Input impedance issue?


Hello all,
I have a PrimaLuna Prologue Premium power amp driving KEF LS50's and a Rotel RB-880 power amp driving 12" NHT subs.
I'm using an NHT X2 crossover on the subs and going direct on the KEF's. I don't like the sound I get on the KEF's going through the high pass on the NHT. They are both being driven by an Audible Illusions L1 preamp. The pre has 2 sets of outputs. Seems when I have one output connected to the NHT and the other to the PrimaLuna the high end on the KEF's drops off. It's there but no sizzle. The only way it seems to sound right is to connect to the NHT via the tape outs on the pre for the subs. Then it's kind of a pain to keep moving the level on the subs all the time. Does any of this make sense? Any ideas? Thanks in advance.
Ben
honashagen
Put a tube buffer in between your preamp and the NTH crossover. The very high input impedance of the buffer will make it almost invisible to your preamp. I think the Musical Fidelity buffer in the square box has a 1 million ohm input imp. that you can pick up for around $200 or less. I noticed the exact same thing on my pre using both outputs, one to the main amp and the other to a powered sub. The tube buffer was my fix and I didn't get it for the "tube" sound, but rather as an impedance matching device.
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
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