical production tolerances.
Photocells have a built-in "Attack" time-constant limiting their
ability to quickly respond to transient signals. When used as a
"Limiter," optical devices are not fast enough for overload protection
— especially in digital-land — where there’s nowhere to go
beyond 0 dB Full-Scale (0-dBFS).

In response to a transient, the photocell’s recovery is non-linear
(initially fast, then slow). When constantly bombarded with light — as
would be the case with heavy compression — the Release time increases,
developing a "memory" that in essence contours the release curve to
be "program dependent." Most users would consider these anomalies
"the happy accident," for it is nearly impossible to make a classic
optical compressor / limiter sound bad.

PW:The API 525 FB compressor
had a circuit called "release time over-ride" that allowed
transients to recover quickly and overloads to recover slower. The
reason was to "prevent the unit from sounding bad with
overuse". I think in those days there were more "tone
police," because the engineers could actually design.

The response-time limitations mostly work in the optical
device’s favor, though as a gain control device, the photo-resistor is
much less flexible than its competitors. It is nearly impossible to speed up
the response time of a photo-resistor short of testing and selecting the
fastest devices. Temperature and time affect these components more than
others, making it difficult to achieve and maintain accurate stereo tracking.
One exception is the Pendulum Audio OCL-2, which uses a proprietary approach
to speed up and "tame" the optical device.

OPTICAL TRANSITION

Recent products — both analog and digital — may have more
signal-processing ability, but in many cases the interface obfuscates the
user’s ability to take advantage of the available power. By contrast,
the LA2-A and LA3-A are simple two-knob devices that
are, in many cases, just right for vocals and bass guitar. They serve as a
reminder that less truly is more. Inside, the aforementioned
"classics" are as basic as they appear on the outside. Only minimal
circuitry is needed, one amplifier to drive the EL panel and another
amplifier to "make-up" (recover) the gain lost by processing. Oh
yeah, let’s not forget the power supply!

Later, in the neo-IC age, the designers of the UREI LA-4 substituted an
LED for the EL Panel. In order to drive the "transmitting" LED, a
"detector" circuit must be added. The perfect lead-in to a more
global concept…

THE BLACK BOX

All Gain-Control topologies can be modeled as a three-terminal "black
box" with input, output and control connections. In order to establish
"control," it is first necessary to convert the AC signal (via
rectification) into a DC Control Voltage (CV) that corresponds to the
variations in signal amplitude. This circuit can be designed to
"detect" RMS or "peak" information plus the ability to
manipulate parameters such as Attack, Release, Ratio and Slope (HEY---MY WIFE
IS ASIAN…). For more information, see theSidebar:
Detection and Side-Chain.

VARIABLE-MU

The highly treasured Fairchild 670 is one vintage example of the
"Variable-Mu" circuit. More recent versions include the Manley Labs
"Variable-Mu" and the Pendulum Audio "6386." In the
latter instance, the model number refers to the vacuum tube used for the
task. No longer in production, the "6386" is a Five-Star military-grade
dual-triode vacuum tube made only by General Electric (GE). All products
currently manufactured are using "New Old Stock" (NOS) tubes, so
the supply is obviously limited.

The vacuum tube predecessor to the classic 1176,

this is the front-end of the Universal Audio 175-B

( version 175-C had a ratio switch like the 1176).

Photo Two is a simplified schematic of the front end of the
Universal Audio 175 "Limiting Amplifier," the vacuum tube
predecessor to the "solid state" 1176 and a Variable-Mu device.
Changing the grid-to-cathode voltage results in a corresponding change in "mu,"
(m ) the gain of the circuit. In this case, the BLUE
line indicates the path of the Control Voltage (CV) — from the detector
and side-chain through a voltage divider — to vary the bias of each
half of the tube.

For "good" performance over its useable range, adjustments are provided
so that each half of the Variable-Mu tube can be balanced to minimize the
amount of CV "feed-through" into the audio path (the red arrow).
From a purest point of view, CV feed through is bad, though it can also be
part of the "sound" that is desired in some cases. One example
would be putting some "attack" back into a kick drum (something
that also happens with noise gates). Although the total amount of Gain
Reduction is limited for Variable-Mu when compared to the alternatives, it is
easier than optical to make a balanced stereo compressor.

FET CONTROL ELEMENTS

Like optical, the Field Effect Transistor (FET) acts as a voltage
dependent resistor. Although it might sound like the perfect answer to a
photo-resistor, it’s not. As a gain control element, the FET is level
sensitive. Very large signals can modulate the device resistance causing a
gain-modulated distortion that is independent of the control voltage used to
manipulate the gain. (Translation: overload the input of a FET-based device
and expect some funky, most likely undesirable sound.)

