The MANLEY VARIABLE MU® LIMITER COMPRESSOR 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. It 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? The unique 5670 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.
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!
You might notice that the Variable Mu® Limiter Compressor has a ganged input control, but do not jump to conclusions that it is mono-unfriendly. Track away! There are separate threshold and output controls to make compensations with plus you can always adjust your individual source levels elsewhere, right? The advantage of the stereo input control becomes dramatically clear when you switch to LINK mode, and that’s what our Variable Mu® Limiter Compressor does better than anything else: final mix, 2-track, or mastering limiting and compression. Like one reviewer put it: “It’s like pouring a bowl of sweet cream over the mix.” Mmmmmm. Yummy. Give your music a big hug.
- MANLEY input & output transformers with nickel laminations in mu-metal cases with flat frequency response from 20Hz-25KHz
- BALANCED INPUTS & OUTPUTS (600 Ohms)
- Fully differential ALL-TUBE circuitry using one each 5670, 5751 (or 12AX7), 7044 (or 5687), & 12AL5 per channel
- Independently regulated B+ and Heater supplies
- Hard-wire BYPASS switch
- Stepped switch INPUT attenuator as of 10/2011. (older units use Silent dual-ganged potentiometer)
- RECOVERY 5 steps: 0.2s, 0.4s, 0.6s, 4sec., 8sec.
- Variable ATTACK: 25msec-70msec
- Continuously variable THRESHOLD
- LIMIT (4:1 to 20:1) or COMPRESS (1.5 to 1)
- HP SC: High Pass Side Chain Filter -3dB @ 100Hz
- Large ILLUMINATED Sifam METERS (older units before serial number MSLC61642 shipped before 12/2003 use: 26V 1.2W FESTOON LAMPS; Manley's Part Number: VAR016B) Order spare bulbs using our parts order form. (newest units after serial number MSLC61642 shipped after 12/2003 use white LED lighting)
- STEREO LINK SWITCH
- Several units can be linked for Surround (custom order )
- Maximum gain: 35dB
- Max. output: +30dBu (26Vrms) 26dB Headroom
- <0.1% THD @ 1KHz Noise floor: -85dB typical
- Power Consumption (120/240VAC): 80 watts
- Unit is factory set for 100V, 120V or 220-240VAC operation for original destination country's mains voltage.
- Operating Mains Voltage changeable with power transformer changeover switch and fuse value change.
- Mains Fuse Value for 100~120VAC operation: replace with 1.25A as of 10/2011
- Mains Fuse Value for 220~240VAC operation: replace with 0.6A (600mA) as of 10/2011
- Mains Fuse Type: MDA or MDL SLO-BLO Time delay 1 1/4" x 1/4"
- Mains Voltage Frequency: 50~60Hz
- Dimensions: 19” x 3 1/2” x 10” (chassis occupies 2u) Power transformer protrudes 3.5" out the back of the chassis.
- Shipping Weight: 23 lbs.
FREQUENTLY ASKED QUESTIONS
WHICH TUBES SHOULD I USE IN MY VARIABLE MU® LIMITER COMPRESSOR? 6386? 5670? OR 6BA6 TBAR MOD?
Yes, the newer units use the 5670 tube instead of the 6386. By now the availability of the original USA GE 6386 is poor; we don't have any left at all, and what we do have are not usable due to noise, microphonics, bad side-to-side match, etc.
DOES THE 5670 SOUND DIFFERENT?
Well, up to about 6db of limiting it's about the same. After that point, the 5670 version tends to sound more "squashed" than the original 6386 version. Some like it better, some don't-- depends on what you're trying to do. To solve all these problems, Paul came up with a really good solution: the T-Bar Mod which uses a pair of 6BA6 pentodes wired as single triodes to replace each dual triode 5670 (or 6386). the 6BA6 TBAR Mod is the preferred system to use in the Manley Variable Mu® for reasons of ability to perfectly match each phase-halve section and each stereo set, ability to select for lo-noise and lo-microphonic sets for a low cost, and because the action of the 6BA6's so closely resemble the smooth 6386 limiting curves.
