dbx Model 21 Disc Decoder
One box that let a conventional stereo play the full dynamic range of a dbx-encoded record - no new amp required.
Overview
The Model 21 existed to solve one problem. dbx-encoded records were unplayable on ordinary gear unless you already owned a dbx tape noise-reduction unit. The Model 21 was the cheap, dedicated decoder for everyone else. dbx's own advertising was blunt about the barrier it removed: no expensive new equipment required, just the Model 21 patched into the tape monitor loop of an existing receiver or amplifier. That single connection point is the whole design. The unit is a decoder, not a processor. It takes the compressed signal off a dbx disc and expands it back to what the master tape carried.
The pitch behind the whole format was dynamic range and surface noise. dbx's ads claimed dbx Discs delivered the full dynamic range of conventional studio master tapes with virtually no record surface noise, and the Model 21 was the key that unlocked it. Owners who already had a dbx tape system - the Model 122, 124, 128, or 224 - could play the discs without a Model 21, since those units had decoding built in. The Model 21 was aimed at everyone else.
The decoder was also a promotional lever. As dbx advertised in the pages of Audio (December 1981, p.49), the company gave a 3BX Dynamic Range Expander buyer a Model 21 free. The inexpensive decoder pulled buyers into the disc catalog.
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