Columbia Broadcasting System (CBS) Audio Processing Equipment

Legendary Audio Processors for Radio Broadcasting

These were the first true audio processors in the world, specifically designed for AM radio broadcasting.
The Volumax is used after the Audimax in the audio chain.
They were famous for the sound of AM Top 40 Radio in the late 1960's and 1970's, as on 770 WABC New York, 930 KHJ Los Angeles, 890 WLS Chicago, 1050 CHUM Toronto, and 800 CKLW Windsor to name a few. SCROLL DOWN TO BOTTOM OF PAGE FOR LIVE DEMO AUDIO!
Audimax - a series of compressors
Model Features Notes
Audimax I 440 All tubes
Audimax II 441 Added tube pre-amplifier to 440
Audimax II 442 Improved version of 441
Audimax II 443 Stereo strapping version of 442 (required two 442's)
Audimax III 444 Transistors + 8056 nuvistor tube Classic AM gain controller.
Audimax III 445 Stereo version of 444
Audimax 4440 All transistorized, smaller, improved S/N version of 444 Classic AM gain controller.
Audimax 4450 Stereo version of 4440

Volumax - a series of peak limiters/controllers
Model Features Notes
Volumax 400 All transistorized, introduced at same time as Audimax III 444 Classic AM processor.  Legendary in performance.
Volumax 4000 Smaller, improved S/N version of 400 Classic AM processor.  Legendary in performance.
Volumax 4300 Added automatic peak polarity sensing/switching IC to the 4000 Classic AM processor, especially for talk/sports programming.  Not only corrected out of phase microphones, but even syllable phase changes! Legendary in performance.
Volumax 410 FM version of 400 The FM versions of the Volumax were not as successful as the AM units.  Aggressive pre-emphasis requirement of FM broadcasting made these units not perform satisfactorily.  They used dual band compressors as a workaround.
Volumax 411 Stereo strapping version of 410 (required two 410's)
Volumax 4100 Smaller, improved S/N version of 410
Volumax 4110 Stereo version of 4100
Volumax 4111 Improved version of 4110

Schematic diagrams:

Audimax I, II 440 441 442/443
Audimax III 444
Volumax 400
Volumax 4300


Volumax 400
Audimax 444
Volumax 4300

The secret of the Volumax is in the time constants:
Volumax 400:  attack = 4500 ohms, 2.2 mfd = 10 msec, release = 90K ohms, 2.2 mfd = 200 msec, plus instant asymmetrical diode clipping
Volumax 4300:  attack = 15K ohms, 0.56 mfd = 8.4 msec, release = 470K ohms, 0.56 mfd = 260 msec, plus instant asymmetrical diode clipping

The Volumax 400 is slightly louder sounding than the 4300.  This is because 1) its release time constant is slightly more aggressive (ie. faster), and 2) there is no positive peak clipping (at the time this model was produced, the FCC did not impose a limit on positive modulation percentage of an AM transmitter!).  The Volumax 4300 is slightly smoother sounding and is suited for talk/sports radio because of the slightly longer release time constant, and the fact that it has automatic phase correction which is great when unknown source microphones are being used in a remote voice hookup.  But both sound almost the same and are loud.

The medium attack time (8-10 msec, 63% time constant) allows percussive sounds to go unaltered straight through, to the diode clippers.
The relatively fast release time (compared to the slower Audimax gain controller) (200-260 msec, 63% time constant) highly compresses the syllables.

The compression is conventional positive and negative peak detection hard-knee threshold with a single R-C timing network with a 10 to 1 gain reduction ratio (for most of the input range) of input versus output. The compression threshold is unsymmetrical if the positive peak switch is in the +115% or +125% positions. There is approximately 4 dB compression when the gain reduction meter is quarter scale ("NORMAL"), or 0.4 dB increase in output for a 4 dB increase in input from when the gain reduction meter starts to register, 8 dB compression for half scale, or 0.8 dB increase in output for an 8 dB increase in input, 12 dB compression for 3/4 scale ("HEAVY"), or 1.2 dB increase in output for an 12 dB increase in input, and 14.5 dB compression for full scale, or 1.6 dB increase in output for an 14.5 dB increase in input.

The asymmetrical clipper is set to begin negative peak clipping at slightly above the full scale level, at approximately 15.5 dB compression, or for a 2.0 dB increase in output for a 15.5 dB increase in input. Obviously, a sine wave going through the Volumax, with the gain reduction meter anywhere on scale will exhibit no clipping, as it should be. Speech or music, however, will have instantaneous peaks that will go well beyond this level even though the average gain reduction shows on scale (less than 15 dB). It is this closeness (1.2 dB between output when gain reduction meter is half scale and the output at the clipping level) of average loudness to the level at which the peaks are clipped by the diodes that give the Volumax its loudness performance.

This was revolutionary, compared to the 1 microsecond attack time peak limiters traditionally used, with no clipping, which basically reduced gain, lowering volume and causing gain pumping.  The Volumax was, and still is, the loudest single band analog audio processor ever designed, and has an especially pleasing sound with 1960's and 1970's rock/pop music as well as voice.

Typical sideband power gain = 6 dB, distortion <0.5%. Frequency response 50-15,000 Hz. It makes everything loud, but adds no harmonic distortion. This result is completely different than amateur radio audio processors. For its simple circuitry, it truly was the most revolutionary audio product of the 20th century. If you were not a teenager in the 1970s, then you missed something on the radio!!

Zero distortion class B driver stage
Tired of close to 10% THD out of the driver stage for your class B modulator tubes, poor frequency response, or even worse, inability to drive the modulator to full plate saturation output?    Here is the answer.  This cathode follower circuit will drive 810's to well over 540 watts output at 2000V loaded plate voltage, with negligible driver distortion.  All you need is voltage...400 volts p-p of clean drive signal, no power required, no variable loading during the sine wave cycle to worry about.

Visit 590/CKEY AM RADIO for a live Volumax 4300 demonstration:
For other demonstrations of the loudness of the Volumax 400 and 4300:
TOP 40 radio station air checks from the late 60's - mid 70's:
CKLW air check
WABC air check
WLS air check