Update 2009: There is a new production 6386 being made by JJ factory. They are very expensive, $120 each! We have not tested these tubes.
Follow the directions below to convert your 5670-powered Manley Variable Mu® to use the 6386 tubes. It's more work to do this than to install our 6BA6 T-Bar mod kits which we charge $500 for (complete and installed with four 6BA6 tubes selected for quad-perfect matching, lo-noise, and lo-microphonics).
If you want us to convert your unit for you, we can. You'll be supplying the 6386 tubes and we will all pray to the Tube gods that they are quiet and internally matched. Maybe buy extras to be sure. Check out our service page for rates and then fill in the RA form to book the service and you can work with Paul on that.
INSTRUCTIONS FOR CONVERTING 5670 TO 6386
- Replace 3K57 resistors connected to pins 4 & 6 of 5670 socket with 6K8 1W.
- Replace 1K resistors connected to pins 2 and 8 with 511 Ohms.
- Replace 10K "balance" trimpot with 500 Ohm trimpot.
- Remove 200 Ohm resistor between pins 2 and 8.
- Replace 33K 1W resistor between wiper of "GR cal" trimpot and the junction of the meter with 22K 1W (resistor is located near the rear corner of the pcb next to the GR cal trimpot).
- Repeat procedure for the other channel; re-calibrate as outlined in the owner’s manual.
WHAT IS THE DIFFERENCE BETWEEN TURNING UP THE INPUT LEVEL CONTROL TO DRIVE THE UNIT INTO LIMITING WITH MORE LEVEL VERSUS USING THE THRESHOLD CONTROL?
The limiting characteristics shouldn't change much, but the distortion characteristics will. The input control on these units is located ahead of the tubes, directly "behind" the input transformer. So as you advance the input control, you're hitting the input tube (which is the gain-varying stage) with more and more signal. Gain reduction is done by making the DC bias on this tube more negative (the threshold control determines how much). The tube has a limit as to how far it can be biased negative to reduce gain before it goes non-linear, and since your audio signal is obviously swinging both positive and negative, the big swings of the audio signal plus the negative gain control voltage will eventually push the tube into the red zone. Tubes like the 6386/6BA6/12BA6 can be pushed a lot farther before the THD builds than tubes like a 5670. Higher signal level also pushes the rest of the tubes and the output transformer harder, which will change the sound character as well.
THE MASTERING VERSION
& CUSTOM MODS
THE MASTERING VERSION
The Mastering Version of our Variable Mu® Limiter Compressor incorporating detented and logable steps built with 1% metal film resistors on sealed gold-contact Grayhill switches is available. NO, we will not mod a "normal" unit to the Mastering Version. The Mastering Version of the Variable Mu Limiter Compressor uses expensive Greyhill rotary switches with gold contacts where conductive plastic pots were used. The steps are determined with a large number of 1% precision metal film 1/2 watt resistors. The best conductive plastic pots only have 10% or 20% tolerance. The ten fold improvement in precision helps a great deal in left-right matching. There is a subtle audible improvement with stepped switches as well. Audiophile HI-FI often uses that technique to wring the last drop of performance out of a preamplifier. And I bet you wanted stepped switches mostly for resets.
The INPUT LEVEL is a five position switch with a generic optimum setting of “0” in the 12:00 position. Each step in either direction is 2 dB. For reference, Unity Gain is "0".
The OUTPUT Attenuators are in half dB steps. Reference Unity is -11.5 or fully counter-clockwise. You might think of these as "gain makeup". They are marked technically, in that “0” or fully clockwise has zero attenuation in the circuit. The tube circuit actually has 15.5 dB of gain. The Input attenuator at "0" removes 4 dB and the Output attenuator removes the last 11.5 producing "unity". With a little compression the "gain make-up" available with the Output Attenuator is very handy.
The Threshold is in half dB steps calibrated to LIMIT mode. In Compress the steps are approximately 1/4 dB. There are 24 steps so LIMIT gets a 12 dB range and Compress has a 6 dB range of adjustment. In some cases it is common to use the Input Attenuator to find a good starting point. Some Mastering engineers find using the combination of Input, Output and Threshold to achieve a little different "drive". Another good reason for stepped gains.
