Speaker impedance patterns - how to read them?


I know this has probably been discussed ad nauseam, but here's my question. I'm grateful for any sensible opinion.

Premise.
A speaker system may have a complex impedance pattern that can vary from 3-ohms to 30-ohms depending on frequency. Some speaker systems with similar impedance patterns may be stated as being 8-ohm loads because the speaker is 8 ohm in an important part of its frequency response, but only there.
A tube amp, as a voltage amplifier, likes a regular impedance pattern. Almost no speaker system has a constant, regular impedance pattern. I am aware that solid state, powerful amps are built to compensate for modern speakers' complex impedance patterns, but there are tube amps around and people who love them. It's difficult to understand what average load a given speaker system will present to the amp.

Question.
When looking for suitable speakers for a tube amp - mine is a PrimaLuna EVO 100, 40w x 2 from EL34s - what must be looked at? How to best interpret the impedance pattern, whenever available? Assuming that one listens at normal levels in a medium-sized room, what are the criteria for matching speaker and amp?

Thanks for your views. M.
martinguitars

Showing 1 response by bdp24

The question posed by @tomic601 was the exact one Roger Modjeski asked when a potential Music Reference customer inquired about the suitability of one of Roger's amps for use with a given loudspeaker. He advised taking a SPL reading at the preferred listening level, a measurement of the amp's output voltage at that level, then extrapolating the required increased power requirements for every 3dB of added SPL.

I have been conversing with a fellow Eminent Technology LFT-8b owner, who has been marveling about how well his moderately-powered tube amp (I do not recall the amp, but it's under 100w/ch) drives the 83dB sensitivity LFT-8b. That's the same sensitivity as that of Maggies, which require and benefit from a high current amp. Sensitivity alone tells one only so much about a loudspeaker.

Maggies are a nominal 4 ohm load (dropping to 3 ohms at some frequencies), the LFT-8b 8 ohm. And if you bi-amp the ET (the speaker is fitted with dual binding posts, one for the m-p panels the other for the sealed box dynamic woofer), the magnetic-planar drivers present an 11 ohm load to the amp, great for tubes. By the way, the owner prefers the LFT-8b to the Maggie 3.7 he previously owned. The most under appreciated, under-acknowledged loudspeaker on the market. $2499/pr. VPI's Harry Weisfeld declared the LFT-8b to have the best midrange he has ever heard, regardless of price. Yet it continues to be ignored by most. Brooks Berdan chose Eminent Technology as his shops' magnetic-planar loudspeaker.