The Phædrus Audio PHAMULUS is now shipping!

PHAMULUS - δμ TUBE COMPRESSOR

FAMULUS n. (pl, ~li) the attendant of a sorcerer

Introduction - Compressors, Limiters and "Limiting Amplifiers"

Compressors and limiters enjoy a fundamental similarity. In a limiter, the knee of the compression curve (the threshold) is set way up the transfer function, after which a very high degree of compression is imparted on the input signal. In a compressor, a lower degree of compression (compression ratio) is imparted at a lower threshold. Most modern compressors provide controls to adjust both compression threshold and ratio, so it is logical to speak of these units as compressor/limiters because they can function as either.

However in older tube-based units, such as the equipment the Phædrus Audio PHAMULUS is based upon, these functions are combined in a gradual characteristic. The user operation of this equipment therefore largely relies on how "hard" the equipment is driven to alter the dynamic range of the incoming signal. By applying low-level drive - by means of reducing the drive via the input attenuator - this equipment will only impart a gentle compression characteristic.

By driving the equipment with a high-level signal (by simply advancing the "input" level control), more and more of the dynamic range will be compressed.

In fact, the distinction between the term "limiter" and "compressor" was not well defined until the nineteen-seventies, so you will often find classic compressors were referred to by their manufacturers as "limiters", "limiting amplifiers" or "compression amplifiers.

The Phædrus Audio PHAMULUS is based upon the classic design of the Altec Lansing 436C Compression Amplifier.

Vari-mu compressors: the Altec Lansing 436 and the EMI RS124

The most famous classic tube-based compressors are of the vari-mu family in which advantage is taken of technology developed for automatic-gain-control (AGC) for radio equipment. In a radio set, the radio-wave strength may vary due to reflections, obstacles; this being especially true for mobile radios in cars and aeroplanes. With amplitude modulation (AM) radio, this is problematic, because a change in the signal strength causes a change in detected output and thereby reproduced volume and this is very disturbing to the listener. This was a major problem with early "wireless sets" and a solution was eagerly researched.

The solution to the problem came in the shape of a post-detector, filter circuit with a time-constant long enough that it removed all the audio modulation and left just an overall DC voltage value dependent on the average amplitude of the RF carrier. This signal was then used to adjust the gain of the radio frequency (or intermediate frequency) amplifiers prior to detection. By this means, the overall value of the radio carrier was held steady and the volume of the detected modulation always remained that of the broadcast signal. How was the variable-gain accomplished? The answer lies in the inherent non-linearity of the valve (tube) which is deliberately exploited in radio valves to offer the AGC function.

All valves (tubes) have a power-law transfer-characteristic. That is to say that a given change in grid voltage does not result in an equal change in anode current over the operating range of the valve. By arranging the RF carrier to work only over a proportion of the overall characteristic and "sliding" the incoming signal over the entire operating characteristic, it is possible with any valve to derive a measure of variable gain, as illustrated below.

Suppose that - for the moment - the radio set is receiving a strong signal. In this circumstance, the filtered AGC control voltage taken after the detector is large and this is so arranged to "slide" the input signal to the left-hand portion of the curve in the figure above. If the signal level drops, initially, the volume of the signal drops too. But, in a fraction of a second (the usual control delay is about 1/4 of a second), the AGC control voltage falls and biases the operating range of the valve to the right of the characteristic, thereby restoring the RF level, and with it, the recovered audio. So important was this development in radio sets, that radio valves are especially designed to maximise the curvature of the transfer-characteristic, these types of vales being known as "vari-mu" valves: the μ (mu) relating to the coefficient of gain.

Another term for vari-mu is "remote cut-off" which describes the fact that the cut-off point for the valve (the negative voltage at which the anode current ceases), is made as "remote" as possible from 0V, thereby prolonging the curvature of the Vg/Ia characteristic. The alternative to "remote cut-off" valve is a "sharp cut-off" device. Audio valves are "sharp cut-off" types; which means that the Vg/Ia characteristic is made as linear as possible. This is one of the reasons why radio valves make poor devices for audio applications.......except as the basis for compressors!

Of course, we might justifiably call "volume compression" by the equally valid term "audio automatic-gain-control" or "audio-AGC", so it is not surprising that the earliest compressors adopted the radio technology and applied to audio signal circuits. The only difference between a volume compressor and an AGC circuit is that - in the case of the former - it is the audio signal itself which is rectified and used as the control signal, rather than the modulated carrier.

