A pair of stereo loudspeakers from Polk Audio with early crosstalk cancellation, sold for $995 per pair in 1984.
Overview
The Polk SDA1 is a stereo loudspeaker system made by Polk Audio. It was part of the company's SDA (Stereo Dimensional Array) series. The idea behind SDA was crosstalk cancellation - a technique meant to reduce interaural crosstalk and give a more three-dimensional soundstage. According to Audio (January 1984), the SDA1 had an original price of $995 per pair, which put it at the premium end of Polk's lineup at the time. A user confirmed buying a pair in 1985, so they were at retail by then.
Design
The SDA1 uses an MTM (mid-woofer-tweeter-mid) driver layout, with no crossover on the woofers and a 6 dB slope on the tweeter. It also has a proprietary "Z-Network" - a Zobel network made of a 55 µF capacitor in series with two parallel 7.5 ohm resistors - meant to stabilize impedance and help amplifier compatibility. The mid-woofers are believed to be the Mw6510 6.5-inch driver, but official specs for the drivers, frequency response, sensitivity and cabinet were never published. Some owners report a sensitivity of 91 dB. That figure is user-reported and not officially verified.
Market
The original electrolytic capacitors in the crossover are known to degrade with age and may need replacing to get the speakers performing properly again. Mid-woofer drivers from this era are also prone to magnet slippage, and owners recognize it as a known problem. The pin/blade interconnect cable between the active and passive speaker is often lost or damaged, which makes setup a headache. Exact current market values aren't documented, but anecdotal reports suggest restored or well-maintained units could bring several hundred dollars.
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