Manley Stereo Variable Mu Limiter/Compressor is a two channel valve compressor in its standard form, with continuously variable conductive plastic potentiometers. The design goes back to the Fairchild topology, where the valve itself acts as the gain control.
Glue you do not hear as compression
On a mix bus the problem is rarely a lack of compression, it is that you can hear it switch on. At two or three decibels of reduction the Variable Mu leaves no pumping and no flattened transients behind, and yet the instruments start playing together. A stepped input attenuator sets how hard you drive the valves repeatably, so transformer saturation is chosen independently of how much reduction you want. The illuminated Sifam meters show gain reduction clearly enough that you never have to guess how much is actually working.
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
- Standard version with continuously variable potentiometers
- Stepped input attenuator for a repeatable drive level
- 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: 5670 x2 in the standard version
- 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
