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Old 11th March 2005, 06:41 AM   #1
dan4 is offline dan4  United States
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Join Date: Mar 2005
Location: WI
Default How do autoformer impedance matching controls work

I am in desperate need to understand how an autoformer based volume control works. Ideally, I would like to get my hands on a schematic. I helped a friend install four volume controls (installed in parallel using the x4 setting to present a combined 8ohm load). I am however perplexed how these controls work and need to understand to resolve a problem.

Any help is greatly appreciated. Thanks.
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Old 11th March 2005, 07:21 AM   #2
AndrewT is offline AndrewT  Scotland
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Join Date: Jul 2004
Location: Scottish Borders
the autotransformer will do both impedance matching and volume control. Which do you need?
For a volume control the autoT can provide either gain or attenuation or a range of both.
Consider an autoT with 5 wires; start, 25%, 50%, 75% and finish.
Connect the input to start and finish then the output can be connected to any pair but with no gain available.
eg. output s/25% gives -12db. s/50% gives -6db, s/75% gives -2.5db, s/f gives 0db. The formula is 20 * log(volt gain). If volt gain is less than 1 then the log will be negative. If you need to specify the transformer tappings then the reverse formula can be applied using the 10^x key on your calculator.
If you connect the input to s/50% then you can get gain (2 tappings) and attenuation.
For impedance conversion use the (volt gain ^2) * impedance to give the new impedance.
Maybe that is what you want, I hope I'm not trying to teach you have to suck eggs, Sorry if I have.
regards Andrew T.
Sent from my desktop computer using a keyboard
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Old 11th March 2005, 10:47 AM   #3
moamps is offline moamps  Croatia
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Join Date: Oct 2002
Location: Croatia

my little calculation:
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Old 12th March 2005, 07:51 AM   #4
dan4 is offline dan4  United States
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Join Date: Mar 2005
Location: WI
Thank you. The wiring diagram provided is what I was looking to obtain. Much appreciated!!!
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