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The CBS Audimax and Volumax

One in a series of occasional looks back at important devices and practices in radio broadcasting.

One in a series of occasional looks back at important devices and practices in radio broadcasting.

In a previous article we spoke of the elimination of the transmitter watch engineer. One important task this person faced was continually to adjust audio levels into the transmitter to avoid over- and under-modulation.

To allow the newly minted “combo man” to focus on creative efforts and to keep average modulation as high as possible, automatic audio level control devices – limiters and compressors – became standard equipment in stations starting in the 1950s, replacing the watchman’s hand.

None of these boxes was more successful during that period than the CBS Laboratories Volumax limiter and the Audimax, a platform-type smart AGC.

CBS Labs was the technical research and development arm of the Columbia Broadcasting System. Headed by lead scientist Dr. Peter Goldmark for most of its existence, the lab was expected to keep CBS technically ahead of its competitors. The company hoped that the sale of some of that advanced technology, such as the Volumax and Audimax, would make the effort profitable as well.


The first units were designed with tubes; but few were sold, as they cost nearly as much as a contemporary Ford or Chevy automobile.

These were updated by the successful, more cost-effective 400 series. Using early solid-state silicon transistors and a balanced amplification scheme, these boxes achieved level control by changing the bias on an intermediate stage’s transistor bases, essentially varying the amp gain.

Well designed and built, they were easy to service, with each subsection on its own PCB. Input and output were transformer-coupled. The overall gain was sufficient to make up for all but the loss of the worst telco audio circuits.

The Achilles heels of these units were the 600-ohm, T-configuration wire-wound variable attenuators on the input and output. They were noisy from day one. Replacements are nearly impossible to find today.

The AM Volumax had a nifty but primitive asymmetrical diode clipper scheme to achieve higher positive modulation.

The FM Volumax was probably the first limiter to level the pre-emphasized portion of the upper end audio above about 2 kHz in a separate sidechain gain controller. The result was a more natural audio leveling that was not bottomed or “plunged” by a random high-end audio component like a record pop (remember records?).

A little lazy, the FM Volumax attenuated any peaks that got by via soft clippers made of complimental zeners on the line-level output.

The 400 series gave way to the functionally similar, yet smaller, single-rack-unit 4000 series. For this compactness, the T pads and input transformer were eliminated.

Mystery inside

It’s hard to kill the CBS 400s. WTIC(AM)’s Volumax was probably the most famous. It carried that station in Hartford, Conn., through at least 10 ratings books as No. 1 against intense competition. Still reliable after refurbishment, it now controls the audio at HRRZ in Juticalpa, Olancho, Honduras.

The Volumax and Audimax are remembered as well for their mysterious sealed circuit modules. In the industrial manufacturing universe, patents and trademarks protect published processes, techniques and devices. As further protection, a firm is allowed to have secret processes and circuits such as the exact formula for Coca-Cola and the sealing glue used in Raychem shrink warp.

CBS Labs chose to encapsulate certain parts of its circuitry in potting compound inside small metal enclosures – probably not an original gambit, but it started a fashion in the electronics industry still seen in some Orban products, among others.

Tame by today’s loudness standards, the Audimax had an effective control window of about 20 dB, with a gated resting position near the mid gain point. The Volumax was a 4-to-1 limiter.

Designed to work as a pair, dynamically these boxes functioned well together, raising the uniform audio and modulation standard of broadcast performance — at least until the Optimod.

Tell us your memories of the Volumax and Audimax. E-mail us at [email protected].