Joule Electra LA -150 ii/ 200 and impedance issues

Thanks, Denis. You understand correctly.

If a single preamp output stage (for each channel) is driving two power amplifiers, either because the preamp has two pairs of output jacks that are jumpered together internally and driven from a single output stage, or if y-adapters are used to split a single pair of output jacks, the load impedance seen by that output stage will be considerably lower than either of the individual power amplifier input impedances.

As you indicated, if a preamp is driving a single power amp the ratio of power amp input impedance to preamp output impedance should ideally be 10:1 or more (at the frequency for which preamp output impedance is highest, and its value at some frequencies may be much higher than the specified nominal output impedance, especially in the case of a tube preamp). A lower ratio may or may not give good results, depending mainly on how the preamp's output impedance varies with frequency (the less variation the better).

If a single preamp output stage is driving two power amps the same principle applies, except that the load impedance that should be used to calculate the ratio is the parallel combination of the two amplifier input impedances. Which as I indicated will be lower than either individual input impedance, usually much lower.

As illustrated by the calculation in my previous post for your specific amplifiers, the combined parallel input impedance of two amplifiers equals the product (multiplication) of their individual input impedances divided by the sum of their individual input impedances.

Regards,
-- Al

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

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?
You may have trouble answering this question:)

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