PW:in the API
525, the signal is padded down 30 dB and then boosted back up with the
FET going to ground on the "—" input of the input
op-amp, controlling the gain to minimize
the effect of gain-modulated distortion. Another reason that that FB compressors were "limited" to 30 dB
of Gain Reduction is that they could only attenuate as much as they were
set up to lose. A reasonable point considering that 30 dB of gain did
not add a whole lot of "extra" noise.

1176: A Very Good Year

The Universal Audio (later UREI) 1176 is a vintage FET compressor /
limiter that is extremely popular with vocalists. The most favored of
versions include an input transformer (eliminated in later production runs,
both black- and silver-faced), a discrete audio path and an output
transformer. The early Allison Research Gain Brain is also an FET compressor
/ limiter that is both transformer-less and unbalanced.

REALITY CZECH

To make the FET work as a gain control element, the signal level must be
kept low requiring more than the usual amount of gain from the "make-up
amplifier." One example is the Input control on a Universal Audio 1176
— a dual-pot configured as a constant-impedance attenuator. To satisfy
the input transformer, the attenuator must be pre-transformer as well
as pre-FET. Extra circuits can help minimize, but not eliminate, the
inherent distortion of this topology. What remains is part of the sound of
any FET-based device. (Discussed in a future article, more than one output
amplifier design was used as the 1176 evolved, affecting yet another facet of
its sound.)

For stereo operation, FETs must first be matched and the circuit must
include adjustments to "find the threshold" of the device so that
it operates within its linear region. The Attack and Release times can be
much faster than an optical-based product.

PW:The 525 could match the
channels quite closely. They used a 5-kohm, feedback resistor (variable)
with the FET connected from the "minus" input of the op-amp (a
"virtual" ground) to ground. By adjusting the gain to match
the unity level, they were almost always gain matched. The only other
problem was distortion matching, which was few and far between,
requiring some FETS to be replaced.

VOLTAGE CONTROLLED AMPLIFIERS

Voltage controlled amplifiers, or VCAs, have been around for a while. They
are the most common method of gain control, followed by the FET. Early VCA
devices did not sound very good, two of the difficulties included matching a
half-dozen or so transistors as well as keeping them all at the same
temperature. Modern VCAs are much better because they are
"monolithic," a.k.a, an "integrated circuit," or IC.
Since all of the transistors are grown on the same piece of silicon, they match
and stay at the same temperature.

PW: They used to be called
"modulators", and the original Blackmer patent was never challenged on this, as no one ever
mixed dc with audio. The original VCA was Class-B and had thermal
relationships to distortion. That is why everyone in this country got
into the habit of leaving everything on! They (THE SILICON) were only grown in the same county, and had to be
potted. You first had to match the transistors, then
pot them.

PW: The Allison VCA was Class-A and
thermal relationships only effected gain. The
early Allison (blue) was also comprised of discrete devices, matched and
potted.

There are two basic types of VCA, the log / anti log
amplifier and the trans-conductance amplifier. For the log / anti-log type to
work well requires well-matched transistors with a "perfect"
relationship between base-emitter voltage and emitter current. Using this
over a large control range can run into problems. Increased gain reduction
results in less current flow in the circuit, which means hi-frequency response
decreases as gain reduction increases. Mis-matched transistors in the VCA can
create a strange character of distortion than may occur at one gain setting
and not another.

The trans-conductance amplifier looks like a discrete differential amplifier
— the gain is manipulated by changing the emitter current. Like the
FET, this type of circuit does not tolerate large signals well —
distortion sets in — and with large amounts of gain reduction it can
also lose high frequency response. Its well-matched transistors must also
have very low noise.

PW: Trans-conductance devices also were
noisy, comparable with a noise level of a good cassette deck.

Modern integrated circuit technology has resulted in great
improvement over the years, but the VCA can still suffer from control feed
through and have different types of distortion at different signal levels. A
major advantage of using VCA devices is that they can have a very useable
dynamic range, which is particularly useful in building automation systems.

PW: That corporation has pretty much licked the problems with
VCAs.

PULSE WIDTH MODULATOR

Using Pulse-Width Modulation (PWM) to control an FET is what Crane Song
uses in both the STC-8 and Trakker compressor / limiters. The technology has
been around for about 20 years, having been used by several companies,
including EMT. With modern technology, a PWM gain-control circuit can be very
fast — from 0 to full gain reduction in 500 nS (0.0005 mS or 0.5 uS)
and an audio frequency response to 60Khz or more.

PWM works as a gate, quickly switching the FET from "off" to
"more-on" by varying the pulse-width. With careful selection of
components and PC board layout it is possible to have very low control
feed-through, a very low noise floor and an audio-path frequency response
that does not vary with gain reduction. This type of circuit requires costly
high-speed components and as a result, there is a fair amount of power
consumption (it runs hot). Distortion is very low and does not change with
gain reduction.

