McIntosh MC901's internal active crossover & speaker's internal passive crossover

When you biwire, you do not send the full amplifier output to the tweeter, do you? Of course not, you’d blow the bugger out. It still goes through the speaker’s x-over. It’s easy to verify with a DMM - just check for continuity. Spendors used to be wired that way, IIRC, and it always puzzled me.

I agree that you should check for continuity before bi-amping, Check hot to hot and ground to ground. I’ve seen all 3 cases:

1. Infinite impedance
2. Both internally shorted (0 Ohms)
3. Ground only shorted

In the case of 1, bi-amping is fully safe.

In the case of 2, no bi-amping at all is safe

In the case of 3, only unbalanced amps with 1 hot output are safe.

In all cases listed the internal crossover remains in place.  It's really a manufacturing choice whether or not the filters are joined internally. 

Want to point out that there’s a number of A’goners who are successfully passively bi-amping. 1 and 3 seem to be the most common situation.

PS - I'm not in any way advocating for bi-wiring.  Just sharing what I know about speaker crossovers and the connectors out the back. :)

@cleeds

What you are missing in your understanding is that the filter sections for each driver are independent in the classic, parallel crossover.

When the external straps are removed the individual high and low pass sections remain in place but the crossovers should not (and usually are not) have any be connected internally.

An external crossover would of course be additive to the internal crossover so it's not quite as good as having a purely external crossover, but it's workable.

Still, the risk of connecting two amps to the same bi-wire speaker is only present in rare cases when they join the grounds internally, and a balanced amp is used. 

Best,

Erik

A biwirable speaker provides a separate path for lo and hi frequencies, but they are still joined electrically at the xover.

@Cleeds In my experience that’s’ almost never the case, but I have seen it happen. Almost all speakers I’ve seen with 2 sets of inputs and external jumpers separate the crossovers internally.  Otherwise, why have jumpers??

You can easily check this in a couple of ways. Remove the jumpers and plug in the woofers. See if anything comes out of the tweeter.

Next, do an impedance check between hot to hot and ground to ground. Should be infinite.

The one case I remember being posted about the speaker only had ground shorted internally. Normally this would be OK unless your amp was fully balanced.

@cleeds 

What exacy do you think a speaker made for bi wiring does?  What do you think happens when you remove the external jumpers?

If you connect both MC901 outputs to a biwired speaker, you'll blow the amp.

Usually not.  Sometimes yes. Depends on exactly how the speaker inputs were wired and whether or not your amp outs are balanced or not.

A properly wired speaker for bi-wiring separates out the crossover sections entirely internally.  Some however short the grounds together, by design or by oversight.

It is not ideal to use both an active and passive crossover, but it is still workable.