The under appreciated impedance curve

Bruce Thigpen stated in his discussion on the design of his magnetic-planar driver, the Linear Field Transducer (LFT), that he could make it with any nominal impedance he chose. Bruce says he chose 4 ohms for the LFT-3, LFT-4, and LFT-6 so as to enable the power amp driving them to provide them with as much power as they are capable of producing (assuming the amp is solid state, of course. SS amps produce more power into lower impedances than into higher. Most tube amps are the opposite.). He doesn’t say why he chose 8 ohms for the LFT-8 (and improved incarnation the -8a and current -8b), but by virtue of it being a nominal 8 ohm speaker, the LFT-8/a/b is more suitable for use with a tube amp than are the magnetic-planar driver 4 ohm Magnepans. By the way, if bi-amped, the LFT-8b’s magnetic-planar midrange driver and ribbon tweeter themselves presents an almost purely resistive 11 ohm load to the power amp, and are therefore an excellent candidate for use with a tube design.

that he could make it with any nominal impedance he chose.

This is true for all cones as well. Off the shelf components are often available at different impedance for exactly the same reasons. Lower impedance = more current = more force against the magnetic field.


However, I have in fact seen Focal create a crossover deliberately to burn power. It had 80 W worth of resistors that didn't need to be there.

I've always over powered my speakers thinking it would take care of any dips.. if any…  I now own a 3-way speaker with a flat curve.. Salk Song3a.. Now I don't have to worry about it anymore..  Yeah, its pretty important to know this measurement.. 

The quick answer: The weaker the amp, the more its output in the frequency domain will look like the impedance curve.

Thanks Eric.  I guess that's a good rule of thumb.  Though I always hear "it's totally different between SS and tube amps how they react to speaker loads" so I think I'm still a bit in the dark.

I have heard various things about matching a tube amps output tap to the speaker impedance.

On one hand some say if you have an 8 ohm speaker, you should be using an 8ohm tap output for your tube amps or you aren't getting all the power from your amp.

On the other, some have said that outputting from the 4 ohm tap will generally entail better sound/less distortion at the expense of less full output power, so *if* you will still have enough power for your speakers from the 4 ohm tap, better to use it even with 8 ohm speakers.

Confusing things more to me are that stereophile measured the CJ premier 12 amps I own as actually dropping power output into higher impedances (139W into 4 ohm speaker load, only 98W into 8 ohm speaker load).  Though the review amps came set for 4 ohm tap output...I don't know if that explains it and if switching the amps to an 8 ohm tap would retain their full power output to an 8 ohm speaker load?

Prof,

It really only takes a little math to understand how it works.


An ideal amp:


[ input voltage x gain ] = Output Voltage


Gain is often around 20x (26 dB)


The reality :


[Input voltage x gain ] = Voltage lost in output stage + Voltage at Speaker terminals


Tube amps will loose more in the output stage. You can analyze it more or less like series resistors.

Vr1 = Vamp out x (   R1 / ( R1 + R2))


Where R1 = Amp output impedance  and R2 = Speaker impedance


Of course, R2 varies based on frequency.

For Solid State, R1 may be < 0.001 Ohms. For tube it is often in the range of 1-2