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API 525 Microphone Preamp

500 Series Discrete Compressor
Part No: API-525

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.

Condition: new

in stock
£1,299.00 £1,082.50 ex VAT
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£1,299.00 £1,082.50 ex VAT


Product Description


The API 525 is a “feedback” type compressor first circulated in the 70s. The user can change dynamics on the fly without level alterations thanks to numerous mechanisms: a provided input (threshold) control and output (make-up gain) control, along with an additional “Ceiling” fine-tune function which can intensify gain reduction (lower threshold) while concurrently raising output level to match.

Attack time is as fast as 15u/s. A compression/limit switch sets ratio at either 2:1 or 20:1. Two switches offer four auto release modes: 0.1s, 0.5s, 1.5s, & 2.0s. An inverse vocal energy curve filter in the detector is injected by a De-Ess function for active sibilance/pop reduction. Primary blue prints and spec control from the API archives have been the basis of the API 525’s design.

High frequency / full bandwidth content is emitted faster than lower frequencies – the frequency dependent release time variation is for natural envelope tracking. Chosen to catch the fastest peaks, attack is immovable without “pumping”. The API 525 applies “vintage” dynamics control and an easy to use multi-function control set which is equally as useful as tracking, mixdown or program compressor/limiter. As is the case with all other 500 series modules the API 525 has a hard-write bypass switch, a balanced input and a pinout. For multi-channel compression, two or more 525s can be chained.

The API 525 Compressor demonstrates the dependability, longevity and uniformity characteristic of all API products and makes use of 2510 and 2520 op-amps.


  • 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


Additional Information


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 micro seconds

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.

Dimensions and Weight

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.