The Stereo Variable MU Limiter Compressor with M/S and T-Bar Mod from Manley Labs is a dynamic signal processor using two single-pentode tubes wired in triode for peak reduction and compression. The tube circuit is constantly re-biased by the vacuum tube rectified side-chain control voltages, resulting in a smooth change in gain.

The Mid/Side modification (also known as the Vertical/Lateral or Sum/Difference) allows for stereo encoding and decoding, as well as image enhancement processing capabilities. Instead of an active circuit, the left switch changes the wiring of the input transformer, while the right switch changes the output wiring. When both M/S switches are activated, the stereo signal is encoded and decoded providing the ability to compress the mid, sum or mono part of the sound on channel one, while channel two's compressor controls the side or difference. The Mid/Side switches replace the high-pass side-chain position on the front panel. The HP/SC are ganged and moved to the power switch position, while the power switch has been moved to the rear of the chassis.

The T-Bar modification uses a converter board with two 6BA6 single pentodes wired in triode mode to take over the duties of the single-envelope dual-triode 6386 or 5670 tubes. Each of the single-pentode tubes are matched for low noise and low microphonics before being installed onto the adapter board. Direct A/B comparisons and testing have found the sound and performance of the dual 6BA6 to be nearly identical to the original 6386 tube. Comparing the dual 6BA6 to the 5670 sounds less "squashed" with fewer artifacts as the amount of limiting is increased.

The compressor setting offers a soft-knee 1.5:1 ratio, while the limit mode offers a sharper knee with ratios ranging from 4:1 to 20:1. With higher limiting settings, the knee will start to soften. Distortion effects can be achieved by turning up the input and lowering the output while using little or no compression. The side-chain high-pass filter uses a 1-pole, 6 dB/octave filter that attenuates 1 to 3 dB at 100 Hz and 4 to 6 dB at 50 Hz. Once engaged, the compressor / limiter will not respond as strongly to low frequency content and will prevent audible "pumping" when processing bass-heavy music.

The input of the Variable MU is ganged, but can still process two independent channels with separate threshold and output controls. With the link switch engaged, the unit operates in true stereo mode and excels at processing 2-track, final mix, or mastering compression and limiting. Handmade in the USA, the unit is housed in a 2U steel chassis with a CNC-machined 1/4" faceplate that is also inlaid with laser-engraved inserts. The Variable MU ships with an IEC power cable.

Sound of the Original Variable MU    Mid/Side Mod Adds Stereo Enhancement    All-Tube Design    6 x Tubes per Channel    Hand-Wound Custom Nickel Transformers    Stereo or Dual Mono    Hardwire Bypass Switch    Side-Chain High-Pass Filter    Large Illuminated Sifam Meters    Handmade in the USA

• M/S modification allows various approaches to Mid/Side recording, playback and processing
• T-Bar Mod replaces the single 5670 tube with two 6BA6 tubes providing the original sound of the Variable MU
• Manley input and output transformers with nickel laminations in mu-metal cases
• Balanced inputs and outputs (600 Ω)
• Independently regulated B+ and heater supplies
• Bypass Switch: Hardwire true bypass
• Input Attenuator: -8 to +8 dB stepped Greyhill switch
• Variable Attack: 25 to 70 ms
• Threshold: Continuously variable
• 5-Step Recovery Switch: 0.2, 0.4, 0.6, 4, 8 sec
• Compression Ratio: 1.5:1
• Limit Ratio: 4:1 to 20:1
• Large Illuminated Sifam Meters: Displays the amount of dB gain reduction
• Stereo link switch

Related Questions and Answers

Q: 1. Hi there. What mounting thread is at the bottom (centre) for the tripod? 3/8 5/8 1/4?

A: The RODE Stereo Bar (B&H # ROSB20STEREO) will mount to a standard 5/8"-27 male microphone stand.

Q: 2. Is the AC adapter included?

A: It is powered by a 9 Volt battery or the included power supply.

Q: 3. Is this appliance voltage 220V?

A: The Manley Labs Slam tube stereo limiter and mic preamp is not 220V. It is rated for 120V use only.

Q: 4. Will this item work with a condenser boom mic (provide phantom power)?

A: I don't think it provides phantom power but it does allow it to pass through? So send phantom power from the recorder (or TX if going wireless) and you should be good.

Q: 5. Will this also work with the Zoom H4essential recorder?

A: The Zoom SSH-6e Stereo Mid-Side Shotgun Microphone for H6essential Portable Recorder is not compatible with the Zoom H4essential 4-Track 32-Bit Float Portable Audio Recorder.

Q: 6. Can this work with iPhone 15 pro ?

A: No the Zoom Am7 Mid-Side Stereo Microphone is made for compatibility with Android Devices with USB-C Connector.

Q: 7. I'm curious if this could work with a phone on the DJI osmo OM4 gimbal? could it balance the phone and would the mic w foam cover get in the way of the gimbal arm?

A: There may be problems with balance as well as with the mic getting in the way of the gimbal arm when the mic is attached to the phone.

Q: 8. Is this compatible with Protools 10?

A: The IK Multimedia T-RackS 5 Dyna-Mu Compressor Software will run in digital audio workstations that support 64-bit AAX, AU, VST2, VST3 plugins. Natively, Pro Tools 10 supports only 32-bit plugins so T-TrackS 5 is not compatible with Pro Tools 10 but is compatible with Pro Tools 11.

Q: 9. The recommended accessories for this microscope include batteries. Could you please advise why batteries are necessary? I assume that they are required for field work where there is no power supply, but this is not mentioned in the specification.

A: This model does not take batteries. My apologies for the confusion. I will contact the Web Team to take this accessory down.

Q: 10. Would this be a good device for evening out the crazy volume levels on my tv?

A: Absolutely! Very simple...just set the Makeup Gain control at the 0 position, and set the Threshold to a level that brings down the extremely loud audio to a level near that of a normal listening level. If you hear the limiter pumping or breathing too much on louder program audio, lower the input level and tweak the Makeup Gain a bit to even things out. It works great for my hard-drive based music collection, which varies greatly in level due to the variety of formats and recording techniques used over the past 4 decades!