Axiom Home Page Forums General Discussion Technical Questions Impedance graphs for bi-amped M80V2 ?

hi,

i missed JohnK's post - that's why i deleted the earlier message.

JohnK - your posting reflects the wisdom i've gleaned from the other articles i've read about bi-amping.

to many, it seems to be a "band-aid" design-wise, and i enjoy the comment about the marketing issue.

i have a set of Polk Audio bookshelf speakers that are "bi-amp capable". i have this image of 2 sets of 12 guage wire running into a set of bookshelf speakers. sort of like Tim Allen's "Home Improvement"

i would be re-assigning 1 unused amp from the 7.1 amp and re-directing it to the fronts. this sounds like passive from the above discussion.

other than heating the living room slightly more quickly, it sounds like there is little to be realized.

Thanks,

Elden

Yes, Elden, that would be passive biamping. Also, and this is more than simply semantics, since it's the cause of most of the misconception about passive biamping, there is no "unused amp" if a full 7.1 speaker setup wasn't being used. The receiver or separate amplifier has only one power supply section, with a limited capacity; the output transistors in each channel act as valves to measure out the needed amount of power from the power supply section, but they have no power of their own to add. So, channeling the power through two sets of output transistors(one previously unused)into the speaker, rather than one, can't increase power in any way.

>>Elden, unless you're planning to use a separate external crossover before the amplifiers(and entirely separate amplifiers)and remove or bypass the M80 crossovers, nothing significant would be accomplished.

Yep. I have never understood why manufacturers promote bi-amping without an electronic crossover. There might be some scenarios where you get a tiny improvement but it never seems worth doing.

>>On the impedance question, the separate low and high frequency sections of a speaker have the same impedance and, as Rick commented, passive biamping doesn't change this.

I'm not sure about this. My recollection is that typical crossovers have woofer/mid/tweeter wired in parallel, each with some series and parallel elements to limit the frequencies going to the driver.

If you break the connection between (woofer plus its crossover elements) and (midrange & tweeter with their crossover elements) I would expect to see different impedence curves for the two sets of terminals, if only because the impedence peaks around resonance are isolated between woof and mid/tweet.

I *think* the impedence at the woofer terminals would be very high at higher frequencies since woofers normally have a series inductor. Not sure what the mid/tweet impedence curve would look like -- the tweeter normally has a series capacitor so impedence would go very high at lower frequencies, but I don't remember what the impedence response of the bandpass RLC on the midrange looks like.

Anyways, as JohnK says this is not an avenue worth pursuing so I'm only posting for the most academic of reasons

Randy,

The original M80ti was tested by Soundstage magazine at the National Research Council's anechoic chamber in August, 2001, and those frequency response and impedance curves are still available on the Soundstage speaker measurements site.

The M80ti's crossover was redesigned in the summer of 2003. All subsequent M80s had the new crossover, which resulted in smoother frequency response across much of the speaker's bandwidth. The new M80, which we later designated the "v2", was tested in December 2005 and you can see the improvements in the smoother "listening window" frequency response curves on the Soundstage site.

The impedance curve for the M80v2 on the Soundstage site is the correct curve for the M80s after the crossover was changed in the summer of 2003, even though we did not incorporate the "v2" into the model name until later on. The curve is different from the earlier M80ti impedance curve, because the crossover of the M80v2 is a new crossover design with different values of components.

Impedance curves do not reflect the acoustic frequency response of a loudspeaker. It shows only how the electrical resistance (impedance) of the driver voice coils and crossover components vary with frequency.

Regards,

Alan,

We thank you for taking the time to clarify the impedance graphs for the M80. It was the electrical impedance that we were after.

John, yes the high and low sections of the speaker are wired in parallel, but that doesn't mean that, for example, a two-way 8ohm speaker has two 16 ohm sections operating in parallel to result in the overall 8 ohm rating. The effect of the crossover is to greatly raise the impedance of each section of the speaker as it passes out of its frequency range. For example, if the effective impedance of the 8 ohm midwoofer was 80 ohms at 5KHz, while the impedance of the tweeter was 8 ohms at that frequency, the reciprocal of the net impedance of the speaker at that frequency would be the sum of the reciprocals of the impedances of the individual speakers, i.e. 1/8+1/80=1/T; 11/80=1/T; T=7.27 ohms. So, two 8 ohm sections in parallel result in about an 8 ohm, not 4 ohm, speaker.

Understood. The question here (I think) is "what happens if you split the crossover into two sections and measure the impedence of one section at a time ?". On M60 and M80 there are two sets of binding posts, each connected to part of the crossover, and normally connected by bridging strips.

I thought the original poster was asking what the impedence would be at each pair of binding posts if the bridging strips were removed, ie if you were measuring impedence for "just the woofer" or "just the midrange & tweeter" sections.

I'm only guessing that the M60/M80 binding posts are wired with one pair going to the woofer and the other pair going to the midrange/tweeter but that seems like a reasonable assumption. They could be wired together internally for all I know

Yes, that was his question. The impedance of each section of the speaker would remain the same within its operating range(e.g. nominally 8 ohms)as it was before.

Ahh, I get it. We're answering different parts of the OP's question.

You're responding to the "will the impedence be higher ?" portion and saying "no, within the operating range of each speaker the impedence will be pretty much the same as indicated on the graph".

I'm responding to the "what will the graphs look like ?" portion and focusing on what the impedence will be OUTSIDE of the operating range for each driver.

The good news is that we're all saying the same thing

Gentlemen,

yes, yes, and yes to all the above.

you've nailed my questions with considerable precision, whist throwing in a bit of mind-reading in the process.

you've answered my question(s), and any others that popped up during the discussion.

Thanks for the careful attention.

Elden