Bi-amp vs Bi-wire ohm question???

Just as a FYI, the B&W 800 series like low distortion high current power and to be bi-wired or a shotgun run of speaker cables. While the nominal impedance may be 8 ohms, these speakers can dip to 2-3 ohms across the frequency spectrum.

I don't see what the crossover has to do with any of it really as it's just a filter for the hz mainly...

I know you can't change the ohm load of the A speaker itself but when you connect two or more speakers together then the ohm load changes and affects the draw from the amplifier.

Consider the example of an 8 ohm speaker that has one woofer, one tweeter, is not bi-wired or bi-amped, and has a simple first order crossover consisting of a capacitor in series with the tweeter, and an inductor (coil) in series with the woofer.

At high frequencies (well above the crossover frequency), from the amplifier's perspective the woofer will essentially not be there, because the high frequencies are blocked by the inductor. At low frequencies, the tweeter essentially will not be there, because the low frequencies are blocked by the capacitor. So at any given frequency (outside of the region of the crossover frequency), the amplifier is seeing only one driver, either the woofer or the tweeter.

Therefore the nominally 8 ohm speaker will have a nominally 8 ohm woofer and a nominally 8 ohm tweeter.

Wiring two complete 8 ohm speakers in parallel does indeed result in a 4 ohm load, but that is because each complete speaker is an 8 ohm load (nominally) at all frequencies.

If you take that 8 ohm 2-way speaker and bi-wire it, nothing changes as far as the load impedance seen by the amplifier is concerned. What changes is that the series capacitor and series inductor restrict high frequency currents to flowing only in the wires to the tweeter, and low frequency currents to flowing only in the wires to the woofer. One advantage that has is that back-emf generated by the woofer will be kept away from the tweeter, and more generally, unwanted interactions between highs and lows may be reduced.

If you bi-amp that same speaker, the amp driving the woofer will see (nominally!) an 8 ohm impedance for low frequencies, and a very high impedance (i.e., a negligible load) at high frequencies, due to the series combination of inductor and woofer. The amp driving the tweeter will see (nominally!) an 8 ohm impedance for high frequencies, and a very high impedance for low frequencies, due to the series combination of tweeter and capacitor. As Bob indicated, that can be determined more precisely if the impedance vs. frequency curve of the speaker is known, as well as the crossover frequency or frequencies, and the rolloff characteristics of the crossover network.

Regards,
-- Al

I don't see what the crossover has to do with any of it really as it's just a filter for the hz mainly...

I know you can't change the ohm load of the A speaker itself but when you connect two or more speakers together then the ohm load changes and affects the draw from the amplifier.

If the speaker is rated at 8 ohm - that should mean that by Ohms law, that the high and low should have a resistance of about 16 ohm each. When you parallel them, like strapping the hi's and low's together to run with one set of wires - it becomes 8 ohm (2 - 16ohm resistors in parallel).

Not true, because there is a crossover network between the speaker terminals and the drivers.

Ideally, an 8 ohm speaker will be 8 ohms at any frequency. (Of course it won't be, and any speaker will deviate somewhat from its nominal rating as a function of frequency, but that is a separate issue).

Therefore if the jumper between the speaker's low frequency and high frequency inputs is removed, looking into the high frequency input (with a high frequency input signal) there will be nominally an 8 ohm impedance, and looking into the low frequency input (at low frequencies) there will also be nominally an 8 ohm impedance. Looking into the low frequency input at high frequencies (i.e., with a high frequency input signal), there will be a very high impedance (ideally infinite). Looking into the high frequency input at low frequencies, there will be a very high impedance (ideally infinite).

None of that is changed by biwiring or biamping. Longhornguy is correct.

Regards,
-- Al

But to answer your question - when you biwire - both of these wires still come together as one at your speaker terminals on your amp - so even if you biwire --- still 8 ohm load....

This is true but if you measure the speaker terminals with an ohm meter you won't get a reading like Bluesbassplayer suggests. You may get an 8 ohm reading from the woofer but the tweeter will measure quite differently. There's more going on in the crossover than a simple resistance reading will tell.
Maybe someone else can explain or provide a source for a good explanation.

If the speaker is rated at 8 ohm - that should mean that by Ohms law, that the high and low should have a resistance of about 16 ohm each. When you parallel them, like strapping the hi's and low's together to run with one set of wires - it becomes 8 ohm (2 - 16ohm resistors in parallel).

But to answer your question - when you biwire - both of these wires still come together as one at your speaker terminals on your amp - so even if you biwire --- still 8 ohm load....

It is my understanding the resistance is native to the speakers and you can't change it (resistance) by wiring.

So, bi-amping or bi-wiring the 804S will get you 8 ohms.