CRANE SONG: Trakker
with 12 Classic Settings

CRANE SONG:
STC-8 Stereo Compressor with interactive Peak Limiter

PW:Dave is real smart. PWM is
the basis of the "Class-D" amplifier. Only a real engineer
would be able to think that up. It’s from having transmitters and
oscillators hammered into our heads at school.

Questions?

If you are unsure about how to set the various parameter options —
Attack, Release, Ratio, for example — it’s not a bad idea to play
it safe. Consider that the optimum range of some gain reduction devices is
limited. Rather than over-use a compressor / limiter in one pass, try
less-aggressive settings. Process the signal twice, once going to tape (for
example) and once coming from tape (or your storage medium of choice).

Combining the fastest Attack with the fastest Release settings can create
undesirable distortion. Most Compressor / Limiters feature a range of Attack
times that are faster than the range of Release times making it easy to smash
transients. (These same transients are also responsible for "image
localization.") To keep the sound "alive" without destroying
it, do the reverse of what is more-often the default.

On the first pass, go for the fastest Release time and start the Attack
time at its slowest setting using a ratio not higher than 2:1. It’s ok
to increase the Attack speed until it just starts to have a dramatic effect, then back off. This will even out the dynamics so that, on
the second pass, conservative but more traditional settings — Fast
Attack, Slow Release — will have a more dramatic and consistent effect.

Being "musical" is not necessarily the most desirable
end-result. Sometimes what might be considered undesirable artifacts could
end up being a cool effect. The best example of this
is the vocal effect (of the pitch corrector) on Cher’s
big summer hit "Do You Believe."

e-mail eddie

Sidebar-1: Detector
and Side Chain

The function of the Detector circuit is to convert (rectify) the audio
signal (AC) into a DC Control Voltage (CV) that corresponds with the
signal’s changing amplitude. The CV is then used to manipulate the gain
of the control device. If this sounds like the workings of an old analog
synthesizer, you are correct. Integrated around the rectifier (Peak or RMS
detection) is a time-constant circuit to manipulate attack and release speed,
some type of threshold circuit (soft-knee, hard knee plus ratio) and perhaps
an analog computer circuit. The side-chain control-voltage should correspond
as closely as possible with the useable range of the gain control element.

Sidebar-2: Feed Me!

There are two basic compressor / limiter topologies: feed-back and
forward-feed, each with their own characteristics, each appropriate for
specific types of signals. The block diagram, Figure-Three, is the easiest
way to understand the difference. I was lucky enough to witness a
demonstration by API designer, Paul Wolff, who included both options on the
API 225L compressor / limiter. The difference is so obvious.

API 225L from the Legacy Series

Forward-feed processors can have attack, release, and slope controls that
are labeled with fixed numbers — so that dialing up and achieving, a
slope of 4 to 1 and an Attack time of 50-mS yields consistent and repeatable
results. Two examples of Forward-feed processors are the dbx 160 and the Gain
Brain 2.

Flipping the switch at the input of the Detector from forward-feed to
feed-back now selects the audio signal, post-processing,
that is, after the gain control device has done its job. Now the compressor
limiter can "see" what it has done and literally tailor the
processing to the signal, now more than ever, a moving target. The feed-back
topology makes the front panel settings — Attack and Release times as
well as the Ratio and Knee — more arbitrary, which to some users can be
slightly disconcerting.

PW: That's why the controls don’t
work the same in both positions.

With the 2500, we have added a patented circuit of
ATI’s. it is a inverse pink noise response
curve filter (3 dD/oct hi-pass) to tip the detector response to equalize the
energy/oct of the signal. We call it THRUST, and it is quite cool. I have
also applied for a patent on using a filter in the linking circuit to reduce
highs or lows or both from the link, eliminating the cross-coupling pumping,
which sounds pretty cool.

The Fairchild 660 and 670, API 525 plus the Teletronix LA-2A and LA-3A and
UREI 1176 are all examples of the "Feed-back" approach to dynamics
manipulation.

Side Chain

Up until this point, we have only considered that the Detector’s
"job" is to create a DC voltage that corresponds to the signal to
be processed and assumed that there will be various tweak-able parameters
such as Attack, Release and Ratio. Most detector circuits are designed to
treat all frequencies equally, but sometimes it is desirable to insert EQ into
the side-chain so that the processor responds, for example, to high
frequencies only as would be the case for a de-sser, a hi-frequency
limiter.

Other possibilities include inserting a high-pass filter so that the
processor would ignore low frequencies. Or, a routing a kick drum to the side
chain input to make the processor "duck" a bass guitar with each
beat. Taking advantage of the side-chain can be very useful when trying to
either create more sonic space or control problem sounds.

End of sidebars.