MXR 136 Dual Limiter

MXR 136 Dual Limiter

The MXR 136 Dual Limiter hit the market sometime around late 1980 (if my detective work is correct). Even some 25 years later it is still drifting along the under currents of forum banter and the second-hand market with a small degree of notoriety. Comments often include words of praise and sometimes even prophesy about future greatness and ‘collectability’ of the MXR 136. I don’t see it becoming the next LA2A or U47, but it is certainly a nice bit of kit that has been described by a lot of people as the poor man’s Universal Audio/UREI 1176 compressor. Featuring two linkable channels for dual mono or stereo operation, the 136 certainly does offer a lot of features and a lot of character for an incredibly low price.

MXR 136 Dual Limiter PCB
Nothing but standard off-the-shelf components.

MXR describe the operation of the 136 in the manual as a VCA (voltage controlled amplifier) based compressor, but when you dive into the design of it there really isn’t a huge amount of similarity to the traditional DBX or SSL style of VCA compressor design. The 136 actually uses a MOSFET controlled by a 220kHz to 320kHz PWM (pulse width modulated) clock signal to chop out parts of the audio at ultrasonic speeds. The premise being that if you switch something on and off fast enough, you can ultimately control its average energy. PWM based compressor designs are shared by other compressors like the much revered Pye compressors, the Great Rivers PWM-501 and a number of devices made by Crane Song.

“If you could operate a switch at a high enough speed you would be able to control the average energy on the output of the switch. This energy could also be feed into a simple circuit that would help make the average more accurate. If the switch was on 50% of the time and off 50% of the time, there would be 50% of the energy at the output of the switch. If it was on 10% of the time and off 90% of the time you would have 10% of the energy at the output of the switch.”
Dave Hill, Crane Song & Dave Hill Designs 

Feature-wise, the MXR 136 Dual Limiter has all the familiar knobs and buttons. The input and output controls allow you to push and pull the audio in and out of compression in a similar fashion to an 1176. To complement these, fully variable attack and release controls are also included to give adjustment over the compression envelope. A 4:1 to Infinity push button drastically changes the ratio from gentler compression to brick wall limiting, and a stereo link switch will share the sidechain signal between the two channels of the dual limiter. It is a little rudimentary as it doesn’t simply allow one channel of panel dials to control both channels in the compressor, but it certainly works. The back panel has XLR & TRS I/O, as well as dedicated sidechain input and output jacks for each channel. Some of these compressors shipped with pin 3 wired as hot on the XLR connectors. Something worth checking out if you do own or plan to get one – the modern standard for the last several decades is to wire XLRs pin 2 hot.

MXR 136 Dual Limiter Leaky Caps
A leaking 4.7uF capacitor between two voltage regulators.

The 136 does seem to be prone to a number of age related problems, but it is nothing a good tech can’t remedy. The dual secondary mains transformer spits out a high voltage – a lot for the 15 volt regulators to handle. As a result the regulators and surrounding components do run at a high operating temperate. This is particularly the case for the 4.7uF capacitors sitting right between the two regulators. I have seen them split and leaking in more than three different 136’s, and it is usually accompanied by dried out thermal paste and brittle brown plastic pins on the regulator heat sinks.

If you do grab one of these units, I would certainly recommend giving it a once over. If it hasn’t seen a new set of capacitors, I would yank out the old ones and put in new ones. Likewise the voltage regulators, applying fresh thermal paste and a new plastic nut and bolt to secure the heat sink. All up it is about $15 worth of components and no more than an hours work for a competent tech, and it will keep the 136 running for years to come.

As far as built quality goes, the 136 was definitely falls under the ‘built to a budget but well put together’ category. All of the ICs are soldered directly to the PCB rather than using IC sockets, the off-board wiring is quite simple with no shielded cables for the XLR or TRS audio connectors, and the component selection is pretty stock standard. In a lot of ways it is a benefit in an older piece of gear. 7815/7915 voltage regulators, NE5532 and TL074 opamps and off the shelf potentiometers and push button switches means the 136 should be repairable and useable for another three decades.

If you keep your eyes peeled, you can often find the for a decent price. They are quite handy little units, and can really give your tracks a bit of raunch. They work great compressing an instrument bus in parallel, and you can do some handy little tricks with it like chaining the left and right channels together in series to smooth out overly dynamic material. Like always, please feel free to comment, ask questions or share your opinions on the MXR 136!

 

MXR 136 Technical Documents

References

  1. Why A PWM Compressor – Dave Hill, Crane Song

Gallery

 

  • MXR 136 Dual Limiter
  • MXR 136 Dual Limiter
  • MXR 136 Dual Limiter
  • MXR 136 Rear Panel
  • MXR 136 Exploded Capacitor
  • MXR-136-Foil-Fuse
  • Replacement Voltage Regulators
  • The old capacitors
  • MXR 136 Dual Limier Re-Cap
  • MXR 136 PCB
  • MXR-136-New-Product
  • Recording-1981-04-OCR-Page-0055
  • MXR 136 Circuit Description

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