Altec Lansing 436C

The Altec Lansing 436C compressor amplifier (above) dates from the nineteen-sixties. The circuit works like this: large negative signal peaks on the anodes of the push-pull output-stage V2 cause double-diode V3 to conduct and drag down the bias volts to the input stage V1. This action is damped by the time-constant formed by R9, P2 and C4 which allows for some adjustment of the compressor's release time. The level threshold, at which the compression action is required, is adjustable too; by means of the simple expedient of biasing the cathodes of the double-diode V3 with a constant bias derived from the slider of P3. The compression action is obtained because, as signal peaks depress the bias volts on V1, the stage "slides" further down the input valve's curved Vg-k/Ia characteristic: the required voltage change for a given change in anode current is reduced and the stage-gain is lowered. The compression control-signal itself remains inaudible because it is applied equally to both input valves: whereas the music signal is applied differentially - in other words, the common-mode control-signal is cancelled out in the differential amplifier. The circuit design of the Phædrus Audio PHAMULUS is based upon this circuit.

The essence of the ALTEC 436 circuit upon which the Phædrus Audio PHAMULUS is based

EMI RS124

A famous, but rare, EMI-developed variant of the Altec Lansing 436 exists which included a modification to the input circuit to include a matched-impedance attenuator and added an output attenuator (both designed to operate at EMI's quaint, 200 ohm interface-standard). Importantly, from an artistic point of view, the original Altec circuit only included a variable release time preset (P2 in circuit diagram above), so this was added as a user-control by the EMI engineers, This greatly increased the scope of the compressor's duties to which has been attributed some Beatles' magic. The entire unit was retro-fitted with an EMI-designed escutcheon which disguised its Altec heritage.

The much vaulted EMI RS124 - at heart, a modified Altec

EMI's variable release user control has been adopted in the design of the Phædrus Audio PHAMULUS such that the control allows for adjustable release times between 0.1 sec (much faster than the original Altec) and 1.3 sec. This faster response widens the applications of the Phædrus Audio PHAMULUS to include compression duties for - amongst other things - drums.

Output attenuator

One criticism often levelled at the Altec 436 is that its output level is very "hot"; making it difficult to interface with other equipment. This is a valid criticism and the EMI engineers wisely added an output attenuator to the Altec chassis for the RS124. The PHAMULUS also incorporates a controlled-impedance attenuator (at a fixed 10dB attenuation), selectable by a front-panel push-switch.

The PHAMULUS front panel

Front panel controls

Note that the Phædrus Audio PHAMULUS also features a front-panel push-switch which enables quick selection between a factory-set threshold level ("compressor" position), and a variable threshold level (via the right-hand preset, labelled "limiter position). On shipping, the threshold preset is set at its maximum which - if never adjusted again - gives a simple way of switching between two regimes one ("compression") suited for tracking: the other (switch depressed) more suited for mastering or applying over a sub-mix. Alternatively, with the switch depressed, the "threshold" preset control may be adjusted for greater flexibility.

The compressor characteristics (as shipped) are illustrated in the figure below. Adjusting the "threshold" preset gives a family of characteristics between these two extremes.

Alternatively, because we are here talking of a "vari-mu" (variable-gain) compressor, it's instructive to see these characteristics in terms of gain at various input levels (measured using static 1kHz tones). This is illustrated here.

The dynamic response of the PHAMULUS is illustrated in the oscillogram below.

Meet the PHAMULI

The Phædrus Audio PHAMULUS has a unique stereo link feature, meaning that two PHAMULI (yes, that's the real plural form of PHAMULUS) may be linked together to form a stereo compressor. Ideal for mastering duties and adding that final "magic"!

Phædrus Audio PHAMULUS Specification

Type: Compressor Amplifier
Gain: +30dB (reduced to +20dB with output attenuator engaged)
Frequency Response: ±1.5 dB, 40Hz to 15kHz
Max Output Level: >+20 dBu (as straight amplifier at 1kHz)
Harmonic Distortion: At 25 db of compression: Less than 1.5%
Noise Level: 74 dB below rated output (—111 dBm equivalent input noise)
Input Impedance: 15k bridging transformer (earth-free)
Load Impedance: Normally bridging: 600 ohms if ordered specially
Maximum Compression: 30 dB
Attack Time: 50 milliseconds
Release Time: 0.1 to 1.1 seconds
Threshold: Adjustable: 0 dBm to -16 dbm output
Compression Ratio: 2:1 at 0 dBm threshold; 4:1 at +16 dBm threshold
Controls: Gain, Threshold, Release Time, Output atten', meter-trim,
Power Supply: 12V AC, 4 Watts
Tubes: 6BC8, 12AU7

6BC8 and 6BZ8 tubes

The tube chosen for the "vari-mu" (δμ) element in the Phædrus Audio PHAMULUS is the 6BC8/6BZ8 tube. This was the tube in the original Altec circuit. The 6BC8/6BZ8 tubes are meduim-mu, semi remote cutoff twin triodes, originally intended for applications in television receivers. Because theses tubes arrived late in the "age of thermionics" and were employed in the most sucessful consumer product of all time (the television), there exist many, many of these tubes as NOS (new old stock) at reasonable prices. This underlies Phædrus Audio's decision to adopt this tube.