The Attack Time has been slightly extended in both directions compared to a regular Variable Mu and divided into 11 steps.
The Recovery 5 position switch is exactly the same as a regular Variable Mu.
The switches specifically are like this:
Input: -4 to +4 in 2 dB steps
Output: -11 to 0 in 1/2 dB steps (24 detents)
Threshold: -11 to +1 in 1/2 dB steps LIMIT(24 detents) and 24 x 1/4 dB steps in COMPRESS
Attack: 11 positions CW (fast) 15mS, 20, 25, 30, 35, 40, 50, 60, 70, 80, 90mS (slow) CCW
Release: 5 positions CW (fast) 200mS, 400mS, 600mS, 4S, 8 Seconds (slow) CCW
HIGH PASS SIDE CHAIN
This modification comes stock on all Manley Variable Mu Limiter Compressors since 12/2009, on both regular and mastering versions.
This mod adds two switches to the front panel, one for each channel, so that when engaged, the side chain will not respond to frequencies lower than 100Hz. (We standardly use 100Hz as the -3dB point. Other frequencies can be custom ordered.) This HP SC Mod can be used with music with heavy bass lines or bass-heavy mixes where you don't want the bass driving the whole action of the compressor.
The filter is a very gentle 6db per octave 1 pole filter, and will typically be down 1-3db at 100 Hz, and down 4-6db at 50Hz. As you decrease the frequency the amount of limiting will decrease also. At the extreme LF (<20Hz) there should be very little gain reduction going on. The whole intent of the filter is to keep very LF stuff (like a heavy kick drum) from activating the compression/limiting so that the overall level doesn't duck with every drumbeat. We can add the High Pass Side Chain option stock boards for an extra $350 NET (or $200 as a kit) to older Manley Variable Mu limiters.
THE T-BAR MODIFICATION
Backstory: The newer Variable Mu units use 5670 tubes instead of the 6386. By now the availability of the original USA GE 6386 is poor; we don't have any left at all, and what we do have are not usable due to noise, microphonics, bad side-to-side match, etc.
So does the 5670 sound different? Well, up to about 6db of limiting it's about the same. After that point, the 5670 version tends to sound more "squashed" than the original 6386 version. Some like it better, some don't - depends on what you're trying to do.
To solve all these problems, Paul came up with a really good solution: the T-Bar Mod. This uses a pair of 6BA6 pentodes wired as single triodes to replace each dual triode 5670 (or 6386). The 6BA6 T-Bar Mod is the preferred system to use in the Manley Variable Mu® for reasons of ability to perfectly match each phase-halve section and each stereo set, ability to select for lo-noise and lo-microphonic sets for a low cost, and because the action of the 6BA6's so closely resemble the smooth 6386 limiting curves.
We can add the T-bar mod to your Variable Mu unit for $500 NET (or $300 as a kit). Read more about Paul's special T-bar mod here.
Also of note - both the Manley NU MU’s LIMIT and COMPRESS reduction curves are very, very similar to those in a T-BAR modded Stereo Variable Mu. Check them out in this graphic.
MID/SIDE (a.k.a. Vertical/Lateral, or Sum/Difference)
This modification opens the door to stereo encoding and decoding as well as exciting image enhancement processing capabilities. For instance, setting to compress only the in-phase information allows the augmentation of the stereo image as the out-of-phase content is left untouched. Or, conversely, if you need a "more-mono" mix for broadcast, or vinyl-cutting for instance, you can set it to kill off more of the out-of-phase info which leaves more in-phase material in the final result. Read more about the MS Mod on page 12 of the Owner's Manual.
Total mod cost for the M-S option is $800 NET added to the base cost.
In order to add both M-S and the HP SC mods, we move the power switch to the rear panel and install both channels' HP SC switch to the center hole where the power switch was. The M-S switches then flank this switch.
Please inquire with us personally for detailed information.