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Amplifier Output Transformers and Output Impedance

Since 4 ohm and 8 ohm speakers are common today, you see amps with 4 ohm and 8 ohm output taps. I've assumed that the 4 ohm tap would present a 4 ohm impedance to the load; likewise with the 8 ohm tap. However, recently I've noticed a few amps measured by Stereophile that have a much lower (like around 1 ohm) output impedance than their output tap indicates.

While this will lessen the interaction with the speaker's impedance, doesn't it lessen the amount of power delivered to the speaker?

Any thoughts on the designer's rationale?
bob_reynolds

Showing 2 responses by onemug

onemug
784 posts
 
Good thread with great information.

Al or anyone else: Besides using the best tap to match up with a speaker, when your amp has the ability to adjust the bias and the manufacturer gives you a range of say 220 to 300 ma, would you want to bias more toward the upper end of that range if the speaker you are using likes current, say like a Magneplanar?
onemug
784 posts
 
Thanks Herman,

I'm using my ears but trying or correlate what I'm hearing with some knowledge.

My amp sounded great at around 240 mA with some low 90 db/8 ohm speakers. Hooking it up to the Maggie 3.6's (84 db/4 ohm) it sounded a little cold with some distortion in the upper freqs. I upped the bias to 300 and it got that magic back. I've been backing it down to see where I lose it again (experimenting as I type).

Do the Maggies, with their contrasting load, take the tubes out of their previous linear range and therefore have to find a "new" sweet spot?
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