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Pendulum Audio 6386 Variable Mu Tube Limiter

The Fairman TMC compressor/limiter created a buzz last year with its Fairchild-reminiscent appearance and sound quality. Pendulum has taken the buzz a step further with its 6386 Variable Mu Tube Limiter.

The Fairman TMC compressor/limiter created a buzz last year with its Fairchild-reminiscent appearance and sound quality. Pendulum has taken the buzz a step further with its 6386 Variable Mu Tube Limiter. Using the 6386 vacuum tube (the same gain control tube found in the Fairchild 660 and 670), the Pendulum 6386 offers the same compression characteristics of the classic Fairchild units with a much higher level of sonic accuracy.

Make up gain is accomplished with a Class A, balanced, transformerless line amp, rather than the push-pull transformer output stage used on vintage variable-mu limiters. As a result, the machine has the advantages of a classic vacuum tube compressor with more accuracy and a less colored signal path.
Product PointsApplications: Studio recording

Key Features: 6386 vacuum tubes; two independent channels with stereo linking; Class A transformerless line amp; balanced I/O

Price: $3,995

Contact: Pendulum Audio at 908-665-9333.


+ Amazing sound

+ Flexible

+ Very fast for a vacuum tube based device


– Power switch is not easily accessible

– Cannot link input or output level controls

The Score: Though expensive, the Pendulum 6386 Variable Mu Compressor/Limiter is flexible, easy to use and
sounds fabulous.

The Pendulum 6386 ($3,995) is an 11.5″ deep 2RU unit that weighs 14 lb. Its rear panel is equipped with two F-XLR connectors paralleled with two 1/4″ TRS jacks for Channel 1 and two inputs and two M-XLR connectors paralleled with two 1/4″ TRS jacks for output. Input impedance is 10 kohms, transformer-balanced, and output impedance is 600 ohms, active-balanced.

Each channel is equipped with a 1/4″ TRS jack for inserting an outboard EQ into the side chain (for de-essing or frequency-dependent compression). Also on the rear panel is the power switch and an IEC power socket for a standard IEC power cord (included).

The unit’s attractive blue front panel is equipped with identical controls for both audio channels – the input attenuator adjusts the signal level to the input transformer; maximum position is nominal operating level; threshold control determines the amount of gain reduction applied to the source material; and the output control adjusts the signal output after gain reduction.

Up to +20 dB of gain make up is available. The output controls on the unit I tested are 10 K linear taper. This gives better selectivity in the 0 to +12 dB range of gain makeup. The unit is optionally available with audio taper pots.

The meter switch selects meter indication of input level, output level or gain reduction. The mode switch selects among the unit’s three modes of operation. In the fast mode, which is my favorite, the attack and release times are 0.5 ms and 50 ms, respectively. The presets mode reverts to the setting selected by the presets switch. In the manual mode the attack and release times are continuously variable from 1 to 40 ms and 0.1 to 2 sec respectively. These times are adjusted by the attack and release controls.

The six attack/release time settings determined by the presets switch are identical to the presets on the Fairchild 670. Presets one to four offer the following selection of fixed attack and release times respectively: 1ms/0.1 sec, 1ms/0.3 sec, 2ms/1.0 sec and 4ms/2 sec. Presets five and six have program-dependent release times, where there is initially a quick release, followed by a longer decay time to zero gain reduction. The attack and release times of these presets are: 2ms/1 to 4 sec and 1ms/0.5 to 20 sec.

The in/bypass switch allows the comparison between the original source and the processed sound, while the link/split switch links or decouples the two channels. When linked, the threshold and dynamics processing of both channels are controlled entirely by Channel 1. Input and output controls remain independent so program balance can be adjusted.

One of the many great features of the 6386 is its power-up sequence, which is designed to significantly prolong tube life. When the power switch is turned on, the 6386 goes through a soft-start sequence, gently applying power to the tubes and stabilizing the circuit before engaging the outputs. The entire sequence takes about two minutes and is completed when the blue LED on the front panel is illuminated.

In use

The Pendulum 6386 naturally falls into place as a stereo bus compressor. I have used the unit as my stereo bus compressor on every mix I have done over the last three months. It sounds absolutely fantastic. Recently I was working in a room equipped with the Manley Variable-Mu and the Alan Smart compressors. I had the Pendulum 6386 and my Focusrite Red 3, which was my previous stereo bus compressor of choice. I spent time comparing the units – all four are wonderful sounding machines but, for this material, the Pendulum was the obvious beast of choice.

Although the machine naturally gravitates toward two-mix compression in a mixing environment, it is equally adept as two channels of mono compression. I have used the 6386 for compression while recording a wide variety of audio sources and have always had satisfying results. The unit is quieter than any other tube compressor I’ve encountered and it manages to provide the vacuum tube warmth that tube circuits are renowned for without adding any negative artifacts or noise.

The 6386 is a bass player’s answered prayer. I found that using the unit to double compress bass works incredibly well. I first compressed the bass through Channel 1 using Preset 3 and then ran the output into Channel 2 and compressed using the fast setting. The result was an incredibly fat, punchy bass tone that completed the bottom end of the track without ever becoming obtrusive. I found this double compression technique to work nicely on vocals as well.

I had extraordinary results using the 6386 to compress piano. It sounds natural and open. The machine allows adequate control of the grand piano’s wide dynamic range without becoming apparently compressed.

While recording acoustic guitar, I discovered that driving the input at a higher level provides additional tube saturation without adding significant noise. This also works well on electric guitars.

I used the 6386 on several other audio sources, including backing vocals, kick and snare drum, cello, drum overheads, synthesizers and drum loops and never had anything less than magnificent results. I’m convinced that there is no sound source this machine doesn’t complement.


Products as timeless as the Pendulum Audio 6386 seem to appear less and less frequently as years pass and manufacturers become more concerned with quantity rather than quality. Fortunately, there are companies like Pendulum Audio who have committed to manufacturing products meeting the highest standard of quality. The Pendulum Audio 6386 Variable Mu Tube Limiter is a modern-day classic.