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Why Bi-wire?

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Posted on August 11, 2003 at 04:33:21
markrohr
Industry Professional

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Joined: July 15, 2002
Or, to include my own bias in the question, “Why does bi-wiring work?”

Some speaker manufacturers provide for bi-wiring, some do not, and a few of those that don’t seem pretty dismissive of the whole idea.

It sure works for me! When I built my current speakers (details below) I made them bi-wirable so I could mess around with it. I expected to make extensive listening tests, but the difference was so obvious that I decided in five minutes that bi-wiring was better, in my case at least. Bi-wired, the speakers were much more dynamic, and the sound opened up spatially in the most remarkable way. It was not close.

So I wonder two things: 1. Why does bi-wiring work? I’ve read several explanations, the most plausible of which (to me) is that it lets less of the woofer’s back-EMF into the tweeter.

And 2. Why do some speaker manufacturers downplay the efficacy of bi-wiring? Is it because bi-wirable speakers are by definition bi-ampable, too, and they’d rather not give their customers the ability (in a horizontal bi-amp setup) to change the tonal balance of their products? Or could some kinds of speakers sound worse when bi-wired?

TIA, gents!

Mark

[FYI my home-brew speakers are smallish, 2-way, sealed-box units with 6” paper-cone Vifa woofers and cloth-dome Scan-Speak tweeters. Crossover is 1st-order at 2.8 kHz, and there is a Zobel network on the woofs. Bass alignment is somewhat over-damped, Q = ca. 0.63.

The wires are 24’ runs of Sound King 10G. Level-match was checked with a volt-meter. The wire was not doubled-up for single wire listening, I just discoed the tweeter wires and strapped the terminals together.]

 

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Re: Why Bi-wire?, posted on August 11, 2003 at 09:12:58
Charles Hansen
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Hello -

I've also had excellent results with bi-wiring. Here are some reasons that some manufacturers don't recommend it:

1) The speaker in question uses a series crossover and cannot be bi-wired.

2) If you use anything except identical wires, the performance of the speaker will be altered in unpredictable ways. There are too many ways the end user can screw things up with bi-wiring.

3) The manufacturer recommends silly expensive speaker wire and if you bi-wired with that stuff, the wire would cost more than the speakers.

4) The designer is deaf and can't hear the difference between different speaker cables OR bi-wiring.

You can probably guess which manufacturers fall into which of these categories.

Best regards,
Charles Hansen

 

DId wonders for my system!nt, posted on August 11, 2003 at 09:32:25
NEAR SOTA
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cd

 

Re: Why Bi-wire?, posted on August 11, 2003 at 11:10:41
So I wonder two things: 1. Why does bi-wiring work? I’ve read several explanations, the most plausible of which (to me) is that it lets less of the woofer’s back-EMF into the tweeter.

Dunno. Haven't seen any explanation thus far which has made much sense.

Though some time back when I started tinkering with solid state single-ended amplifers which often use a large electrolytic output coupling cap or caps to block the DC offset of the amp, I saw that bi-wiring could afford the opportunity to bypass the main output coupling cap(s) and drive the high-pass section directly from the amp's output.

That's about the only scenario that's made any sense to me at least from a technical perspective.

se




 

Re: Why Bi-wire?, posted on August 11, 2003 at 19:38:37
john curl
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Charles, does the WATT 1 have a 'series' xover? Or is it only 2 terminals for some other reason?

 

I'm not sure..., posted on August 11, 2003 at 21:21:56
Charles Hansen
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because the Wilson crossovers are potted. However, I suspect that it's reason #3. Some of those "network" style speaker cables that Wilson recommends cost up to $30,000 a pair!

Best regards,
Charles Hansen

 

Re: Why Bi-wire?, posted on August 12, 2003 at 06:54:55
markrohr
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Joined: July 15, 2002
Thanks much for your answers, Charles, although #3 & 4 will undermine the faith I have in the integrity of the high-end community! ;-)

I forgot about series crossovers. I had thought they might be pretty rare in production speakers since they must be a pain in the neck--or at least incredibly time-consuming--to develop.

I hope more will chime in on why bi-wiring works.

Thanks again,

Mark

 

Don't despair...., posted on August 12, 2003 at 08:31:52
Charles Hansen
Manufacturer

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just because a few manufacturers don't offer bi-wiring for the wrong reasons. There are many more that don't offer bi-wiring for valid reasons, and many, many more that do offer bi-wiring.

