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The earliest Manley Variable Mu® mono units: Manley Limiter Compressor Amplifier 1 1/2 R U version built (1992 - 1994) Manley 10dB Limiter Compressor Monoblock 1 R U version built (1995 - 1997) Features And Specifications | Suggested Retail Price  Mono Variable Mu® Limiter Compressor Owner's Manual  These Mono versions of our Stereo MANLEY VARIABLE MU® LIMITER COMPRESSOR are no longer made, but they were one channel versions of our later stereo unit, and the predecessor to it.    The Stereo Variable Mu® has been 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 typeof  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. They 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 compliment 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. Hutch had a large part in the Stereo Variable Mu design, taking out mostly all of the funkiest things of the mono units. 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.

MANLEY Mono VARIABLE MU® LIMITER Used 6386 tubes for gain control 12AL5 or 6AL5 for rectifiers (depending...) 12AX7 gain stage tubes Usually 12BH7 for output tubes MANLEY input & output transformers with pure nickel laminations in mu-metal case with flat frequency response from 20Hz-60KHz BALANCED INPUTS & OUTPUTS Earliest units used Sowter transformers Fully differential ALL-TUBE circuitry Hard-wire BYPASS switch in 1U units Silent conductive plastic INPUT attenuator Selectable RECOVERY 5 steps Variable ATTACK less than 0.5 msec.  1 1/2U units usually did not have Variable Attack Continuously variable THRESHOLD Switch for LIMIT or COMPRESS modes Front panel mounted METER ADJUST Sifam METERS STEREO LINK SWITCH Maximum gain: 24 dB Max. output (into 600 ohm load): (+20 dBm) Power consumption (120/240VAC): 80 watts Operating mains voltage: Units are purpose built for original destination country's mains voltage:    100V, 120V, or 220-240VAC as indicated on the serial number badge. Power transformer must be replaced in order to change mains operating voltage. 120VAC units may NOT be rewired to put the primaries of the power transformer in series for 220-240V operation or a large radiated field of hum will develop. If changing locations/voltages, the power transformer must be replaced with the dedicated one for the voltage at which it will operate. Mains Voltage Frequency: 50~ 60Hz Dimensions: 19" x 10"  (occupies 1u or 1 1/2u depending on which one) Shipping weight: 18 lbs.

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

VARIABLE MU^® is a registered trademark of Manley Laboratories, Inc.