These Mono versions of our Stereo MANLEY VARIABLE MU® LIMITER COMPRESSOR were one channel versions of our later stereo unit, and the predecessor to it.

Manley Limiter Compressor Amplifier 1½ R U version 

Manley 10dB Limiter Compressor Monoblock 1 R U version

The Stereo Variable Mu® was our best selling product for many years. It is one of the very few compressors that has become a real standard in Mastering studios and contributed to most hit records over the last decade and probably the next. "Mu" is tube-speak for gain, and Variable Mu is our registered trademark for this limiter compressor. 

This type of limiter works by using the "remote cut-off" or re-biasing of a vacuum tube to achieve compression. The precious vintage Fairchild 670 also uses this technique and is one of few all-tube compressor to do so, that we know of. Even the side-chain has glowing rectifier bottles. How's it work? In the Mono Variable Mu units, the unique 6386 dual triode is at the center of the peak-reducing and compression action constantly being re-biased by the vacuum tube rectified side-chain control voltages which cause this tube to smoothly change its gain. Just like that. (These mono units were built back when we had 6386 tubes!)

The COMPRESS mode is soft-knee 1.5 to 1 ratio while the sharper knee LIMIT mode starts at 4 to 1 and moves to a more dramatic ratio of 20 to 1 when limiting over 12dB. Interestingly, the knee actually softens as more limiting is used. Distortion can be creatively used by turning up the Input and turning down the Output while using very little or no compression. See the gain reduction curves here! 

The earliest mono units were built at the VTL factory between 1991 and 1993. The very first prototype version of our limiter was this one sold to BOP Studios in Mmabatho, Bophuthatswana. It was called "PEAK-LIMITING LINE AMPLIFIER" and was built in our standard early 90's steel 1U chassis with the stainless-steel top. You can see the 0A2 gas regulator and the 12AL5 dual rectifer. This unit used the 6386, 12AX7 gain tube and 12BH7 output tube. The flatpack transformer supplied one of the B+ rails while the toroid supplied heaters and the other B rail. There was a switch on the back of this unit to change the output transformer secondaries from series to parallel hookup for different source impedances of 50 vs 200 Ohms. The 4-pin Switchcraft XLR used 2 pins for provision to hook a remote PPM 1mA meter to and two pins for linking to another channel. 

The production units followed shortly thereafter with a new side-chain circuit engineered by house-tech Richard Schroeder. The first production LIMITER COMPRESSOR AMPLIFIER's were housed in a 1 1/2U billet chassis and featured a more complex vacuum tube regulated B+ rail. Sowter made the input and output transformers for these units. The output control was sometimes a big ol' Langevin 600 Ohm attenuator. If you see a big gold cylindrical thing there, then that's what you have. There were no ATTACK controls, however, a mod was later available to add a 3-position switch for this function. BYPASS on these earliest units were not hardwire type bypass controls. A bit odd. RECOVERY control was a 3-position switch; we used to joke "slow", "slower", and "slowest"... The large Sifam meters were not illuminated. We hadn't gotten to that yet back then. Tube complement on these was 12AL5 (I think... might be 6AL5... just replace 'em with whatever is already in there) for the rectifiers, 6386 (of which there are no more) for the gain control, 12AX7WA (usually the Yugoslav Ei ones) and then the 12BH7 for the output tube.

After we moved over to the Manley Labs factory in 1993, we built a handful more of the 1 1/2U units before the STEREO unit came out. After the Stereo unit was out, then the mono unit got a rework. The 1U units in our extruded side chassis with the removable perforated metal tops and bottoms became a real mono version of the then-new stereo one with the ATTACK pot and 5 position RECOVERY (RELEASE) switch. Because the Stereo Variable Mu® was selling so well compared to the mono version, after two years we killed off the mono version and put him in the mausoleum. 

1 1/2U version mono Variable Mu® Limiters(with three position Attack time mod) shown above in Ed Tuton's rack.