1. Introduction

The X903 is a cult compressor module which will add just the right amount of analog punch needed! Equipped with smart RMS detection, the X903 will treat your sound to high-end, authentic analog compression and limiting. With the X903’s Negative Ratio capabilities, you can even inverse the dynamics of audio material, pulling back the transient peaks for a unique special effect. All in all, the X903 is a natural-sounding compressor capable of managing dynamics in great ways!

2. Features

  • Natural-sounding compression even at relatively large amounts of gain reduction.
  • Infinite and negative compression make unusual special effects possible.
  • RMS detection interprets the signal level as the human ear does for superior compression and limiting.
  • Continuously variable Compression Ratio and Threshold settings.
  • LED Gain Reduction metering.

3. Original Hardware Specifications

Maximum Input Level

Maximum Output Level

Frequency Response
20Hz-20kHz (+/- 1dB)

2nd Harmonic 0.05%.
3rd Harmonic 0.2% typical at infinite compression, 1kHz, 0dBm.

Input Noise

Attack Rate
15ms for 10dB above threshold
5ms for 20dB above threshold at 63% gain reduction

Variable -40dBm to +20dBm

4. Layout

1. In Button

Activates or bypasses the compressor. If the diode above is lit, the compressor is active. If it’s not, the compressor is bypassed. The In button also doubles as the BP button from the AFX Rack.

2. Threshold Control

Adjusts the sensitivity of the compressor.

3. Ratio Control

Varies the amount of compression from 1:1 (no compression) through to Infinity:1 (infinite compression). You can also go into negative compression ratios, producing inverted dynamics and a sense of “punch”.

4. Output Control

Controls the output gain from -20dB to +20dB.

5. Gain Reduction Meter

The LEDs indicate the amount of compression in dB.

5. Usage Tips

5.1 Threshold

The Threshold control sets the level at which the X903 starts compressing the signal you are processing.

At low compression ratios, a very low threshold setting can gently reduce the overall dynamic range of your audio.
Higher compression ratios coupled with low threshold settings provide levelling for instruments and vocals.

High threshold settings are generally used for limiting. Compression at low threshold settings tends to sound less natural at ratios of 10:1 or greater.

The X903 uses a soft knee at the compression threshold. This means it gradually increases the compression ratio from 1:1 towards the selected ratio as the input signal rises through the threshold region.

With this compressor, the threshold is defined as the point approximately midway between fixed gain and the point at which the curve levels off to the selected compression ratio. At the Infinity:1 setting, the maximum permitted output level is 5dB above threshold with the Output gain set to 0.

5.2 Compression Ratio

A setting of Infinity:1 means the input signal must reach an infinitely high value for a 1dB increase in output level to be produced.

High compression ratios (6:1 or greater) and low threshold settings reduce dynamic range and prevent output signals from substantially exceeding the threshold setting. Low compression ratios (4:1 or lower) affect dynamic range to a lesser degree.

High compression and low threshold are typically used to tighten up the sound of the bass guitar, snare drum, or vocal.

At the extreme clockwise Ratio Control rotation, a -1:1 Output to Input ratio is achieved. This way, the X903 reduces the output level by 1dB for every 1dB that the input signal increases above the Threshold setting. Thus, the X903 output will start rising in level with the input until it begins to approach the Threshold setting.

At this point, output level will begin decreasing with further increase in input level. At a relatively low threshold setting, the dynamics of some instruments will become inverted for an effect similar to tape being played backwards.

6. Contacts

Having difficulties with X930 or anything else Antelope? Get in touch with us by the following means:

European Direct Support Line
+44 2039578550
10:00 a.m. – 10:00 p.m. (EET), Monday – Friday
USA Direct Support Line
+1 734 418 8661
7:00 a.m. – 6:30 p.m. (EST), Monday – Friday
Online (live chat and e-mail)
Antelope Audio Support Page

7. Glossary

Adjusts how quickly the compressor starts to work after the signal exceeds the compression threshold.
Controls how soon the compressor will stop after the signal goes below the compression threshold.
Controls how much compression is applied. For example, a 3:1 ratio means that every time the input signal crosses the threshold by 3dB, output signal will be attenuated 1dB.
The Threshold is the level above which the compressor considers the signal too loud and starts applying compression.
Knee and Attack have a lot in common, but they are not the same. Like Attack, Knee controls how the compressor will react once input signal passes the threshold. Hard Attack makes the compressor engage immediately while Soft Knee shapes its attack time to make it slower and smoother.