Aphex 124B
2 Channel Audio Level Interface

OUR PART #  124B

The industry standard for interconnecting Consumer and Pro level stereo audio equipment.

  • Allows use of consumer (–10dBv) gear with pro (+4 or +8dBm) systems
  • Two-way buffer so consumer and pro systems can operate properly
  • Acts as a signal splitter, providing two high quality balanced outs from an unbalanced "Y" connector

A Clean and Reliable Solution for Bidirectional Level and Impedance Matching

The Aphex Model 124B Audio Level Interface is designed to allow use of -10dBV consumer hi-fi equipment with +4 or +8dBm professional/industrial/broadcast audio systems. It provides an extremely clean, reliable two-way buffer so both systems can operate at maximum performance levels, matching impedances and operating levels.


Servo-balancing means that high common mode voltages, such as induced hum from a nearby AC line, are compensated for, preserving headroom. The input impedance of 100K Ohms allows bridging over a 600 Ohm balanced or unbalanced line with negligible loading effects. Front panel switchable 600 Ohm load allows for input line termination when required.


Servo-balancing allows the outputs may be used balanced or unbalanced (single-ended) at any time by simply grounding either unused pin. The servo compensates for the voltage loss on one side so there is no 6dB loss of level. The outputs are also protected against short-circuits, so nothing is damaged if the wrong pins are grounded. A front panel indicator will flash red when the balanced output level is 5dB below clipping.


  • Two-way interface amplifier for connecting any unbalanced device such as a cassette deck, equalizer, CD player or DAT deck to the IHF inputs and outputs. The balanced inputs and outputs go to a studio console, patchbay or other line level equipment. The different gains and impedances are matched for optimal performance.
  • Line driver. The servo-balanced input and output stages of the 124 can be used as a very effective line driver for long lines. The output stage provides a short-circuit proof, low impedance source that will be relatively unaffected by the impedance of the cable. Gain may even be taken to boost signal level. In this application, either the unbalanced IHF or balanced high level inputs may be used. For the latter, the IHF inputs and outputs are simply jumpered.
  • Signal splitter. The 124 may be used to provide two high-quality balanced outputs from a single unbalanced mono signal A"Y" connector is used to split the incoming signal to each of the IHF unbalanced inputs.



  • Bandwidth: 5 Hz – 100 kHz (+0, -0.5dB)
  • Dynamic Range: 99 dB
  • THD: 0.005
  • Channel Isolation: > 70dB
  • CMRR (typical): > 45 dB


  • IHF Input Connectors: RCA Jacks
  • IHF Input Impedance: 18 kOhms
  • IHF Input Level: -10 dBV; peak +15dBV (ref. 1V)
  • Balanced Input Connectors: XLR 3-pin Female
  • Balanced Input Impedance: 100k Ohms with front panel 600 ohm termination switch
  • Balanced Input Level: +4 dBm (+8dBm via internal jumpers); peak +30 dBm


  • IHF Output Connectors: RCA Jacks
  • IHF Output Impedance: 150 Ohms
  • IHF Output Level: -10 dBV, peak +18dBV (ref. 1V)
  • Balanced Output Connectors: 3-pin XLR-M
  • Balanced Output Impedance: 65 ohms
  • Balanced Output Level: +4dBm Front panel adjustable ± 6dB, peak +25dBm

Other Specifications:

  • Power Requirements: 100, 120, 220, OR 240 VAC, 50-60Hz
  • Dimensions: 1.75" H x 8.25" W x 7.25" D (44.5 x 209.5 x 184.2 mm)
  • Net Weight: 3.5 lbs (1.6 kg)

WARNING: Cancer and Reproductive Harm - www.P65Warnings.ca.gov.

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