In other words, don't throw out the baby with the bath water. There are already enough people on this forum that do that already. We don't need any more.

Best regards,
Charles Hansen

 

'Cause It's Unlawful to Steal Wire..., posted on August 12, 2003 at 08:45:38
Todd Krieger
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Location: SW United States
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Albert Von Schweikert had a rationale that "field interference" from a cable's "self inductance" caused sonic degradation, and such interference between low and high frequency signals would be reduced with bi-wiring, because such electric fields would then be "separated."

 

Thanks--just kidding, actually nt, posted on August 12, 2003 at 08:53:17
markrohr
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.

 

Ok, so here's my theory..., posted on August 12, 2003 at 15:51:19
Commuteman
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Most amplifiers have some form of feedback implemented by sampling the output at or close to the speaker terminals. Amp works to keep the output voltage close to the original (or a multiple thereof).

Bi-wiring avoids any interactions between currents (and attendant fields) of the high- and low-pass sections of the speaker because the two sections only meet at (or near) the place where the amp can exercise control over the signal. In other words, the only wiring common to woofer and tweeter is now inside the amp's feedback loop.

Single wiring allows currents flowing to and from the woofer (actually to the low-pass xover section) to modulate the signal presented to the high pass xover section because the cable has a non-zero impedance.

I'm still not convinced this is right, so I need to think about it some more....

Perhaps there is an analogy to good grounding techniques. I was taught to do the thought experiment of putting a small resistor in place of every ground connection in a piece of electronic equiment and looking at the ways in which things like capacitor charging currents can modulate ground voltages.

Peter

 

doesn't superposition contradict this?, posted on August 12, 2003 at 16:23:53
Commuteman
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My Physics is definitely rusty, but I thought that the principle of superposition suggested mutiple signals (e.g at different frequencies) can travel in a medium (like a cable) without interacting. This means that there would not be magnetic field modulation.

That's what I'm still struggling with -- a credible explanation for the interaction between HF and LF signals in the cable.

Doesn't mean I think it doesn't happen -- just that I can't explain it to my own satisfaction (yet).

Peter

 

Re: Ok, so here's my theory..., posted on August 12, 2003 at 16:29:55
john curl
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You are a hard man to convince, Peter. Now get it together for you to drop by for a short time, so that we can catch up on things. I promise not to make things into a time consuming situation, for your family's sake, if for no other reason.

 

Re: doesn't superposition contradict this?, posted on August 12, 2003 at 19:57:27
Lymp Baygul
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Assuming perfect wire, yes. However, the signal to the speakers is high current, and the slight amount of inductance and resistance the cable presents becomes signifigant, and has to be considered part of the crossover network. By bi-wiring, each side of the crossover is connected to the feedback loop of the amplifier seprately, so the amp can damp out inductive kick back from the woofer before it gets to the tweeter.

I am not aware if anyone has tried to compare speaker cables with low and high inductance to see if the effects of bi-wiring are dependent on this. I would think a braided design such as Kimber Cables would not show as signifigant a difference as straight designs do.

The liniear inductance of cable is also why fancy power cables do what they do.

 

Re: doesn't superposition contradict this?, posted on August 12, 2003 at 21:25:10
Todd Krieger
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Superposition does not take into account "non ideal" devices and effects, including the "field" effects the AVS alluded to. (Other effects include ESR, leakage, mutual inductance, etc.) It is a good tool to approximate the characteristics of circuits, based on "ideal" devices. Especially circuits with *multiple* voltage and/or current sources.

 

Re: doesn't superposition contradict this?, posted on August 13, 2003 at 19:12:35
dave slagle
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Location: NYC
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well my physics is non-existant, but the concept of skin effect where higher frequencies travel along the outer shell of the conductor could be another way to say what you suggested???

the change in resistance is quite trivial by the math with the audible frequencies we are concerned with... but amps of current through the trivial could add up to something. (see crossover coments elsewhere in the thread)

you also say "can travel" is that the same as "does travel"??? just tryingto get the 99 cent physics lesson i slept through in highschool wondering "when will i ever use this junk"

now histery and english... them was naps well spent.

dave

 

Re: Why Bi-wire?, posted on August 14, 2003 at 06:30:13
TomWh
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Maybe it is as simple as more wire vs less. I am not going to tell you I have the slightest idea what electrons are doing. You might want to try running the double cable to one post and bridging the posts to see if the high vs low frequency theories float.

