B&W "bi-wire vs. bi-amp" Question

[STORMINORMAN]

Have a B&W speaker that I use as the centerchannel speaker in a HT system. This speaker already has terminals which allow what B&W describes in the Manual as a "bi-wire" option.

My pre/processor (Rotel) is pretty flexible, as you can imagine. It is currently running C, RF & LF, RR & LR through an ADCOM 5 channel power amp and a powered sub.

I have available a separate quality power amp for the RF & LF channels in the 250 RMS range, another quality amp with 100 RMS for the LR & RR channels My specific question is as follows:

Can I split the Center Channel feed (RCA cable female into 2 male) from the pre/processor and use another similiar quality stereo power amp (@125 RMS per channel) and run one wire pair to each the two B&W terminals after I remove the bridging strap? This amp does have adjustable L&R channel gain control on the front panel of the amp, BTW.

Do I REALLY need an electronic crossover or will the internal crossovers protect the speakers. It would seem to me that if they are bridged to begin with both are receiving 100% of the program material. If bi-wired, they are still each receiving 100% of the program material, are they not? We are not talking ear splitting volume levels here, just an upgrade from the ADCOM rather limited power output. The current system sounds very good on Blu-ray soundtracks and doesn't seem to ever be lacking in volume, I'm just looking to use some of the older quality separates I've accumulated over the last couple years...

And be able to drive the front speakers (in Stereo) with authority when playing LP's and some of the older two channel material I have.

 

[jliedeka]

Using both sets of binding posts doesn't really eliminate the crossover. The woofer and tweeter circuits are just separated. The tweeter still has it's high-pass parts (and L-pad and whatever else) and the woofer still has it's low pass components (plus probably Zobel, etc).

Bi-amping may or may not gain you anything when you repurpose internal amplifiers like back surround amps to aid your front LR speakers. It depends on whether the power source - transformer/capacitors/rectifier - can provide enough current for loud passages. If so, the question becomes whether or not two amplifiers per loudspeaker really provide more current than one. Another consideration is the woofer will use more power than the tweeter so that amp won't be pulling a ton of current.

If you have two unused amplifiers, it's worth trying, especially if your speakers are power starved in your listening environment.

If your center channel really need more power, you are probably better off using a monblock amp in place of, or in addition to, the internal one.

In general, bi-amping is a waste of time. In most cases what you really need is a more powerful amplifier. I'm building speakers and none of them will have more than a pair of binding posts. I wish manufacturers wouldn't put two or more pairs of binding posts on their speakers because it leads to suckers thinking they have to buy double the speaker wire.

Jim

 

[STORMINORMAN]

Thanks!

Jim, thanks for the prompt reply!

The amp for the front L&R speakers is an Yamaha MX-1000 rated at 260 RMS into 8 ohms. The amp I'm probably going to use for the Center Channel B&W will be an MX-630 or possibly an M-50 which are both in the 125+ RMS range.

I kinda' hoped this would provide more than enough umph! to drive my speakers. Your comments about the crossovers, etc. all still being in place are much welcomed. I also thought the fact that there was an adjustable gain on the front panel of the M-50 (and hopefully the MX-630 as well) could allow for more fine tuning re: relative volume levels between the woofer and midrange/tweeter sections of the B&W.

There has to be a lot of room for improvement between, say... 50, 60 or 75 RMS to all 5 channels and 260 L&R, 125 + 125 for the center and 100 (or so) for the rears? But no real problem in setting up the power amps in the suggested bi-amp (vs. "bi-wire") configuration?

Thanks, again!

Norm

 

[TLS Guy]

Jim, thanks for the prompt reply!

The amp for the front L&R speakers is an Yamaha MX-1000 rated at 260 RMS into 8 ohms. The amp I'm probably going to use for the Center Channel B&W will be an MX-630 or possibly an M-50 which are both in the 125+ RMS range.

I kinda' hoped this would provide more than enough umph! to drive my speakers. Your comments about the crossovers, etc. all still being in place are much welcomed. I also thought the fact that there was an adjustable gain on the front panel of the M-50 (and hopefully the MX-630 as well) could allow for more fine tuning re: relative volume levels between the woofer and midrange/tweeter sections of the B&W.

There has to be a lot of room for improvement between, say... 50, 60 or 75 RMS to all 5 channels and 260 L&R, 125 + 125 for the center and 100 (or so) for the rears? But no real problem in setting up the power amps in the suggested bi-amp (vs. "bi-wire") configuration?

Thanks, again!

Norm

If you plan on using different amps for left and right and center, you can easily run into phase problems. By no means does the phase of the output of an amp have to be in phase with the input.

So if your amps spec sheet is silent on the matter you will need a generator and scope to check the phase of the amps. This applies to amps from the same manufacturer. I can attest to 180 degree phase shifts between models from the same manufacturer.

 

[STORMINORMAN]

I think you may have lost me on the "180 degree" phase shift...

Left and right front channels powered with MX-1000, make sure they are wired correctly (+ vs. -). Run RCA output for center channel through adaptor (male to 2 female plugs), then connect male to male RCA cables to the 2nd power amp, maintain (+ vs. -) to 2 sets positive and negative terminals on B&W center channel.

Wire rear channels through 3rd power amp, and maintain (+ vs. -) speaker wire configuration.

Where do I run the risk of being out of phase? What amp specs are you referring to vis-a-vis an " 180 degree phase shift" and how to I test for them?

I really do appreciate these responses as this is a little bit new to me regarding the setup.

 

[TLS Guy]

Left and right front channels powered with MX-1000, make sure they are wired correctly (+ vs. -). Run RCA output for center channel through adaptor (male to 2 female plugs), then connect male to male RCA cables to the 2nd power amp, maintain (+ vs. -) to 2 sets positive and negative terminals on B&W center channel.

Wire rear channels through 3rd power amp, and maintain (+ vs. -) speaker wire configuration.

Where do I run the risk of being out of phase? What amp specs are you referring to vis-a-vis an " 180 degree phase shift" and how to I test for them?

I really do appreciate these responses as this is a little bit new to me regarding the setup.

You can't assume that a positive deflection at the input will result in a positive deflection at the output of any amp. It is about a 50/50 chance whether an amp is phase inverting or not. This is seldom mentioned in the specification sheet and so makes a pitfall for the inexperienced.

The only way I know of to test for this is to put the inputs on the x plates and the output on the y plates of a scope and look at the phase between input and output. I always do and have done this when I have different amps powering different sections of s speaker and when using different amps in a surround system. I can assure you that you can never assume the input of any amp is in phase with the output. This variation occurs as there a lot of topologies for the phase splitter stage before the output stage. The negative terminal must be ground, as otherwise the owner could inadvertently easily damage the amp. However, unfortunately the red and black terminals do not guarantee a positive deflection at the positive terminal, with a positive deflection at the input.

This is a big reason I counsel the inexperienced against biamping.