API Audio 525

Discrete Compressor

SKU: 525 Categories: , , , Tags: ,


API Audio 525 Features

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 blueprints 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 that 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
Noise: 98 dB below +10 dBu output, at 10 dB of Compression
Harmonic Distortion: 0.5% Max, 30 to 20 kHz
Attack Time: 15 microseconds
Release Times: .1, .5, 2.0, 2.5 seconds
De-essing: Inversion of the Voice Energy Curve
Slopes: Compression, 2:1, Limit, 20:1
Metering: 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.
Size: 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.

Additional information

Weight 3 lbs
Dimensions 6 × 8 × 2 in