Manley Stereo Variable Mu Limiter Compressor with T-Bar Mod is the standard version of the compressor with a replaced gain control stage: instead of two 5670 dual triodes it runs four 6BA6 pentodes wired as single triodes.
Four pentodes instead of two triodes
Up to roughly six decibels of reduction the 5670 valves behave much like the original 6386 of the design this compressor descends from. Past that point, though, the sound starts to squash in a way you do not want on a bus. The T-Bar modification swaps them for four 6BA6 valves and restores the smooth curve of the original, so you can push deeper into reduction without the material sounding choked. The arrangement also delivers more gain, and separate pentodes can be matched in pairs for noise and microphony, which was far harder with dual triodes.
The valve itself as the gain control
A variable mu circuit has neither a VCA nor an optical cell. Gain changes through the bias voltage of remote cutoff valves, so the reduction is not applied to the signal, it is how the amplifying stage behaves. That produces the characteristic response: the harder you push it, the gentler the curve looks, instead of a hard knee at the threshold. The mode switch gives 1.5:1 in Compress and 4:1 to 20:1 in Limit, attack is continuously variable from 25 to 70 ms, and recovery has five settings: 0.2 s, 0.4 s, 0.6 s, 4 s and 8 s. A high pass filter in the sidechain at -3 dB @ 100 Hz keeps the low end out of the detector so the kick does not drive the whole mix.
KEY FEATURES
- T-Bar modification: four 6BA6 pentodes instead of two 5670 triodes
- Smooth limiting curve maintained past 6 dB of reduction
- Two channel compressor and limiter with an all-valve path
- Compress mode at 1.5:1 and Limit mode from 4:1 to 20:1
- Attack continuously variable from 25 to 70 ms
- Recovery in five steps: 0.2 s, 0.4 s, 0.6 s, 4 s and 8 s
- High pass filter in the sidechain at -3 dB @ 100 Hz
- Manley IRON transformers with nickel laminations
- Channel linking for stereo operation
- Hardwire bypass for comparison against the unprocessed signal
- Illuminated Sifam gain reduction meters
TECHNICAL SPECIFICATIONS
- Variable gain valves: 6BA6 x4 in the T-Bar modification
- Amplifying valves: 5751 or 12AX7 x2
- Output valves: 12BH7 x2
- Sidechain rectifier: 12AL5 x2
- Frequency response: 20 Hz to 25 kHz (±0.1 dB)
- Maximum gain: 35 dB
- Maximum output: +30 dBu with 26 dB of headroom
- THD: below 0.1% at 1 kHz
- Noise floor: -85 dB typical
- Balanced inputs and outputs, 600 Ohm
- Controls: conductive plastic potentiometers, continuously variable
- Dimensions: 19 x 3.5 x 10 inches, 2U; power consumption 80 W
