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Autoformers (again??!)
Autoformers (again??!)
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Old 14th October 2003, 07:15 PM   #11
PRR is offline PRR  United States
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> Can I use one as the volume control in a passive pre-amp.


These are wound for use between a loudspeaker amp and a loudspeaker. Load impedance about 8Ω, source impedance very much lower. Probably the inductive impedance of the unloaded autotranny dips to 8 ohms at 20 Hz. Your CD deck's output impedance is more like 100Ω. Output level will be low and possibly distorted, and bass will be even lower.

Also as Dave says: the minimum useable attenuation setting is probably higher than you want as a main volume control. Steps may be larger than we'd really like too.

There are line-impedance tapped autoformers, but not at the $20-$60 level. Sowter has nice 10K:10K cores for 86.63, but you will have to find a 2 pole 24 position switch.

The main advantage of a transformer over a resistor attenuator is that it does not waste power. If you feed a 100 watt amplifier into an 8Ω pot to reduce speaker level, the pot burns up. With the autotranny, the transformer action reflects a higher load to the amplifier, drawing less power, and delivers 90+% of that power to the speaker. It runs cool.

At the 0.000,1 Watt levels that lines work at, overheated pots are not an issue. And pots are linear to DC and to high voltages and frequencies.

> What exactly is the z out of a cd player, and where would I find this out?

You could measure it. But the fact is: you never want to come close to loading most modern outputs in their actual source impedance. They have a rated load impedance, and you don't want to go lower than that. For hi-fi box line outs, usually 10K, though some will happily drive 2K or even 600Ω.

The internal impedance may be as low as 47Ω, but if you load them in 47Ω you will get half-output and probably gross distortion. The low output Z gives authoritative drive into long cables and no change in output level between 10K and 1Meg loads, but is not the load that the box can drive.
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