The Variable Mu Limiter Compressor is the GLUE that holds the mix together while crystallizing it into a professional, final product. All-tube, all transformer, an all-American legend. High Pass Side Chain mod now comes standard.

FREQUENTLY ASKED QUESTIONS

FAQ #1: WHICH VERSION OF THE STEREO VARIABLE MU DO I HAVE?

FAQ #2: 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.

FAQ #3: 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.

Read more about Paul's special T-Bar Mod here.

Update 2009: There is a new production 6386 being made by JJ factory. They are very expensive! We have not tested these tubes.

You can read an informative thread about the JJ6386 on Gearslutz to decide if you want to them instead.

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 (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. Contact our Service team for more information.

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.

FAQ#4 Is it hardwired and bypasses everything from Input to Output or does it still kind of go through the unit in some way, you know, like, through the electronics in the unit at all?

The BYPASS switch is a hardwire BYPASS switch which means it is not going through any circuitry in bypass mode. IN goes straight to OUT.

FAQ #5: 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.

FAQ #6: WHAT IS the deal with the LINK switch? Everything is shifted over to the one side.

The LINK switch only links the side-chain voltages to encourage similar action of the control voltages. ALL of the front panel knobs remain separate meaning you have to turn each channel’s knobs individually as you normally would when using the unit in dual-mono or separate mode. Set the unit up in dual mono mode, adjust your INPUT, OUTPUT, ATTACK, RELEASE, and THRESHOLD knobs up on both channels, then throw the LINK switch. (Obviously only the INPUT knob is stereo-ganged across both channels. No other controls are electrically or mechanically ganged.)

FAQ #7: THIS THING ISN’T WORKING. I JUST BOUGHT IT. FOR THIS AMOUNT OF MONEY, IT NEEDS TO FUNCTION.

OK, listen up: the XLR INPUTS and XLR OUTPUTS are TRANSFORMER-COUPLED so that means you HAVE TO have something connected to all of the pins. 

PIN 1: GROUND
PIN 2: HOT (+) POSITIVE
PIN 3: COLD (-) NEGATIVE

You cannot "float" Pin 3. You can GROUND it if you are driving it from an unbalanced source. But you cannot hook it up to "nothing".

Pay attention when interfacing with some of these interfaces like your Apollo. If you are using TRS - XLR cables, make sure that it is wired:

TRS TIP = HOT (+) to XLR Pin 2
TRS RING = COLD (-) to XLR Pin 3
TRS SLEEVE = GROUND to XLR Pin 1

or 

TS - XLR cable, make sure it is wired:
TS TIP = HOT (+) to XLR Pin 2
TS SLEEVE = GROUND to XLR Pin 1 AND Pin 3

FAQ #8: Hi, I can hear the sound from inside the unit itself, I can compare it to the sound of a cartridge playing a vinyl as your amplifier is turned down. Have you ever noticed that?

Yeah we have! The sound you hear is mechanically coming from the output transformers. Most output transformers in tube guitar amps or hifi amplifiers also do the same thing, but you usually just can't hear them singing because of music coming out of the speakers. This is normal.

FAQ #9: HELP MY FUSE BLEW! Do I need a new transformer?

Probably not. It’s probably the heater rail’s 22uF cap or the RL252 bridge rectifier that failed, specifically, the heater bridge if "TCI" branded is suspect. Disconnect both of these components and then replace the fuse. And apply power back tot he unit. Transformer humming=excess current draw. If the transformer doesn’t blow its fuse and the transformer stops humming, you found the problem. We have an uprated bridge rectifier for older units and we recommend that it be relocated to the the side of the chassis for better heat sinking (as the modern units are).

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

CUSTOM MODIFICATIONS

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 to older Manley Variable Mu limiters – please contact our Service team for more information.

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 – contact our Service team for more information, and 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. 

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.