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.

Murphthelab:

Thanks a lot. In your opinion what type and what size of cap it the best for my Keilidh?

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".

Murph...,

Your suggestions seem to be fine but you're still facing a "double-degradation" using passive crossovers on two sides amplifier and speaker-embedded.

A capacitor tended to bring more distortions than any active element so mainly a lot of engineers try to avoid their introduction unless for smoothing in PS or forward or reverse correction of RF in input and feedback circuitries. A further note that capacitor isn't an ideal and "strong" filter as you may expect and more or less will pass portions of "unwanted" amplitudes in such interaction.

The more complicated part that is not probably covered in EE books due to complexity of calculations and tremendous time to be spent for testing is integrating built-in speaker crossover or actually complexed speaker load with built-in crossover with filters tended to be used on amplifier side.

There is always signal degradation even if the filter is purely passive. Consequently, the quality of the caps and resistor will have an impact on the sound, since the signal traverses an equivalent C-R circuit.