Requisite Audio L2M Mastering Limiter MK III


In it’s 12th year of production, the L2M is an optical limiter based on classic designs of the early 1960s and is the beneficiary of many significant improvements, representing what a truly refined optical limiter can be. We classify the L2M as a mastering limiter because it was designed with the specialized requirements of the mastering engineer in mind, though, the L2M is equally at home in tracking and mixing applications. Among the more notable performance features of the L2M are it’s sonic excellence, versatility and stable frequency response while limiting program material. When providing unobtrusive compression/limiting action, there is no undulating bottom or loss of openness, even in the heaviest of limiting situations. Corrective EQ, following the L2M, is simply not required. With the L2M, your work stays intact, just as you need it to.



Output level/make-up gain is accomplished using a precision 24-step attenuator (one for each channel). Range of settings is from -6dB to +17dB in 1dB increments. The L2M is shipped with UNITY calibrated to within .1dB. Each channel is calibrated by it’s respective UNITY ADJ trimmer on the back of the unit.


Left to run freely, vacuum tubes are not linear and exhibit a considerable amount of harmonic generation and other forms of distortion. For over half a century, negative feedback has been engineered into circuits to “improve” the linearity of vacuum tubes when used in audio gain stages. However, best readings on a meter don’t always equate to the ideal sound, whatever that happens to be. By making LIMITER FEEDBACK an adjustable feature, harmonic content and texture are put into the hands of the audio engineer and not the guy with the THD meter.


The L2M will provide equal compression/limiting at all frequencies by responding to the entire spectrum of the program material. When LIMITER RESPONSE is set fully clockwise, frequencies below 1-kHz gradually require more energy to initiate gain reduction. For frequencies below 100Hz, gain reduction initiation is down approximately 10dB. This arrangement is ideal for general program material & especially material with a great deal of bass energy and/or broad bass dynamics. The result is a marked smoothing of mid and high frequencies (above 1-kHz) and a tight/punchy bass response.


Mid/Side on the L2M is effective for tracks recorded using stereo micing technoques.


Having a total range of 6dB, STEREO LEVEL RIDE is ideal for manual adjustment of level transitions from various sections of the program material within the analog domain. Stereo tracking of this dual-ganged pot is extremely accurate and holds a tolerance within 0.2dB throughout it’s entire range.



  • In/Bypass
  • Output Level – 1dB, 24-Step Attenuator
  • Limiter Feedback – 1dB, 11-stpe Attenuator
  • Ratio 10:1/3:1 – Limiting/Compression
  • Limiter Response – Continously Variable
  • Peak Reduction – Continously Variable
  • Mid
  • Side
  • Stereo Level Ride
  • Link – Left/Right
  • Meter Select – Gain Reduction, +4dB, +10dB
  • Zero Adj – Gain Reduction Meter Zero Adjust
  • GR Track – Gain Reduction Tracking Calibration
  • Link – Left/Right
  • Link – Left/Right
  • Meter Select – Gain Reduction, +4dB, +10dB
  • VU Adj – Calibration for Volume Units Meter in dB
  • Stereo Adjust – For Balancing Left/Right Peak Reduction
  • VU Meters


  • Ground Connector – Chassis and/or Circuit Ground
  • Power Supply Inlet (choice of Tube or Solid-State Supply)
  • Transformer Balanced XLR Output (pin 2 HI)
  • Limiter Link – 1/4″ Jack for linking additional PAL Limiter (not included)
  • Tube Channel – Protective Structure for easy Tube replacement and Ventilation
  • Transformer Balanced XLR Inputs (pin 2 HI)


  • Solid-Core Copper/Silver Wire with Teflon Dielectric
  • Handwired, Point-To-Point Throughout (only Meter/Phantom on PCB)
  • Custom Input and Output Transformers
  • Overbuilt – using highly rated, quality components



The RTS-4A is built on classic, no compromise design and techniques. It is tube regulated, tube rectified and capable of powering any Requisite Audio vacuum tube audio processor in current production. It is expandable from one to four channels. Filtering is accomplished using custom made poly/oil capacitors and a heavy duty audio frequency choke.


Standard with the RTS-4A is the GZ34 (5AR4) tube rectifier. DC filtering is via custom AF Choke input & custom made oil & polyprophylene capacitors.


Power Supplied: B+, Heater, Meter Driver, Lamps. Power Required: The RTS-4A can operate on 115V or 230V mains.

  1. Single channel RTS-1A
  2. Dual Channel RTS-2A
  3. Three Channel RTS-3A
  4. Four Channel RTS-4A


The OPS-2D is a single or dual-channel solid-state power supply designed to power all current production Requisite Audio vacuum tube audio processors: L2M MkIII, PAL Plus MkIII, OS1, SM 162 Summing Mixer and the SMX 1748 Summing Mixer Expander. The OPS-2D can be configured for single or dual-channel operation.


Standard with the OPS-2D are ultra-fast, soft-recovery diodes with balancing resistors and polypropylene bypass caps. DC filtering is via Nichicon audio capacitors with polypropylene bypass caps. Power Supply coupling is via AF Chokes.


Power Supplied: B+, Heater, Meter Driver, Lamps. Power Required: The OPS-2D can operate on 115V or 230V mains.

  1. Single channel
  2. Dual Channel