A feature often found in high-end radios of the 1930s and 40s was the dynamic volume expander circuit. This circuit, basically a feedback loop, was used to restore some of the broadcast program’s volume range lost due to compression. In order to keep a radio broadcast within the transmitting station’s assigned bandwidth (typically ten kilohertz), engineers had to use volume compression to limit the audio level of the program material. This tended to make it seem as if softer passages were louder and vice-versa, resulting in a constant volume level and a somewhat stilted sound. With a volume expander circuit, the reproduced program could be made to sound more “natural”. The circuit was also effective on recorded music, since recordings were subject to the same type of volume compression.

The method was quite simple. The circuit begins with a vacuum tube used as a plain audio amplifier, usually placed electrically near the main input. The incoming audio signal is amplified, and then must be changed into a direct current controlling voltage. This would be accomplished with a rectifier, most often a twin diode tube. Thus, a controlling voltage was produced that varied with the intensity of the audio signal.

The controlling voltage then passed through a load resistor and filter capacitor. The values of these components would be adjusted to give the best time constant – which governed the rise and fall of the voltage, and thus the attack and decay times. Noisy “pops” in the final audio output were most often due to improper time constants.

Finally, the controlling voltage was fed to the volume expander control tube, which also received input from the original audio signal. This tube was ostensibly another audio amplifier tube – but the controlling voltage from the rectifier tube governed its amplification. If that voltage increased or decreased, the amplification of the control tube increased or decreased accordingly.

The total effect, to the listener, was that as the audio passages got louder or softer, so did the overall apparent loudness of the system. This restored, to an extent, the original dynamic range of the program, resulting in more faithful reproduction. It was as if someone were twisting the system’s volume control in accordance with the program.

The use of dynamic volume expanders died out in later years, but the circuit reappeared in some of the larger European table radios of the 1960s. Today, audio processing of that sort is frequently done with software. For example, the popular DFX plug-in for Winamp has a “dynamic boost” control that mimics the functionality of the old volume expander circuits.


SOURCES

RCA Receiving Tube Manual, Techincal Series RC-22, 1956
Rider, John F. Radio Troubleshooter's Manual. New York: John F. Rider Publisher, 1938.

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