Bi amping with Tube and Solid State

You would like to use tubes on top to enjoy the lush, liquid sound. However, you also want the low end slam of SS. Passive bi-amping (some may call it "Fool's Bi-amping" does not have all the advantages of active bi-amping. However, in your case, a key advantage is obtained. Despite presenting full-signals at the input to both tube and SS amps, you will definitely reduce the load burden experienced by the tube amplifier. It will not "see" nearly as much of the speaker's low end load. This is where the majority of current and power are consumed. Therefore, your output tubes can run somewhat cooler and you will not drive your tube amp towards clipping as quickly at high volume.

One tweak that will help this configuration further is the addition of a High-pass filter to the tube amplifier's input. This can be nothing more than a capacitor soldered behind the input socket. The size of the cap is chosen according to the cut-off frequency you desire. Now the tube amp does not have to internally deal with the low end spectrum, thereby enhancing performance further. You have now reduced smearing and other harmonic distortion effects.

Active bi-amping not only requires an active cross-over feeding the independent amplifiers, it also requires a viable means of bi-passing your speaker's embedded x-over.

First you must know the cross-over frequency of your speaker (low end). On my Martin Logan SL3's, it is 250 Hz. Therefore, I would choose something less than this to make sure I preserve the integrity of the internal x-over circuit inside the speaker. So in my case, I could choose 180 Hz. You want a simple, single-pole low pass filter. Therefore, a single capacitor will suffice. You also need a resistor if the input port is not already using one. The capacitance is determined according the relationship between cutoff frequency, resistance and capacitance. You'll need to dig out your EE book or use a tech for your case. The cap will likely be something < 1 uF.

A key point is that this is a purely passive filter, below the x-over frequency. Therefore, there is no degradation to the signal whatsoever. Cap quality is less of an issue, use something "reasonable".

In the context of cross-overs and bi- (or tri-) amplification, the word active simply refers to the fact that no energy from any one amplifier is used on more than one target driver. While this is a good thing (listen to a Meridian active speaker system for verification), it is still achieved via filtering. That is, the active cross over, which acts on the line level signal, uses capacitive components to filter the signal and thereby introduce its own contribution to "signal degradation" and distortion (however minimal).

My suggestion above on the HP filter is a means of reducing the energy outside the pass-band of the speaker's x-over circuit. I stand by the concept that the consequent performance improvement should outweigh any effect that an RC combination will have on signal frequency components above the corresponding x-over's cutoff frequency.

Notwithstanding this, I concur w/ Gmorris that using quality caps and resistors is prudent, at the very least. At the same time, man-o-man I wish this would all be easier... ;-)