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API Audio 525 500-Series Compressor Used

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Single space compressor for the 500 series rack units.


Discrete Compressor, Re-Issue


  • Exact reissue of API's original 525 from the early 70's
  • Peak detecting "feedback" compressor/limiter
  • Variable Threshold and Output levels
  • Unique "Ceiling" control combines key functions
  • Analog VU meter for monitoring gain reduction
  • 4 Switchable release modes
  • De-Ess function
  • Traditional API fully discrete circuit design
  • Active Balanced Input/Output
  • Hardwire bypass

Originally released in the 70's, the API 525 is a "feedback" type compressor. An input (threshold) control and output (make-up gain) control are provided, along with an additional "Ceiling" fine-tune function which can increase gain reduction (lower threshold) while simultaneously raising output level to match, so the user can alter dynamics "on the fly" without level changes. A compression/limit switch sets ratio at either 2:1 or 20:1. Attack time is as fast as 15u/s. Four auto release modes are offered via two switches, 0.1s, 0.5s, 1.5s, & 2.0s. A De-Ess function inserts an inverse vocal energy curve filter in the detector for effective sibilance/pop reduction. The 525 design has been taken from the original blue prints and spec control drawings from the API archives.

Release times vary with frequency, with high frequency / full bandwidth content released faster than lower frequencies for natural envelope tracking. Attack time is fixed, chosen to catch the fastest peaks, without "pumping". Equally useful as a tracking, mixdown or program compressor/limiter, the 525 utilizes "vintage" dynamics control and an easy to use multi-function control set. The 525 has a hard-wire bypass switch, a balanced input, and a pinout that is the same as all the other 500 series modules. Two or more 525s can be linked for multi-channel compression.

The API 525 Compressor makes use of the 2510 and 2520 op-amps and exhibits the reliability, long life, and uniformity which are characteristic of all API products.



Input Impedance:

> 30 K Ohms, Balanced

Output Impedance:

100 Ohms, Transformer coupled, Balanced

Operating Levels:

-15 dBu to +20 dBu

Maximum Gain Reduction:

25 dB


98 dB below +10 dBu output, at 10 dB of Compression

Harmonic Distortion:

0.5% Max, 30 to 20 kHz

Attack Time:

15 micro seconds

Release Times:

.1, .5, 2.0, 2.5 seconds


Inversion of the Voice Energy Curve


Compression, 2:1, Limit, 20:1


Compression or Limiting-Illuminated Analog dB meter,

shows Gain Reduction

Frequency Response:

±.5 dB, 30 to 20 kHz

Power Requirements:

±12/18 Volts/DC, @80 ma.


1.5" X 5.25" X 7.5" Deep

Size (Boxed for Shipping):

4.5" X 6.5" X 10"

Actual Weight:

1.77 lbs.

Shipping Weight:

2.17 lbs.

MPN: API 525

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Retail Price: $1,295.00
Discount Price: $735.00

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