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In Reply to: RE: Depicting high impedance capability for SET's posted by saki70 on September 01, 2015 at 07:52:25
The impedance issue is an amplifier issue, not a real speaker issue, and often not an actual issue at all. There are only a few specific and unusual circumstances where a higher impedance per se is better.
Modern speakers (those designed in the last 40-50 years...) are usually designed for low output impedance amplifiers. A relatively flat impedance spectrum reduces the response changes cause by high source impedance. This is most evident in the deep bass, where the impedance fluctuations are hard to reduce, and cause lumpy bass. Some speakers are overdamped, and work well with SETs which reduce damping; a very very few are actually designed around typical SET output impedance.
These two problems meet in the middle, where using a 16-ohm speaker on the 4-ohm tap of the output transformer reduces the source impedance. Unfortunately it also reduces the available output power.
Follow Ups:
Hi Paul,
What if the nominal speaker load is at 4 Ohms and never dips below 3 Ohms and in the mid-90's dB efficiency, and paired is with a zero fb SET amp of let's say 6W that only has 8 or 16 Ohm taps on the OPT. So only either taps could be used... What other implications are there besides being relatively underdamped in the response? Thank you.
If it has 8 AND 16 ohm taps, connect the 4 ohm speaker between those two taps....
If you only have one tap (2 terminals), then your max power capability will be cut in half at least and your damping factor, too.
Hi,
Thanks for the reply. I guess It would be the worst case scenario on paper (your last sentence.) I just have to do empirical testing to subjectively assess things.
The output tap is chosen to get the best power/distortion combination. A 4 ohm speaker on an 8 or 16 ohm tap will have higher distortion and 2 or 4 times worse damping; it might have greater peak power - or might not - depending on the design choices.
Thanks Paul.. I guess it also boils down on the OPT design especially on the secondary windings.
Best regards..
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