I love the idea of active bi-amplification with high powered solid state amplifiers for the bass and lower powered amplifiers – maybe solid state Class-A, or (if you want to be really silly) valves for the midrange and treble. IMO, valve amps have no place in modern audio, but doing bass duty is surely dumb. Nonetheless, when relieved of that burden the output valves ought to last longer and the output transformers are much less likely to be saturated. And compared with bi-amplification using passive crossovers which filter by providing an extra frequency-dependent impedance load on the amplifier, the amplifiers of an actively crossed system only amplify their respective filtered portions of the line level signals for efficient delivery to the drivers. Intermodulation distortion is reduced. Tri-amplification takes it a step further but for many (not me), that is considered as taking the path of diminishing returns.
This was my first active cross-over. It’s a straight-forward application of an ESP Project 09 24dB/octave Linkwitz-Riley PCB with a 15-0-15 power supply and muting relays. It has OPA2134PA opamps.
It’s set at 150 Hz for woofer-to-mid in a pair of old Jamo Concert VII speakers in which the passive bass/midrange filters were disabled – retaining their midrange/tweeters sections:
These are very dynamic speakers and were awarded Loudspeaker of the Year in 1989 by an Australian hi-fi magazine. The clamshell push-pull (isobaric coupled) bass drivers in a 6th order band-pass configuration with concentric ports extend fairly flat to 25 Hz with a sharp cut below that. The standard passive crossovers had the two sets of input binding posts grouping the woofers with the midrange (isolating just the tweeter) which was a bit odd. Sections of the passive board were disabled for the active filter to take over between the woofers and midrange, so while doing that I used a hacksaw and jumper wires to group the midrange with the tweeter and isolate the woofers. I also swapped an electrolytic capacitor for a polypropylene one and bypassed the tweeter protection fuse which was not necessary with the tweeter now grouped with the midrange and on a low-powered valve amplifier.
Standard passive crossover board:
A C-J MF2500 power amp (which I modified years ago to MF2500A spec.) powers the bass drivers and a 22W C-J MV55 (which I modified to operate in low power push-pull “triode configuration”) powers the midrange and tweeters.
The speakers are by no means high-end by modern standards, but now sound better than they did.