Bi-Wiring & Bi-Amping

Bi-wiring does absolutely nothing. Electrically it moves the jumper connecting the high and low frequency crossover filter inputs in the speaker from the back of the speaker to the receiver. That may seem to be of some benefit, but since electron waves move through wire at a speed of roughly 6 million feet per second it doesn't do anything other than wasting copper.

Bi-amping uses one amp for the highs, one for the lows, and that is beneficial. But if you don't bypass the passive crossover in the speaker most of those benefits are lost, making it almost as much a waste of copper as bi-wiring.

Bi-wire=waste of time/money/resources. Bi-amp=nice sound improvement on 2 channel systems. Most everyone has a separately powered sub now so bi-amping makes not alot of sense to me. Bi-amping is to keep one amp powering the lows, the other amp for the highs. This is done so all the power isn't being used for bass which then the highs suffer. But with 5.1, 5.2 etc... really no need to bi-amp as the minimal bass the AVR allows thru does not affect the highs at all so really unnecessary UNLESS as I stated earlier, you are running a dedicated 2 channel system. Just my take on the subject.

I agree with Bill. Not much use unless you're going with active crossovers.

I would not agree with this at all. There are many benefits to active bi-amping using DSP based sytems. Even with HT multi-channel systems. Certainly, not cheap.

http://www.avsforum.com/t/1458680/is-there-a-reason-more-of-you-arent-running-active

Active crossovers and separate amps. In the OP's case, he's going to be limited by the single powersupply in his AVR regardless.

It's not a matter of no need, it's a matter of not being able to take advantage of altering the crossover frequency and filter slope with an active crossover with the passive crossover still in place. Nor do you get the lower distortion and elimination of insertion loss that occurs when there's no passive crossover. IM distortion is reduced, but that in and of itself isn't worth the bother.

+1 Most AVRs do only "passive" bi-amping so there's not even any real headroom gain...

Which you would only do if the wrong frequency/slope was chosen by the designer of the passive crossover. This is a non-issue Bill and has nothing at all to do with bi-amping.

Someone find me the distortion introduced by the crossover. As Bill himself is fond of pointing out: the issue for distortion is the driver... and again this has nothing to do with biamping.

The big issue with passive bi-amping is that you are still suffering the db-adjust done to the more efficient driver by the crossover.
Also: if there's a db-adjust designed to extend frequency response, you are suffering that too.

An example. Imagine I build a speaker with a 93db tweeter and a 90db woofer which was -3db @30hz.
In the crossover, I'm going to take 50% of the power for the tweeter and try to discard it (turn it to heat).

Now imagine that I want this thing to run "flat" to 25 hz, which is -6db naturally on the woofer.
In that case I will use the crossover to make most of the woofer -3 db above (about) 30 Hz, (half the power), and I will have to make the tweeter -6db to match this new 87db sensitivity.

In active, I would do none of those thing. I would instead play with the voltage pre-amp for the woofer, and reduce the gain on the tweeter amp. I'd have a speaker with different efficiencies at different frequencies; but it would use (generally) far less power.

If it's passive it's not bi-amping. The marketeers strike again.

True dat. When I got back into this game I responded to a couple of bi-amping posts and immediately discovered I had no idea what they were talking about as I had never heard of "passive bi-amping" the way the AVR makers use it. Then I was confused as to why anybody would do such a thing... All bi-amping I have done, pro or home, has involved a line-level crossover and separate amps. Passive to me is a passive line-level crossover instead of active...


@JerryLove: Crossover distortion -- Well, they do add some, but unless it is defective it is far less than the drivers, I agree you won't hear it. The main advantages of getting rid of the crossovers are direct driver control by the amp, which does lower distortion somewhat but is very system-dependent, and the ability to use much steeper crossover slopes with much finer control (esp digital crossovers) to better isolate the drivers so they don't deal with as much out-of-band energy.

Don't forget about phase shift. The marketeers. They advertise being able to double the power delivery to the speaker. They don't mention that it doesn't make any difference in the sound, as voltage swing remains the same, but that's why they're marketeers.

Someone who was using real data to evaluate phase shift would acknowledge that the signifigant studies (Toole) show that relative phase shift has no effect on perceived sound (until and unless we start hitting issues like phase cancellation)
You yourself have pointed out the advantage that, when the amp is driven to clipping, it is most likely LF and that bi-amping would mean that the tweeter amp was not likely clipping at the same time.

Of course, a better/easier/cheaper solution would have been a better amp.

When passively bi-amped using an AVR both amps see the same signal and both amps would be clipped (voltage-wise the signal is the same).

There must be a crossover before the amp to benefit the tweeter amp.

Err... no. Current draw is different either side because both amps see a different resistance.

1. I said nothing about current draw and in fact noted voltage swing is the same for both amps. Power will be different depending upon the frequency content (spectrum) of the signal and load impedance of the speaker across frequency.

2. "Different resistance" why? If it were an ideal 8-ohm speaker, it would present 8 ohms across the bandwidth, and when you bi-amp each section (bass and treble) should still present an 8-ohm load to each amp. Now in fact I agree, but the impedance of speakers tends to vary wildly over frequency. Peaks and valleys exist across the spectrum. Some are flatter than others, some have the lowest impedance at LF, others at HF.

3. None of that changes the voltage clipping that can occur because both amplifiers see the same signal at their inputs. It may be reduced slightly because whichever amp is delivering lower current will have less drop across the internal output devices, but that is usually a minor player and so I neglected it.

I think I may have confused you because when I said each amp sees the same signal I was speaking of the input signal, not the load. An AVR that does not include an internal crossover and thus sends the same signal to both amps will cause both amps to clip at the same voltage (assuming identical amps). Approximately.

