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The Alice 538R Rackmount OPTO Stereo Compressor is a full size mains powered 1U rackmounted version of our popular 538T OPTO stereo compressor from the 500 Lunchbox series.


The 538R has OEP transformer balanced inputs and outputs, a side-chain input and dual VU meters.


  • Ted Fletcher’s historic opto compression design
  • Perfect ‘Hard-Wired’ by-pass for instant comparisons
  • Wide range ratios from gentle to brutal
  • Accurate UV compression metering on a real meter
  • Extreme stereo width control
  • Side-chain
  • Shielded hand-matched audio transformers


The ‘538R’ is designed for creative compression effects that are difficult to achieve with plug-ins. It’s capable of extreme compression just as Ted Fletcher’s original Joe Meek SC2 V5; amazing drum sounds! It’s also super-quality ‘Levelling Amplifier’ with the sparkle of a well-designed opto compressor.



The 538R is a 1U rackmounted unit using mains voltage.


Inputs & Outputs

Left and Right transformer balanced inputs and outputs are via hand matched OEP canned transformers

Side-chain via a 6.35mm stereo TRS connector


Width Control

In order to retain perfect symmetry between the left and right channels of the stereo equaliser the audio signal is processed into ‘sum and difference’ mode (that is L+R = ‘sum’, L-R = ’difference’), the two audio signals are then processed by the EQ controls before being re-processed back into left and right format. By going through this process the left and right channels remain perfectly symmetrical with no image shift, any minor difference between the processed sum and difference signals show up only as small deviations in width of the stereo image. A further useful advantage of the sum and difference processing is that the width of the stereo image is controlled by the volume level of the ‘difference’ channel so by adding a gain control the stereo width can be accurately controlled. With the width control at minimum the audio output is mono. When the control is half way, the processing accurately recombines the audio into left and right. If the control is increased further, the image is extended beyond the original stereo.



Easy-to-read stereo analogue meters reading audio (UV) or compression depth (comp)

Alice 538R Rackmount OPTO Stereo Compressor

  • INPUTS · 10K impedance differential balanced.
    OUTPUTS · Floating balanced 50 ohm.  Max balanced output approx. +26dBu
    GAIN · Maximum 24dB
    NOISE · At least 80dB below operating level. Dynamic range exceeds 100dB.
    HARMONIC DISTORTION · Generally within 0.005%. 2nd order harmonic distortion rises at lower frequencies under compression.
    AMPLITUDE FREQUENCY RESPONSE · Line in 20Hz to 20KHz within 1dB.
    AUDIO CROSSTALK · the 538 uses M/S processing (sum and difference) with ‘side’ content variable via the width control. With width set at 100% L/R crosstalk can be better than -50dB.
    COMPRESSOR · Photoresistive servo operated feedback type· Ratio non-linear;  minimum approx. 1.2 to 1 maximum approx. 8 to 1. Sidechain filtered for correct bass compression. Attack time 1mS min. 10mS max. (variable) Release time 200mS min 1.5S max. (variable)
    CONNECTIONS · via Neutrik XLR and TRS connectors
    POWER · 85 to 285v AC
    HOUSING · 19″ 1U rackmount unit
    Supplied with 2m IEC cable

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