Passive bi-amping works ?

Martin Logan's cross-over (not sure which speakers you have) with passive bi-amping allows each (hi / lo) connections to only draw power in that bandwidth. While active bi-amping usually yields superior sonic results, this passive bi-amping is very effective and less expensive than having to also buy an active cross-over. You do get significant benefits of passive bi-amping in this configuration over higher powered amps with bi-wire. The reason is the electrostatic panels can go to relatively low impedance. Depending on the music there may at the same time be a large bass spike (kick drum or something equally demanding). This asks for a large amount of power to the woofer, while the impedance is very low. This is an extremely difficult demand to the amplifier. By bi-amping (passive) these two demands are separated, so one amp can be handling the electrostatic (low impedance) demands and the other can be handling the bass (high power/impact).

Not so fast. With passive bi-amping, both amps still see the full range of the input signal. Yeah, you get twice the power if you use one more amp like you had before but you don't get the benefits of limiting the bandwidth of the INPUT signal to each amp. This is what active bi-amping is all about and the benefits in that case are considerable.

Now having said that, passive bi-amping can be attractive with some speakers. If the high & low pass sections on the ML's are isolated from each other when configured for bi-wiring, then, during passive bi-amping, each amp will only see 1/2 of the x-over instead of the whole thing. As such, your amps will probably deliver more current to the individual drivers than a single amp would of the same combined wattage looking into the entire x-over. This also means you will have slightly better control over each driver as well.

Also, as is usually the case for electrostatic panels, as the frequency goes down impedance goes up, sometimes considerably. If this is so with the ML's, then the amp would be asked to supply more voltage than current at lower frequencies. This may reduce the stress on the amp running the panels and may be audible. Similarly, as the frequency rises, there is probably an increase in the impedance of the low frequency section. It won't be as dramatic as the rise of the panel, but it too may help to reduce the stress on that amp and may also be audible.

When I passively bi-amped my ML Quests by using another Forte 4a (yes, that amp did a great job with the Quests), I immediately noticed a step up in quality, with all the usual suspects (slam, soundstage, transparency...)going up a notch. The basic charater of the combined amps was the same as a single amp, just noticably more refined and dynamic. The point here being, if you have an amp that you like with your ML's but want more power, just add another amp.

If you do decide to passively bi-amp, one thing you need to pay attention to is to use a HIGH QUALITY splitter (assumming your pre-amp does not have an extra output set)and use identical, high quality interconnects all around.

Good Luck

1953 makes a good point which may have not been clear in my first post. The input to the amp is the full bandwidth (as 1953 correctly stated), but the load is only the hi/low that it is going to, so the demands on the amplifier as I stated in my first post are limited by the passive crossover. It will, as 1953 mentioned, make a substantial improvement.

Like other things audio, it's controversial, but like mono-blocs, bi-amping allows for use of (potentially) very short speaker cables. Many claim significant improvements with the use of short speaker cables.

My amps will be back from SMc Audio (for upgrading & matching) next week, and I'm going to use them for passive bi-amping (Vandersteen speakers cannot be active bi-amped without screwing up the internal X-over up) as recommended by Vandersteen and SMc. I'll post a short report when I've gotten some time on the system. Cheers. Craig.