It appears that some of the speaker selector switches that claim impedance matching or amp protection just put a small, high power resistor in series with each channel (left and right) to limit how low the resistance that the amp sees can go. With a 3 ohm resistor in series and from 1 to 8 pairs of 8 ohm speakers the resistance (actually impedance...) would vary from 11 ohms for one speaker (8+3) to 4 ohms for all 8 (3+8/8). That seems pretty good and is a lot less complicated than the series/parallel switching for the other unit posted. If you want to have a minimum of 8 ohms just use 7 ohms for the added resistance. Max would be 15 ohms. If you want to allow all speakers to be open put an 8 ohm resistor in place of one set of speakers.
The down side is not being able to adjust the individual levels, the heat generated in the resistors at high volume levels, and limiting the power delivered to much less than the amp output (when all the speakers are on). The sound output will also vary with how many speakers are on. But it is simple and cheap so if you can live with the limitations it may work fine for you.
Hum. My speaker selector does indeed have a 50W 3 ohm resistor at each channel, and looking at the circuit board it does appear to be in a series with the amp input.
So maybe I only have 1 ohm of resistance with the speaker wiring, but the amp sees 4 ohms with the 3 ohm resistors already in the series. How do I test for this? Turn all relays on so the speaker resistance is 1 ohm, then measure the resistance at the amplifier input and it should read 4 ohms?
EDIT - just tested it out and this is exactly the case. There is a 3 ohm resistor wired in series between the amp and the parallel wiring of the speakers within the speaker selector. With all six zones on, 8x 8 ohm speakers, I see 1 ohm at the speakers and 4 ohms at the amp. With 4x 8 ohm speakers on, I see 2 ohms at the speakers and 5 ohms at the amplifier. With 2x 8 ohm speakers on, I see 4 ohms at the speakers and 6 ohms at the amplifier. So I think my original idea worked.
Next idea - I want the elk to make voice announcements through some of these speakers via output 1 on the M1G. I am initially thinking about a source sensing a/b switch. The sinks goes into a and the elk output goes into source sensing b, and the switch output to my speaker selector. How is this idea? Any other ideas?
Edited by Galun, 04 April 2011 - 09:54 PM.