Impedance of biwire-able speakers

Just a simple question (I think) - is a 4 ohm, bi-wireable speaker still considered a 4 ohm load if it is truly bi-amped? In other words, if I take out the jumpers and get a second amp, is the load for each amp still 4 ohms? Or is it now 8 ohms... or 2 ohms... or does it depend on the crossover/speaker design? Thanks.
It will vary with the individual speaker design. Given that most speakers vary impedance with frequency, it is quite possible to have two completely different loads presented to each amp. The one thing that will be consistent though is that the amp will typically see a much higher impedance at and around the resonance point(s) of the woofer and / or the vents ( if used ). It is not uncommon for a large dynamic woofer in a vented alignment to produce an impedance peak of 30 - 100 ohms. Needless to say, power transfer and control over the driver is greatly reduced in that region due to this. It is for this reason that larger woofers / designs using multiple woofers tend to benefit the most from bi-amping as the amp can now cope more efficiently with the higher levels of reactance and reduced power transfer without dragging the other frequency regions into the mess. Designs using smaller woofers with less excursion and / or designs with controlled impedance may not benefit nearly as much from bi-amping. As a general rule, most sealed and stuffed designs have a MUCH lower impedance peak at resonance, increasing both power transfer and control. Then again, sealed designs do have make slightly longer excursions than a vented design, negating some of those benefits due to slightly higher levels of reflected EMF. Sean
Sean hits the nail on the head. This is a question for your speaker manfacturer assuming they would tell the truth. Nominal impedance is sort of an average of impedance across the frequency spectrum. You probably can find a frequency vs impedance curve either from a review, your manfacturer or maybe your speaker manual. Once you determine the crossover point you can look at that curve and get an idea of what impedance you are facing . Actual impedance is determined based on the amps output impedance together with the speakers impedance(Thanks, Raul). Hopefully your speaker will have a relativey smove curve.
The other problem is you really need to have an separate volume control for each amp. It would be helpful to know what your amp and speakers are.
Congratulations on recognizing a point I never thought about.
Greg: To take that a step further, the nominal impedance of the speaker cable is also inserted in series with the speaker as part of the load. This is why some cables are more neutral in some systems than others. Running your .08 ohm output stage from your solid state amp into a 60 - 100 ohm loudspeaker cable to feed your 4 - 8 ohm speaker seems a bit ridiculous to me, but hey, what do i know??? : ) Sean
Thank you both, Sean and Gregadd. I am new to audigon and am finally delving into higher end audio for the first time. I have been a 'wanna-be' audiofile since my college days in the 80's, but have let marriage and kids take the front seat. I am asking about speaker impedance for two reasons. I do understand that speaker load is dynamic and subject to several factors. But, several years ago I fried one channel of my Phillips FA80 integrated amp by driving 2 pair of speakers simultaneously - (1) 4 ohm pair (KEF Q15) and (1) 6 ohm pair (MB Quart somethings...). So if I get the amp fixed and want to use both channels to drive the pair of bookshelf KEF Crest 2's I just bought, I want to make sure I dont burn it up again. Second, I have found a mint pair of KEF 104.2's and will be shopping for new components to drive them; but in the mean time don't want to mess my receiver up if I 'bi-amp' them with both channels (of my Onkyo TX8511 receiver). I am on a tight budget for a while, but refuse to do without my music. I want to experiment and learn, but can't afford to run out and buy a hi-end preamp and amps right away. I have just received my Canare 4S11 cable and want to have some fun! Thanks for your time.
You're losing me here. Bi-amping a pair of stereo speakers would mean using four channels of amplification i.e. one channel for the left woofer, one for the left tweeter, one for the right woofer, one for the right tweeter, etc... Is this what you are talking about or can you better describe what it is you're interested in doing? Sean
Allow me to get touchy feely. To be an audiophile all you need is a desire to get the most out of recorded music. I started out with a table radio listening to FM. As I write this I am at my home office desk listening to my mini system(purchased from Best Buy under $1k).
Secondly, you are not talking about bi amping. If you are running two pairs of speakers off one set of speaker connections, you either run them in parrallel or in series. Parrallel means you connect both set of speaker wires at the same reciever speaker taps. Series means you connect one speaker to your receiver then connect the other to the first speaker. If the resulting load is to low you can cause your amp to oscillate and even blow. If the resutlant impedance is to high, you frequency reponse curve will look like a roller coaster(Raul, thus is true with tube and ss)Courtesy of another Audiogoner, here is how you calculate it:12-01-04: Elgordo
For speakers in parrallel it's R1 X R2 divided by R1 + R2, where R is the speaker impedence. So two 8 ohm speakers would be 8X8=64 divided by 8+8=16 for an answer of 4 ohms. In series it's additive so two 8 ohm speakers would be 16 ohms.
From your description you speakers were hooked up in paralell. Doing the math in my head that yeilds 2.4 ohms. This could indeed cause some amps to oscillate. In sereies the answer would be 10 ohms. Too high, but probably will not damage your amp. Onkyo made some of the best receivers and maybe able to handle a low load. Check your manual.

I violated a rule: The right answer is usually the simplest one.
Sorry for the poor explanation. Yes, I'm talking about using 4 channels of amplification just as Sean described, from one receiver. Each speaker post would be hooked up to a seperate output post on the receiver. Thanks again.
Bi amping means TWO AMPLIFIERS. If you have a reciever with multi channel or multi zone you probably will not have a problem.
Don't worry. As you can see, I enjoy giving those explanations.
Duece1: What model receiver are you using? Is it a stereo receiver with multiple speaker jacks or is it a multi-channel surround sound receiver? Sean
Thanks guys. I am using an Onkyo TX-8511 stereo receiver (no A/V or surround- it has two sets of speaker jacks (A & B) and switches to play one pair (A), the other pair (B), or both pairs of speakers (A and B). There is also a switch on the back for me to select speaker impedance - one position is for when I have 8 ohm speakers hooked up to A OR B; and the other position is for when I either have 4 ohm speakers hooked up to A OR B, or when I have 8 ohm min speakers hooked up to A AND B. This is how I got to my original question - are my new 8 ohm bi-wire speakers still acting like 8 ohm speakers when I wire the tweeters to A and the woofers to B? (When I had two pairs of speakers (1 pair 4 ohm, 1 pair 6 ohm) I never played A AND B together.)
This would be bi-wiring not bi-amping and it might not be advantageous at all.

To see if the wiring configuration of the receiver is in series or parallel, try hooking up the woofers to channel A and nothing to Channel B. Then select A+B and see if you hear anything coming out of the woofers. If you do, the speaker jacks are wired in parallel you can use both sets of speaker terminals as you intended. How much of a gain you get from this, if any, may not be worth the added cost or trouble. The only way to find out would be to try it out. Either way, it wouldn't hurt anything. The setting would still remain at 8 ohms.

If you do the A+B test and you get no sound out of the woofers, you shouldn't use both sets of terminals for the speakers. That's because the outputs are wired in series, which means that the signal would have to go from the receiver out to the woofers, back to the receiver and then out to the tweeters. This would create a time delay with increased signal smearing.

Hope this helps.... Sean
Sean my guess is the A+B is parrallel. That means the amp is seeing a 4 ohm load. The fact that they have only an 8 ohm tap for using A and B also suggests it is parrallel. This means that the amp is not designed for anything less than a four ohm load.

I agree with Sean there is probably no sonic benefit from what you want to do.
Thanks guys - that's exactly the kind of feedback I needed.

Have a great holiday,