Denis, I took a look at the PF review you referred to. The comment in the review about 100K, which I quote just below, makes no sense to me. Frankly, I don't think the author has any understanding of how gain and impedances inter-relate.
The combined impedance of your 100K and 47K loads, when paralleled, is (47 x 100)/(47 + 100) = 32K.
Even if the output impedance of the LA-200 were as high as 32K, which would be utterly absurd, there would only be a 6 db reduction in overall gain as a result of the impedance incompatibility. For more reasonable output impedances, and the 32K load impedance, the gain reduction would be negligible. And although I haven't researched the gain and sensitivity of your particular power amps, 8 and 12 db line stage gains are in the right ballpark for most (although not all) applications these days, especially if digital sources are being used.
That said, the 150 MkII has an output impedance that is specified as being selectable to either 400 or 1200 ohms. Conceivably it is a good deal higher than those numbers at some frequencies, especially in the deep bass if its output is capacitively coupled. And as you indicated there doesn't seem to be an output impedance spec for the 200. If the two sets of output jacks that are provided on the 150 (and perhaps also on the 200) are driven from the same output stage, and if the worst case (highest) output impedance of that stage at any frequency is more than a small fraction of 32K, frequency response flatness (not gain) may be adversely affected.
I would suggest that you ask Joule what minimum load impedance is recommended for the two preamps, and if two pairs of individually buffered output jacks are provided. More often than not, when two pairs of output jacks are provided on a preamplifier they are driven from the same output stage, and are simply jumpered together inside the rear panel. In which case the single output stage (for each channel) would see the combined 32K load presented by your two amplifiers, rather than either of their individual input impedances.
Regards,
-- Al
Matching it [the LA-200 line preamplifier] to an amplifier with impedance input more than 100,000 ohms is critical to obtain significant gain in the system. The circuit is set for 8 dB gain, but can be adjusted up to 12 dB. I left mine at the stock setting. Bear in mind that low impedance amps wont be driven to their full output. This may or may not be important to you. But, it is a consequential characteristic of the LA-200 that limits its universality.First, under any circumstances that are remotely within reason what may be affected to a significant degree by having too low a load impedance will be frequency response, and possibly phase response, not gain. Secondly, a blanket statement that "low impedance amps won't be driven to their full output" makes absolutely no sense, because different amps differ widely with respect to how much input voltage they require to be driven to full power.
The combined impedance of your 100K and 47K loads, when paralleled, is (47 x 100)/(47 + 100) = 32K.
Even if the output impedance of the LA-200 were as high as 32K, which would be utterly absurd, there would only be a 6 db reduction in overall gain as a result of the impedance incompatibility. For more reasonable output impedances, and the 32K load impedance, the gain reduction would be negligible. And although I haven't researched the gain and sensitivity of your particular power amps, 8 and 12 db line stage gains are in the right ballpark for most (although not all) applications these days, especially if digital sources are being used.
That said, the 150 MkII has an output impedance that is specified as being selectable to either 400 or 1200 ohms. Conceivably it is a good deal higher than those numbers at some frequencies, especially in the deep bass if its output is capacitively coupled. And as you indicated there doesn't seem to be an output impedance spec for the 200. If the two sets of output jacks that are provided on the 150 (and perhaps also on the 200) are driven from the same output stage, and if the worst case (highest) output impedance of that stage at any frequency is more than a small fraction of 32K, frequency response flatness (not gain) may be adversely affected.
I would suggest that you ask Joule what minimum load impedance is recommended for the two preamps, and if two pairs of individually buffered output jacks are provided. More often than not, when two pairs of output jacks are provided on a preamplifier they are driven from the same output stage, and are simply jumpered together inside the rear panel. In which case the single output stage (for each channel) would see the combined 32K load presented by your two amplifiers, rather than either of their individual input impedances.
Regards,
-- Al