Spectron musician ii hybrid monoblock question

Good info, thanks very much for your knowledge. I'll defiantly give ARC a call and pose that question to them 

I would expect that the input impedance of each of the amp’s inputs is the same regardless of whether the amp is being used in mono or stereo.

The 10x rule of thumb guideline for impedance compatibility should ideally be applied at the frequency for which the output impedance of the component providing the signal is highest. Most impedances are specified at a mid-range frequency such as 1 kHz. It is very common for tube preamps to have output impedances at deep bass frequencies that are much higher than that specified value, often 2K or 3K or even 4K ohms. That rise at low frequencies results from the output coupling capacitor that is used in the majority of tube preamps. In those cases a significant degree of deep bass rolloff may result if the load impedance is less than 10x that higher value, depending on how much difference there is between that low frequency output impedance and the output impedance at higher frequencies.

If as is often the case the component’s output impedance at 20 Hz is not known, and is not indicated in published measurements (such as Stereophile often provides), then to be safe a considerably higher ratio than 10x should be used, perhaps even 50x or 75x IMO. Especially if the component is tube-based.

In any event, to assure optimal results I would consider ARC’s 20K recommendation to supersede any such guidelines. The question, though, is whether or not that figure applies to the balanced outputs as well as the single-ended outputs. As I indicated I would have some concern that the corresponding figure for the balanced outputs might be 40K (i.e., 20K per leg). You might want to pose that question to ARC.

If the Spectron has a balanced input impedance of 50K, given that the LS-17SE’s two XLR output jacks (per channel) are most likely driven by the same output stage (and are probably just wired directly together inside the rear panel), driving the two inputs of each monoblock from those two jacks (or from one of the jacks using your existing splitter) will result in a load impedance seen by the LS-17SE of 25K.

Regards,
-- Al

Sorry I hit send too soon
i thought the thinking was the Amps input imp should be 10x higher than the preamps output (I could be wrong). Or maybe I am not considering the fact that I'm using the amps in mono? Would the amps input impedance be different in stereo vs mono? I couldn't find anything concrete but I'll see what I can research 

For the ARC:
INPUT IMPEDANCE
120K ohms Balanced, 60K ohms SE. 

OUTPUT IMPEDANCE
600 ohms Balanced, 300 ohms SE Main (2). 20K ohms minimum load and 2000pF maximum capacitance. Outputs (3): 

spectron has an input of 50k ohms

Yes, that will function ok. However I’m not sure that the resulting sonics would be optimal in the case of the LS17-SE and some other ARC line stages, due to impedance compatibility issues.

In the case of the LS17-SE (as well as many other ARC line stages) the recommended minimum load impedance spec is 20K. And although the specs I’ve seen don’t make it clear, I wouldn’t be surprised if that was for the unbalanced outputs only, and if the corresponding number for the balanced outputs is twice that amount, or 40K. And I also suspect that the two balanced output jacks of the LS17-SE (for each channel) are driven by the same output stage, which would mean it would be driving both inputs of the Spectron, and therefore seeing a load impedance corresponding to half of the input impedance of each of the Spectron’s inputs.

So if the input impedance of the Spectron Musician ii is spec’d at less than 80K the resulting sonics might not be optimal with the LS17-SE (and a number of other ARC line stages), especially in the bottom octave where the LS17-SE’s output impedance probably rises significantly relative to its specified nominal value.  And that concern would apply regardless of whether you connect to one output jack per channel using your present balanced splitter, or connect to the two jacks with separate cables.

Regards,
-- Al