APHEX 320D Compellor Stereo Audio Level Controller

APHEX 320D Compellor® - Automatic compressor / leveler-limiter, two channel with digital and analog I/O, two channel with digital and analog I/O.

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Item: 320D


Since the introduction of the original Compellor in 1983, no other audio level controller has met with such success.

Around the world, in broadcasting, recording, or sound reinforcement, the Compellor is specified more than any other level controller.

Dual Mono or Stereo
The 320D converts between acting as two independent mono processors and one linked stereo processor by front panel selection. This is true with both analog and digital I/O.

Analog or Digital In, Analog and Digital Out! 
The 320D serves as a fine A to D converter with processing. When you use the analog input, the digital output contains the Compellor's processing and is therefore ready for use in almost any application. Conversely, the 320D serves as a fine D to A converter. When you use the digital input, the analog output contains the Compellor's processing, and is properly levelled and packaged.

Analog and Digital Audio Interfaces 
The professional analog audio inputs and outputs are fully balanced and adapt to consumer or professional operating levels at the touch of a button. Analog or digital source selection is made at the rear panel.

For digital interfacing, the 320D contains a high quality 24 bit low noise codec to receive digital audio and transmit digital audio. Once the incoming digital audio is converted by the DAC section, it passes through the compellor as normal audio. The audio output is then converted back to digital by the ADC section of the codec.

The rear panel AES3 jacks are standard professional 110 ohm interfaces. There is no word clock input or output. Synchronizing is done from the AES3 input signal or an internal clock. When a digital input is present, the output is synchronized to the input sample rate. If no digital input is present, the digital output automatically locks to the internal default sample clock. The user can specify the sample rate by changing the internal crystal. Spare crystals for 32kHz, 44.1kHz, 48kHz, and 96kHz are supplied with the unit free of charge.

The model 320D codec translates the -20dBfs level of the digital audio source to the 0dB reference within the Compellor circuits. The average output levels will target around -20dBfs. The selectable peak limiter's threshold translates to -6dBfs. Thus, no digital audio output peaks will rise above -6dBfs when the limiter is on. The combination of averaging the level around -20dBfs and holding peaks below -6dBfs packages the audio perfectly for natural sound quality and for most transfer or transmission purposes. For example, the digital audio would be ready for direct input toan MP3 encoder, HD recorder, Broadcast digital STL, .wav file, etc.



  • Frequency Discriminate Leveler - No bass pull-back in music
  • Dynamic Release Computer
  • Dynamic Verification Gate - adds greater transparency
  • Silence Gate - Locks gain when audio stops
  • Process balance - Lets you have any combination of leveling and compression
  • True Bypass - no electronics in bypass
  • AES3 I/O up to 96kHz
  • Analog I/O - Fully balanced professional
  • High Reliability - Famous Aphex Engineering
  • Stereo & Dual Mono - Use as two separate channels or linked for stereo
  • Source Select - Choose digital or analog input
  • Dual Outputs - Use BOTH digital and analog at the same time with either input selected

Typical Uses

  • Broadcast pre-processing
  • MP3 Encoding
  • Recording
  • Installed and Mobile sound
  • DAW input
  • Mastering
  • Satellite uplinks
  • Home Stereo
  • Background Music
  • Ham Radio
  • Tape Duplication
  • Cable TV

Leading Aphex Technology
The Compellor circuits are totally unique. By throwing away all conventional design ideas, we created audio processing with unprecedented sound transparency, yet is exceedingly effective at controlling levels. Because our designs are patented, no other company has been able to match the performance. Algorithms like our "DVG", "DRC", and "FDL" have yet to find any rivals.

Unmatched Reliability
Aphex engineering and manufacturing produces products that last and last without need for repairs or calibration. Some Compellors have literally been in 24 hour use in the same rack for a decade, or longer! When you specify a Compellor, you know it will do its job flawlessly and give you peace of mind.

Digital Advancements The first Compellor was introduced in 1983 as an analog product. Because of their advanced technology, most of those older units are still in regular use. Since that time, newer models have been produced, still in analog form. Digital technology of today promises many improvements in user interfaces, and sometimes in sound quality, but not in audio processing algorithms. No digital Processor can match an analog Compellor for sound quality and effectiveness. Nevertheless, many of today's systems need a Compellor that can interface to digital audio.With the Model 320D, Aphex brings you just that. You get all the advantages of the analog processor with AES3 connectivity.

Compression - Leveling - Limiting: A Very Brief Tutorial
Two properties of audio that can cause problems are the average level (the "loudness") and unpredictable peaks that can clip. The Compellor handles both problems for you, fitting the sound into an ideal sound envelope package.

The Compellor's "Frequency Discriminate Leveler" (FDL) section with the "Dynamic Verification Gate" (DVG) and the "Silence Gate" (SG) slowly corrects the volume level of an audio stream with very complex algorithms that hide its work. It works differently at different frequencies, for example. It locks up when the sound stops and resumes instantly when sound reappears. When you listen, you know the levels are made more even, but you don't perceive the levels are shifting.

The Compellor's Variable Slope Compressor with the "Dynamic Recovery Computer" and the "DVG" work on top of the leveling platform to add adaptable compression that helps you match the texture of changing audio sources. Again, you don't hear "pumping" or other audio artifacts, just very natural sound that is evenly consistent in loudness.

The Compellor's limiter acts on peaks that shoot beyond typical audio crest factors. Extremely peaky sounds will be brought into an envelope that is easy to handle by any analog or digital recording or transportation medium including digital codecs.