Question about external crossover with tube preamp

I'm setting up a new 2-channel system, and in my setup, I'm looking to use a tube preamp. I also want to use an external crossover of some kind, so that the sub handles the low frequencies, with the mains handling the higher frequencies. I'll probably cross at around 80 Hz.

My question is this: I know that when matching a tube preamp with a SS power amp (or a SS subwoofer amp), it's important to check that the Zout on the preamp is sufficiently low compared to the Zin on the amp. Im thinking it should be better to connect the preamp directly to the power amp, rather than connecting it to both a power amp and one (or two) sub amps, particularly since the Zin on sub amps can be rather low.

However, it seems like most people are crossing using an active crossover, which goes between the amp and preamp. This requires that the preamp signal go to both the power amp and the sub amp, which I'm trying to avoid for the reason above.

Is it generally a bad idea to use an active crossover with a tube preamp, or is it "okay" for some reason, given the impedance characteristics of typical equipment?
If you want to cut off your main speakers at 80 Hz you will have to have either an active crossover or a passive attenuater between the amp and pre. The option is to run the main speakers full range and run a parallel set of signal leads to an active or passive crossover driving the sub amps. I use REL subs, which give many options and I recommend. There are other powered subs which do similar things. I think any of them would work well with a typical tube preamp. With an external crossover it would depend on WHICH crossover and WHICH preamp but most should work well, consult the makers of the units under consideration for best advice.
I should have added that I also wanted to avoid running the main signal through an electronic crossover, as I have always heard a degradation in the sound quality when I did so. Others disagree. With the RELs I set the subs to start rolling off at 25 Hz at the top end. I think 60 is too high, let alone 80 , for a sub used for music reproduction. HT has different parameters.
If you use an active external crossover, the preamp output goes to the external crossover only. The crossover then drives the amp and subwoofer. The preamp only "sees" the external crossover. So you need to match impedances between the preamp and the external crossover.
Interesting Bob, I wasn't aware of that. Is this also true for a non-powered unit, like the Marchand XM46?
Yes, assuming the external crossover is a bass management controller, i.e., has both a low pass filter whose output connects to the subwoofer and a high pass filter whose output connects to the main speakers via their amps. The connections will be exactly the same whether or not the filters are passive or active.

I seriously would consider the potential for problems with a passive unit. An active one should have relatively constant input and output impedances and the outputs will be driving the loads not your preamp. Their should be no impedance matching problems with an active crossover and any of the solid state amps (including the sub's). You'll have to be aware of impedance matching between the preamp and the active crossover. You need to know the worst case output impedance of the preamp not the typical value at 1 kHz.

With a passive unit, the effectiveness of the filters will depend on the impedance interaction they experience with their source and load impedances. Their load impedance, the input impedance of the amps, should be relatively constant and not a problem. The filter can be built with that knowledge. But, their source impedance, the output impedance of the preamp, will likely vary with frequency -- increasing in the bass region. The filter can be built knowing the worst case impedance of the preamp, but it will still have an issue with the impedance changing.

There is also the insertion loss of the passive crossover.

Ultimately, the variation of the output imedance of the tube preamp has to be taken into account. It is less of an issue with an active crossover.


I use exactly the type of set-up you're describing. A tubed ARC preamp (LS 25) feeds an NHT X-2 active x-over. The NHT high pass out (signals over 75hz) feeds an ARC VT 130SE. The main speakers are usually Ohms, but I also ocassionally rotate in Maggie or Verity speakers.

The low pass out of the X-2 feeds a Velodyne SMS-1 subcontroller which allows for digital room correction and fine tuning of the high cut filter characteristics for the sub's top end roll-off. This signal, in turn, feeds 2 Rythmik 12" subwoofers.

The results have been enormously satisfying in a very tough room and the NHT X-2 has little to no (to my ear, anyway) impact on the main signal path.

As always, YMMV.

Good Luck

That sign-off should read:

Good Luck


(Last I checked, anyway)