Input impedance issue?

I'll second Koestner's comment.

As in many and I suspect most preamps which provide two pairs of outputs, the two sets of output jacks on your preamp are most likely wired directly together just inside the rear panel. So the output stage of the preamp is therefore driving both the NHT crossover and the PrimaLuna power amp. While the PrimaLuna probably has a high input impedance, the NHT's input impedance is only 10K, so their combined input impedance, as seen by the preamp, is probably in the area of 8 or 9K. The output impedance of your preamp is specified as 1.2K, and mostly likely rises to considerably higher values than that at some frequencies. An output impedance that is high in relation to the load impedance, and that ALSO varies significantly as a function of frequency, will result in the kinds of issues you are perceiving.

I'm not sure if the Musical Fidelity buffer (the X-10v3 I believe) is still being made, but if you can find one it would be a good solution. Also, another member here who had a similar issue has reported an excellent experience having a tube buffer custom made for him by Tom Tutay of Transition Audio Design, in Florida I believe.

Good luck. Regards,
-- Al

ZD, yes, a suitably chosen solid state buffer will of course do the job well, and perhaps (depending on the specific models being compared) better than a tube-based buffer. But a tube-based design that is intended to be used as a buffer perhaps stands a greater chance of providing an optimal combination of high input impedance and low output impedance, especially the high input impedance part.

Burson Audio used to make a well regarded solid state buffer, the AB-160, which may be findable pre-owned. I am not specifically aware of any other high quality equivalents that are currently produced, but there certainly may be some.

And hey, perhaps a good solution would be to simply buy a used minimalist/no frills solid state or tube line stage preamp, which provides high input impedance and low output impedance, and insert it between the existing preamp and the NHT crossover.

Best regards,
-- Al

On paper, at least, the Silk Audio buffer and the similar Yaqin model look like they would probably be reasonable choices. Although I do have a bit of concern about the lowish 60 db A-weighted signal-to-noise spec on the Silk unit. And I would suspect that the Yaqin is no better in that regard, with its stated 75 db performance (no weighting specified) just being better on the basis of "specmanship."

Normally I don't attach any significance to signal-to-noise specs, because they can be defined in numerous different ways that are usually not indicated. But in this case the 60 db A-weighted number is low enough to raise a bit of a red flag, although it's quite possible that in a subwoofer application it won't matter.

Also, I note that there is no spec on output impedance. Which I would assume is low at mid and high frequencies, but depending on the value of the coupling capacitor that is most likely used at its output could conceivably rise to unacceptably high values in the deep bass region, which is what is important in this case.

Good luck. Regards,
-- Al