Sansui QS-500 (c. 1972)

The QS-500 was Sansui's early-1970s way to get four-channel sound without buying a new receiver. It was a decoder/adapter with two rear-channel amplifiers built in. You kept your existing stereo system, added the QS-500 and a pair of rear speakers, and the unit handled the decoding (or synthesizing) of the rear channels. Audio-Collection (p.242) called it an "adapter-rear-channel-amplifier combination unit," with controls and metering for front, rear, and front-to-rear balance.

Sansui's ad in Audio (January 1972, p.5) led with the price, $209.95, and the pitch that converting to four-channel didn't have to cost a fortune. By June the same ad listed $214.95 (Audio, June 1972, p.17). The features were three balance controls for front-rear and left-right, separate decode and synthesize positions, two- and four-channel tape monitors, alternate-pair speaker selection, and electrical rotation of the speaker output. That last one shifts the sound field by 90 or 180 degrees.

That's about all the surviving record holds: period ads and an equipment-guide entry. No service manual or definitive production figures have turned up so far. The QS-500 belongs to the first wave of quad add-on components, before full four-channel receivers made the adapter approach less necessary.