PHAMULUS - δμ TUBE COMPRESSOR

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

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

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


Read Sound on Sound magazine's review of the PHAMULUS.

Altec Lansing 436C and the RS124

The Altec Lansing 436C δμ compressor amplifier (above) dates from the nineteen-sixties. The circuit of the Phædrus Audio PHAMULUS is based upon this circuit.

Abbey Road RS124

A famous, but rare variant of the Altec Lansing 436 is the Abbey Road studio RS124 which included a modification to the Altec input circuit to include a matched-impedance attenuator. Importantly, from an artistic point of view, the original Altec circuit only included a variable release time preset, so this was added as a user-control by the Abbey Road engineers too. This greatly increased the scope of the compressor's duties to which has been attributed some of The Beatles' magic. The entire unit was retro-fitted with a house-designed escutcheon which disguised its Altec heritage.

The variable release user control has also 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 Abbey Road 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 - internal view


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.











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