Joule Electra LA -150 ii/ 200 and impedance issues

Al, As always, your insights are extremely valuable and in the past have prevented me from bad decisions, like biamping on single terminals:)....but are you saying that, if a preamps outputs are paralleled, which yoy say is common, bi-amping may cause issues without our knowing it because the combined impedance load is too low for the preamp? Isnt it conventional wisdom that the ratio should be 10:1 of impedance output:input? Maybe this isnt often achieved and maybe bi-amping may adversely affect frequency response and, on the whole, be deleterious instead of additive? Do I misundertand you? This is surely a fundamental question?

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