+1.

The crossover creates different resistance so only LF or HF driver current will flow on each corresponding side. It will be the current flow demand that causes an amp to clip.

I haven't got any finger puppets handy to help explain this to you... so instead watched this vid the talks about the role of voltage, current, and resistance. http://youtu.be/J4Vq-xHqUo8

Bi-wiring looks cool.

Unless you have a surplus of lower than ideal speaker wire lying about, its just neat looking

Amplifiers are limited by their voltage rails, not just "current flow demand". The voltage out of both amplifiers is the same (within reason) in a passive bi-amping system so both amps' voltage clip at the same level. The crossover does not "create different resistance". I understand Ohm's Law, KCL, KVL, Maxwell's equations, etc. I don't need finger puppets, thanks; I'll suffer through with my various EE degrees and decades of experience.

+1. I bet you even know that crossovers do not "creates different resistance"'

Thanks! Yup. But who can argue with the 'net?

Onwards and... something... - Don

Which is funny because Bill, in the past, here on AVS, has argued that limiting clipping to one driver was an advantage of passive bi-amping.

Chest thump much?

Ok. Are the LF and HF paths on a shared crossover run in serial or parallel.

Remind me again: If I run two electrical paths with the same resistance in serial, is the resistance the same as each individually or does it double? How about in parallel? The same or half?

It's good to hear that clipping is caused by the input signal though. Now I know I can run 50 speakers off one amp without clipping; afterall, the input voltage is constant. Yay!

The other guy started it, with statements that showed a lack of understanding of basic principles. Chest thumping? No, I was trying to make clear that I understood what was going on. Should have known better, on the 'net presenting credentials inevitably raises hackles and results in derision. Seems strange, but that's the way it is...

You are missing the point of the crossover; it crosses over from one driver to another at a particular frequency. Except right at and near the crossover frequency the two drivers (for a two-way) are never driven by the same signal at the same time. The LF signals go to woofer, which for sake of argument is 8 ohms. The HF signals go the tweeter, again assume 8 ohms. If it were a perfect crossover and the drivers ideal, there would be no bump and every frequency would see 8 ohms. Put another way, if you put in a low tone, it sees only the woofer, which is 8 ohms. Put in a HF and it sees only the tweeter, again 8 ohms. Put in a tone at the crossover frequency, and it sees both, but the crossover splits the energy so you can in fact maintain 8 ohms.

In the real world all sorts of things happen to the impedance and crossovers are usually designed for best sound, not the flattest impedance, but hopefully you get the idea. The two drivers are not really in parallel as you are implying (I assume you know they are not in series).

As for the input signal comment, I do not know if you do not understand what I was trying to say or merely being insulting. Assuming the former, of course eventually current demands will drag the amp down, often by dropping the voltage rails. They are not independent in a real amplifier. However, most audio amplifiers are designed to be near ideal voltage sources, with very low output impedance and high (ideally infinite) current output. Increasing the current will cause the power supply (voltage rails) to sag, and of course there is voltage drop across the output devices that does increase with current. However, barring your 50-speaker example, for reasonable loads the amplifiers clip when the input signal reaches a level that the output signal can no longer follow and the amp becomes nonlinear. When most people speak of clipping, the picture they have in mind is of a signal that is clipped (chopped off) at the top and bottom of the voltage waveform, what you might see on an oscilloscope. And, for most audio amplifiers driving most speakers, that is a reasonable picture.

When you passively bi-amp the same input signal goes to both amplifiers. The output power may be significantly different, and indeed typically the HF signal requires less output power. However, the voltage are roughly the same at the outputs of the LF and HF amps since their inputs are the same; the HF amp is still putting out the LF signal since there is no crossover before the amp. A large LF signal that clips the LF amp, will also clip the HF amp because it is also outputting the large LF signal. Yes, there is essentially no LF current and thus power is low, but the voltage headroom is still compromised by the LF signal.

Example: If there is a two-tone input such that at the output we have a 10 V peak HF signal riding on a 100 V peak LF signal, just two tones, then the peak output voltage is 110 V. If we have two identical amplfiers with 99 V +/- rails, then passive bi-amping that drives both amps with the same signal, clips both amplifiers. It does not matter that there is essentially no power at LF from the HF amp, it runs out of voltage headroom and clips. Just like it does not matter that the extra HF signal on the LF amp requires essentially no HF power (or current); if the amp had +/-100 V rails, that extra 10 V HF signal would still cause the LF amp to clip. Does that help?

Under significant power demands, there will be more drop across the output devices and in the power supply (now assuming non-ideal supply), but that is usually not a large factor, and I said earlier I was in fact neglecting that extra drop. I do not think it is significant in most cases; losing a few Volts of headroom is not a large change in power.

HTH - Don

yup waste ...

dont need to bi wire with good cable like this!

Don't bother trying to educate him, Don. He's not here to learn, he's here to argue. Block him and invest your time with those who do want to learn something.

Good advice. Hopefully if he is not interested, others may get something out of my babbling. In the meantime, onwards and... something... - Don

p.s. Same thing happens on a trumpet forum I help moderate; seasoned pros get called out by some HS or college kid, then after trying to explain what is really going on (i.e. the real world) and facing continual denial and derision, get frustrated and leave. Just not worth their time and effort to deal with those who have infinite time to make their life miserable and are often clueless about the person and wealth of knowledge they are dismissing. I am by no means saying I have that kind of knowledge to offer, but I am glad my professional rep does not depend on the 'net...

Ethan Winer has a name for them, see near the bottom of the link:
http://www.ethanwiner.com/articles.html

Trust Ethan to come up with something like that... I like the spray idea.

thats a great link..

cheers..