API 527

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The 527 is like a trip in the back machine, with a familiar old school VCA sound!

The API 527 takes it's place alongside the family of API VCA based compressors, the 225L and the 2500 Stereo Bus Compressor. Anyone familiar with those units will immediately be at home with the 527. Features common to the line like "feed forward" (NEW) and "feed back" (OLD) gain reduction methods selectable on the front panel, provide a choice of "that old way", or "the new way" of compression, for the highest level of flexibility in signal gain control. The "old way" or Feed-Back method is what most of the classic compressors used for the gain control circuit. The "new way" gain reduction is more typical of the newer VCA type compressors that rely on RMS detectors for the gain control voltage.

There is a "SOFT"/"HARD" switch for an "over-easy" type compression resulting in a very natural, uncompressed sound or a typical sharp knee type that lends itself to a much more severe limiting effect.

The Threshold control sets the initial operating parameter. The 527 can be operated with signal input levels from +10 dBu to -20 dBu.

The 527 features an Output fader control that ranges from minus infinity to +10dB, and output capability is +28 dBu through our 2520 discrete amplifier and a large API output transformer. The output level remains fairly constant regardless of the threshold or ratio control, much like the "more/less" Ceiling control on the API 525 compressor. This allows for live adjustments without any noticeable gain changes in the program level.

The patented THRUST function can be switched in and out via the front panel as well, applying a high pass filter before the RMS detector circuit that preserves that punchy bottom end.

Attack and Release are fully adjustable on the 527. Release time can be adjusted by rotating the inner concentric RELEASE knob. Release time constants: .3 sec to 3 sec. Attack time is adjusted on the outer concentric ATTACK knob, which ranges from 1 millisecond to 25 milliseconds.

The 527 is designed for individual channel use or, through the use of the LINK switch, two units can be combined for Stereo applications via a motherboard bus connection.

The 527 Compressor/Limiter makes use of the 2510 and 2520 Discrete Op-amps and exhibits the reliability, long life, and signature sound which are characteristic of API products.


  • Audio circuit uses the 2510 and 2520 Discrete Op Amps with transformer output
  • Continuously variable detented Threshold control
  • Continuously variable detented Attack and Release controls
  • Continuously variable detented Ratio control
  • Output Fader level control
  • 10 segment VU/Gain Reduction meter with selector switch
  • Overload LED
  • "Link" switch for multiple unit interdependent compression linking
  • "New"/"Old" switch for feed-forward or feed-back compression
  • "Hard"/"Soft" compression curve knee switch
  • Patented THRUST® switch for frequency dependent side chain control
  • "IN" switch with hard relay bypass when in the "Out" position


  • Input Impedance: High Level In 160K ohms
  • Low Level In 80K ohms
  • Output Impedance: High Level Out 85 ohms
  • Low Level Out 50 ohms
  • Nominal Operating Level: -2dBu or +4dBu
  • Maximum Input Level: +26dBu
  • Maximum Output Level: +26dBu
  • Output Fader Control Gain: -Infinity to +10dB
  • Attack Times: 1 millisecond to 25 milliseconds (typical)
  • Release Times: 0.3 seconds to 3 seconds (typical)
  • Compression Ratios: 1:1 to 1:Infinity
  • Threshold Control Range: +10dBu to -20dBu
  • Metering: 
  • 10 RED LED VU scale: -20VU to +3VU (positive illumination
  • indicator)
  • RED GR scale: -1.5dB to -23dB (negative illumination indicator)
  • RED Overload LED: +27dBu (clipping)
  • THRUST® Filter Action: -10dB/Decade slope, unity @ 1kHz
  • Frequency Response: +/- 1dB 20Hz - 20kHz
  • Total Harmonic Distortion: < .2% THD, any level below clipping, any compression setting
  • Signal to Noise: Unweighted -100dB below clipping (+26dBu@0.1% distortion, no compression)
  • Power Requirements: +/- 15-18VDC @130mA
  • Conforms to Automated Processes, Inc. VPR Alliance size and connectorization layout specifications.
  • This unit is made to install into the 500 Series frame VPR rack, lunchbox®, or any API
  • console with 500 series slots.
  • Size: 1.5" X 5.25" X 7.5" Deep
  • Size (Boxed for Shipping): 4.5" X 6.5" X 10"
  • Actual Weight: 1.75 lbs.
  • Shipping Weight: 2.15 lbs.

Our ONE WITH EVERYTHING ZEN WARRANTY extends the standard manufacturer warranty by one year, see our ZEN WARRANTY page for details.

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