On the bench: CBS Audimax 4450 Stereo Compressor / Leveller

The Audimax range was developed by CBS for AM & FM radio broadcasting. It’s a compressor / leveller designed to create a consistent level from mixed sources and less-than professional operators. But it can be creatively abused to dramatic effect: ” I noticed that excessive input creates huge compression density and also makes these things pump like mad!”

The Audimax was often paired with a CBS Volumax limiter which was used to drive the radio transmitter to peak modulation – but not over. Overmodulated transmitters could cause interference in other bands and the FCC levied hefty fines for offenders

At the time broadcasters vied for the loudest signal in their area and there was a constant struggle to achieve the loudest, punchiest signal. The Audimax and Volumax – dubbed “The Max Brothers” – were the CBS way of achieving this

Levellers like the 4450 use a slow release time to avoid pumping. Later models, like this one, had an internal three-way switch to select release times. I’ve modified this to have much more aggressive release times to accentuate the effect

Attack times were set by the CBS compression module, an encapsulated “secret” module developed by their engineers. Schematics are now available

There’s a fair amount of background info available for these:




CBS Audimax 4450 compressor board 4450 – deep 1U box with lots of separate circuit boards

CBS Audimax 4450 internal shot shows power supply, control board plus left & right compressor boards. The grey encapsulated units are proprietary CBS timing modules

CBS Audimax 4450 compressor board

CBS Audimax 4450 control board & power supply before recapping

CBS Audimax 4450 front panel preset pots – to be replaced with regular pots & knobs