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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 dynam= ics processing is very four-dimensional and not-so-well understood. Conversat= ions 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" unt= il taking into consideration the VU meter’s slow response time. By adj= usting for less than a full dB of (displayed) Gain Reduction, the "6386&quo= t; came alive, making me realize that older products with VU meters (Fairchi= ld, UREI, Teletronics or Neve) might also benefit from a kinder and gentler approach. (Translation: more is going on inside than meets the eye outsid= e.)
For any device with a mechanical VU meter — fr= om 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 assis= t 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 stubbo= rn brain and an ego can easily get in the way of sonic perception — ye= t 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).
With all of that in mind, I welcomed an e-mail that =
came
from Crane Song’s David Hill (former designer of OOPS! An investigation into our vintage audio heritage is helpful both for t= he "happy accidents" that occurred (and endured) as well as gainin= g a greater knowledge of what made these great boxes tick. This article is an overview, with another planned to cover "just" the supporting a= mplifier 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 lig=
ht
dependent resistor (a.k.a. photo-resistor or photocell) as optical receiv=
er
(as shown in Photo-1a and Photo-1b, respectively).
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.
Table One: Topology Gain= Reduction "Window" ** Note: The "approximate" range of Gain Reduction is typic= al for what is practical for each topology.**
SPEED OF LIGHT A photocell, responding to light, decreases its resistance as the amou= nt 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 pro= tection — 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-line= ar (initially fast, then slow). When constantly bombarded with light —= as would be the case with heavy compression — the Release time increas= es, developing a "memory" that in essence contours the release curv= e to be "program dependent." Most users would consider these anomali= es "the happy accident," for it is nearly impossible to make a cla= ssic optical compressor / limiter sound bad.
The response-time limitations mostly work in the opt= ical 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 track= ing. One exception is the Pendulum Audio OCL-2, which uses a proprietary appro= ach 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 contras= t, the LA2-A and LA3-A are simple two-knob device= s 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 min= imal 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 "b= lack box" with input, output and control connections. In order to establi= sh "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 milita= ry-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.
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, th= e BLUE line indicates the path of the Control Voltage (CV) — from the dete= ctor 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 a= re 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, i= t 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 lev= el 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 devi= ce and expect some funky, most likely undesirable sound.)
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 run= s, both black- and silver-faced), a discrete audio path and an output transformer. The early Allison Research Gain Brain is also an FET compres= sor / 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 11= 76 — a dual-pot configured as a constant-impedance attenuator. To sati= sfy the input transformer, the attenuator must be pre-transformer as w= ell as pre-FET. Extra circuits can help minimize, but not eliminate, t= he 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 face= t 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 th= at it operates within its linear region. The Attack and Release times can be much faster than an optical-based product.
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 matchin= g 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.
There are two basic types of VCA, the log / anti log amplifier and the trans-conductance amplifier. For the log / anti-log typ= e 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 res= ponse decreases as gain reduction increases. Mis-matched transistors in the VCA= can create a strange character of distortion than may occur at one gain setti= ng and not another. The trans-conductance amplifier looks like a discrete differential amp= lifier — 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.
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 system= s.
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 cos=
tly
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.
If you are unsure about how to set the various parameter options ̵= 2; 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 (f= or example) and once coming from tape (or your storage medium of choice). Combining the fastest Attack with the fastest Release settings can cre= ate undesirable distortion. Most Compressor / Limiters feature a range of Att= ack times that are faster than the range of Release times making it easy to s= mash transients. (These same transients are also responsible for "image localization.") To keep the sound "alive" without destroyi= ng 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 effe= ct, then back off. This will even out the dynamics so th= at, on the second pass, conservative but more traditional settings — Fast Attack, Slow Release — will have a more dramatic and consistent eff= ect. 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 o=
f this
is the vocal effect (of the pitch corrector) on Sidebar-1: Detec= tor 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 sp=
eed,
some type of threshold circuit (soft-knee, hard knee plus ratio) and perh=
aps
an analog computer circuit. The side-chain control-voltage should corresp=
ond
as closely as possible with the useable range of the gain control element=
.
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 t= he 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 t= hat 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 repeata= ble 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, pos= t-processing, that is, after the gain control device has done its job. Now the compress= or limiter can "see" what it has done and literally tailor the processing to the signal, now more than ever, a moving target. The feed-b= ack topology makes the front panel settings — Attack and Release times = as well as the Ratio and Knee — more arbitrary, which to some users ca= n be slightly disconcerting.
With the 2500, we have added a patented circuit of ATI’s. it is a inverse pink noise respon= se 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 red= uce highs or lows or both from the link, eliminating the cross-coupling pumpi= ng, 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 dynam= ics 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.
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