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In Reply to: RE: Fisher 80AZ zmatic posted by stltrains on April 19, 2011 at 18:06:20
Quite possible that there's a oxide film on the switch contacts so that an ohmmeter won't make it conduct - wouldn't be a problem on a power switch, as 117V would instantly burn it away. Anyway, the grounded end of the pot is the same as "off" anyway.
How it works: the negative feedback is a combination of voltage feedback (from the 16 Ohm terminal) and current feedback (the drop across a sample resistor, with three settings for 4, 8, 16). Voltage feedback lowers output impedance, curent feedback raises it, so the variable mix of the two can give any desired output Z. Most modern speakers are designed to work with low source impedance, so you'd probably leave it at zero.
Follow Ups:
Tom can't thank you enough for your explanation of zmatic. I did not get to looking into the.switch today played with filter caps. Mike
hey-Hey!!!,
the big Heathkit W6m also had a similar FB combination. Neat stuff, but I eliminated it with the rest of the amp/pi tubes and circuitry in favour of a pentode LTP.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
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