Joule Electra LA -150 ii/ 200 and impedance issues

So, before biamping, this calculation and these data should always be done and obtained, or you could end up with worse sound at great expense? WoW!?

Yes, unless the preamp provides two pairs of outputs that are driven by separate output stages (i.e., that are "individually buffered"). Some preamps do that, but most don't. With a preamp having that provision, the 10x guideline would be applied in the normal manner.

Interesting that, in all the various bi-amping posts, I've never come across this concept. Am I blind, is it less practically important or are others equally unaware of it?

It won't be an issue with most solid state preamps, due to the low output impedance they usually have. The high input impedance of most tube power amps will also reduce the likelihood of it being an issue, if both amps are tube-based. And if, as is common with tube preamps, their worst case output impedance within the audible frequency range occurs at 20 Hz (due to the impedance rise that occurs in the deep bass region as a result of the output coupling capacitor most tube preamps use), the problem may not be readily perceptible if the frequency range of the speakers (as modified by the room) or of the source material doesn't extend down to very low frequencies.

Regards,
-- Al

BTW, I'll mention another potential biamping issue that isn't generally realized.

In the case of a preamp driving a single power amplifier, if the output impedance of the preamp is high (at high frequencies), and the interconnect cable is long and also has high capacitance per unit length, the result can be a slight but perceptible rolloff of the upper treble. Although in most circumstances that effect won't be great enough to be audibly significant. But in the case of a preamp driving two power amps from a single output stage, the possibility of that kind of issue increases significantly, because the combined (total) capacitance of the interconnect cables to BOTH amps will affect the signal seen by the high frequency amp.

So in the situation where a single preamp output stage is driving two power amps, and the preamp has a highish output impedance, it becomes especially important to minimize the TOTAL of the lengths of BOTH interconnects, and/or their capacitance per unit length.

All of these considerations, btw, are also applicable to the situation where a preamp is driving both a power amp and a powered subwoofer.

Regards,
-- Al

Springbok, yup it's just like Al said. Take a look at some of my posts which discussed how I loaded my ARC Ref 3/5 preamp's 2 outputs: Main 1 -- drives the main amp. Main 2: -- drives a single subwoofer. ARC recommends that the combined load impedance presented to the Ref 3/5 NOT be less than 20K Ohms.

Using Al's formula, I realized that I overloaded my preamp. In my case, I found a gent who custom designed a built a buffer device for me that presents Main 2 with a much higher impedance load. The device also (i) sums the left and right channels for the subwoofer without shorting the Mains and (ii) enables me to tap both loads into the preamp's balanced outputs.

It's a cool device and was NOT terribly expensive.

Jud Barber says, in response to my e-mail, that his preamps are designed to work best with impedance 25K ohms and above. So I should be safe. thanks for your help, Al and Bifwynne