The MXR Dual Limiter was designed as a flexible two-channel dynamics processor for recording, mixing and sound-reinforcement applications. Each channel functions independently when the front-panel Stereo/Dual switch is set to Dual, while Stereo mode links the control voltages of the two channels so that gain reduction occurs equally across both sides of a stereo signal.
Each channel provides Input, Output, Attack and Release controls together with a switch selecting either a 4:1 compression slope or an effectively infinite limiting ratio. Rather than providing a conventional threshold control, the Input control determines the signal level at which compression or limiting begins. Increasing the Input setting makes the processor progressively more sensitive to incoming level.
The Output control establishes the level leaving the processor. At its minimum setting, the output level of a signal undergoing limiting is approximately -30 dB, increasing to approximately +10 dB at the fully clockwise position. The control therefore allows the processed signal level to be matched to the equipment following the limiter.
Attack is continuously adjustable from 0.5 ms to 50 ms. The circuit automatically modifies the effective attack according to the magnitude of the gain change, allowing smaller changes to be treated more rapidly than larger changes at a given control setting. Release is adjustable from 0.1 to 5 seconds and incorporates programme-dependent behaviour intended to reduce low-frequency distortion when relatively rapid release settings are used.
The gain-reduction display for each channel consists of a series of LEDs operating from right to left as progressively greater attenuation occurs. The threshold indicator illuminates when at least 1 dB of limiting is taking place, with subsequent indicators representing increasing amounts of gain reduction. The display remains active regardless of the position of the channel In/Out switch, allowing settings to be monitored while the processor itself is bypassed.
Each channel has an In/Out bypass switch. In the Out position, the output connector is routed directly to the corresponding input, allowing the signal to pass even if AC power is removed.
One of the more unusual aspects of the MXR Dual Limiter is its internal gain-control system. MXR described the heart of the processor as a voltage-controlled amplifier using switching devices operating at 250 kHz. These switches are pulse-width modulated according to the control voltage. As incoming level rises, the control voltage increases and the pulse width becomes narrower, reducing the VCA gain. A four-pole output filter then removes components above 26 kHz produced by the switching process.
The input stage uses a differential amplifier intended to reject common-mode noise, hum and radio-frequency interference. A feedback network associated with the Input control determines the level presented to the VCA, while an input filter removes frequencies above 36 kHz before the switching gain-control stage. After the VCA and filtering stages, the signal passes through the Output control and a buffered output amplifier.
Rear-panel Detector Loop connections extend the capabilities of the processor considerably. The send and return connections permit external devices such as equalisers to modify the detector signal without altering the main audio path. MXR specifically described using this arrangement for frequency-conscious compression and de-essing. Boosting frequencies between approximately 2 kHz and 8 kHz in the detector path causes those frequencies to trigger greater gain reduction, reducing sibilance while leaving the programme signal itself otherwise un-equalised.
The detector loop could also be patched to create alternative compression slopes. Although the front panel provides 4:1 compression and infinite limiting, MXR documented detector-loop configurations producing approximately 2:1 and 1.6:1 slopes.
Applications described for the MXR Dual Limiter included vocal dynamics control, PA system protection, instrument compression, multitrack mixing and control of transient-rich sources. The infinite-ratio position was intended primarily for limiting, while the 4:1 setting allowed the unit to function as a compressor where maintaining some programme dynamics was required.
The rear panel provides both XLR and 6.35 mm phone connections, with the phone input connected in parallel with the XLR input. Balanced and unbalanced sources can be accommodated, with an input impedance of 20 kΩ and output impedance of 200 Ω.
Features
- Two independent compressor/limiter channels
- Dual-mono or linked stereo operation
- 4:1 compression and infinite limiting ratios selectable from the front panel
- Additional approximately 2:1 and 1.6:1 ratios obtainable through detector-loop patching
- Independent Input and Output controls for each channel
- Continuously variable Attack control from 0.5 ms to 50 ms
- Continuously variable Release control from 0.1 to 5 seconds
- Programme-dependent attack and release behaviour
- LED gain-reduction displays for both channels
- Individual In/Out bypass switching
- Linked control-voltage operation for stereo processing
- External detector loops for frequency-conscious compression and de-essing
- 250 kHz pulse-width-modulated voltage-controlled amplifier system
- Differential input circuitry for common-mode noise rejection
- Balanced 20 kΩ inputs
- Low-impedance 200 Ω outputs
- XLR and 6.35 mm input connections