Radio Broadcast Technical Consulting and Sales
 10032 Island Drive, Brainerd, MN  56401

  CBS Audimax III 
Modifications

by Mark Persons

Here is a 1970's vintage CBS Audimax III Automatic Level Control after having undergone shop modifications.  Those modifications include replacing the 8056 Nuvistor tube with a field-effect transistor.  Two resistor values are changed and then the unit is tuned to match the original performance.  Audio does not go through the Nuvistor.  It is a control amplifier for the transistor audio stages.

The Audimax is a gated audio gain control typically used in radio studios to adjust audio levels within a +/- 10DB range to a +/- 1.5 DB range making up for operator or automation errors.  CBS did an excellent job designing this unit with the components available at the time.  

You will notice blue color just to the left and up from the front panel meter.  That is a ten-turn control which replaces the original output level control.  The original controls are subject to becoming intermittent and are currently unavailable.  We often use ten-turn screwdriver-adjusted controls on the input and output of the Audimax with excellent success.  Those can be single-turn controls that can be turned by fingers when required.

This photo shows a board we add.  It is a phase rotator to keep positive and negative audio peaks even.  Without this feature, it is not uncommon to see audio peaks on the positive side 4 DB lower than the negative side.  For broadcasting, where processing is run to the maximum, it is very important to start out with audio that is symmetrical.

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