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Polk Audio Speaker Repairs

One fascinating thing about the world of speaker repair is the amazing variety of strange and interesting designs I come across.

A local young man, who had previously brought me a suspect tweeter to test (it was bad, Polk sold him a replacement), brought me both of his SDA 2A speakers, concerned that the "spatial effect" driver in one was not working properly.

Essentially, the effect driver plays the bass frequencies from its own side, and the midrange from the other side, but out of phase. This is acheived by using a special cable to connect the two speakers to each other. It serves to carry the "+" signal from each speaker to the other, and depends on the amplifier in use having outputs with a common ground.

The layman's test that this is working is to move the balance control all the way from one side to the other, and when only one speaker is playing, the effect driver on the other side should also play.

It turns out I can't easily test this here with music, since my Adcom GFA 555 amplifier does not have common ground outputs.

I had to ponder for a bit and sleep on the problem, until I remembered that my test amplifier, a Crown DC300, is common grounded, and serendipitously, usually has each channel running radically different signals - one is a 1300 Hz or so tone used in part of my tweeter testing, and the other is a low frequency sweep, from about 20 to 150 Hz, used in part of my woofer testing process.

He did not have the original special cable, and so had replaced the special connectors with binding posts to be connected red to red and black to black, although they weren't soldered (I was hoping that was all the problem would be).

So, after soldering those reasonably well-twisted connections, I wired them up, facing away from each other, and played my two signals through them, muting them alternately to see if the effect driver in the other speaker was doing what it should. They both were, so I had him come over to witness.

The crossover schematic, adapted from a 6/20/86 Polk document found on the internet, is below the photos.

polk sda 2a

polk sda 2a

Key to Crossover Symbols and Component Values (in a new window)

polk sda 2a schematic

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