Enjoy Tom

 

Yes, superposition does contradict it., posted on August 14, 2003 at 10:08:37
jneutron
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Unfortunately, it is not superposition that must be considered.

For a wire, when skin effect occurs, the current will be excluded from the wire interior, in gradations of course.. So, a skinning current will have no internal inductance, hence no energy storage. Whereas, the lf info does not, and does have internal energy storage.

If an audio lf signal is pulled across zero by a hf signal, the internal stored energy will be pulled out by the slew rate of the hf information..it is not clear yet if that released energy is consumed in the re-distribution of the current density profile, or if it reacts externally, possibly time shifting the leading and trailing edges of the hf information, thereby blurring the interpreted soundstage. That would require time shifts in the 5 to 20 microsecond realm.

See, that wasn't so hard, now was it??

Cheers, John

 

Not when there is another factor, posted on August 14, 2003 at 10:31:13
Jon Risch
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Contributor
  Since:
March 1, 1999
That other factor is motor/generator action within a cable, and the related problem of the cable being vibrated by the sound output from the speakers.

The current through the two bi-wire sections is different, and so the cable carrying the HF current will not get motor/generator modulated by the LF currents.

Current flow in the two bi-wire cables in a simple 2nd order crossover is shown at:
http://www.geocities.com/jonrisch/page7.htm

The other factor that I think bi-wiring addresses, is that of woofer to tweeter isolation, and I go through this aspect at this page:
http://www.geocities.com/jonrisch/biwiring2.htm
and the following one.


Jon Risch

 

Re: Ok, so here's my theory..., posted on August 14, 2003 at 10:32:46
Jon Risch
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Contributor
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Been there, done that , made a chart.

See:
http://www.geocities.com/jonrisch/biwiring2.htm

Jon Risch

 

Nice job, posted on August 14, 2003 at 14:30:53
Commuteman
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The comments you made about the relationship between amplifier output Z, cable type and IM attenuation sound like a good model for predicting which combinations are likely to benefit from bi-wiring.

Obviously, the xover topology is also an important factor, since it will affect the accuracy of your simplified circuit for one driver driving the other, and the range of frequencies over which biwiring would reduce IM. I would imagine that the series impedance of most xovers sections around the xover point is somewhere near the impedance of the driver, which should be much higher than the cable and amp output impedance. Doesn't this reduce the potential benefit of biwiring?

Peter

 

this seems more likely, posted on August 14, 2003 at 14:41:24
Commuteman
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Location: Bay Area California
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than interaction between the fields.

The acoustic coupling of the drivers you described is the first explanation I have seen of driver A affecting driver B in the frequency range that driver B produces.

In other words, the acoustic output of the tweeter causes the woofer to act as a motor for signals within the tweeter's freq range. This would clearly cause a delayed, distorted version of the original signal to be reproduced by the tweeter.

The magnitude of the effect would be dependent on the xover topology. As an extreme case, there are some speakers that have no low-pass xover on the woofer. In that case, the full magnitude of the induced signal would be driving the tweeter xover, in parallel with the cable and amp impedance.

Fascinating.....

Sounds like time for an ABX test to prove we can't hear any of this.. ;-)

Peter

 

Re: this seems more likely, posted on August 14, 2003 at 15:15:22
Todd Krieger
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I think this is true for *series* crossovers (which cannot be bi-wired, BTW), because there is often a common "node" directly tied to both woofer and tweeter. In fact, I've seen tweeters go out quite often with such crossovers, because I think the woofer's "back EMF" energy can be transferred directly to the tweeter. (This could be why some series crossovers use a "blocking capacitor" in series with the tweeter.)

 

Many thanks, Jon nt, posted on August 15, 2003 at 09:01:57
markrohr
Industry Professional

Posts: 5567
Joined: July 15, 2002
.

 

So Spectron Remote Sense technology obviates the need for biwiring...?, posted on August 17, 2003 at 22:24:13
Rob Thomas
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Location: The West
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...by sensing for the feedback loop at the speaker binding posts?


 

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