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Is bi amping really a trade off?

First a little background. My very modest system consists of a NAD T744 receiver that I'm running as a pre-amp to an old HK PA5800 amp. My speakers are a pair of B&W 685s. I do have a Paradigm PW2200 sub, but don't like to use it, preferring 2.0 stereo.

Just on a whim, I decided to bi amp my speakers, using two more channels from my amp. I had them running bi-wired (a remnant from my Studio 40s) so bi amping them took me all of ten minutes.

The result? The bass tightened up a LOT and went deeper. Although the sound stage stayed about the same, I got a lot more depth. Listening to some Sade, I could not only tell that her sax player was standing to her left, but I could now tell that he was also standing a good bit behind her.

Another observation that I made, though, was relative to the volume level. My usual listening level is at -30. When I turned it up, after bi-amping, I noticed that I had to go to -20 to get to my usual level. Now, I'm not real sure about this because I never really paid much attention, but this is just what I thought.

Anyway, it just got me to thinking. My amp is spec'd as putting out 80W on an 8 ohm load and something around 120 on a 4 ohm load. I wondered how splitting the tweeter from the woofer affected the overall load of the speaker components.

Just to keep things simple, I'll assume that each component in the speaker is running in parallel to the other. If that is right, then (to keep things simple) can I say that each component has a resistance of 16 ohms so that when I run the components in parallel I have a speaker that presents an 8 ohm load?

If all of this is right, then by bi-amping, I've effectively made it so that each channel of my amp is driving a 16 ohm load. Again, if this is true, then I've taken my 80 wpc amp and turned it into something around a 50 or 60 wpc amp, due to the greater load.

Am I making any sense? Or am I just imagining things? Did I trade off power for better sound?
tonyangel

Showing 2 responses by almarg

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almarg
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I'll assume that each component in the speaker is running in parallel to the other. If that is right, then (to keep things simple) can I say that each component has a resistance of 16 ohms so that when I run the components in parallel I have a speaker that presents an 8 ohm load?

If all of this is right, then by bi-amping, I've effectively made it so that each channel of my amp is driving a 16 ohm load. Again, if this is true, then I've taken my 80 wpc amp and turned it into something around a 50 or 60 wpc amp, due to the greater load.
No, the low and high frequency sections of the speaker are not in parallel, due to the presence of the crossover network in the speaker.

The amplifier section that is driving the speaker's woofer will NOMINALLY see an 8 ohm load for the very low frequency components of the signal, and a very high impedance for the very high frequency components of the signal. Therefore that amplifier section will have to supply little or no current and power at very high frequencies.

At frequencies in the crossover region, which for the 685 is specified as being centered at 4 kHz but is probably fairly broad, the impedance seen by that amplifier section will gradually transition between the nominal 8 ohm value and being very high.

The opposite situation will exist for the amplifier section driving the tweeter, i.e., it will have to supply little or no current and power at low frequencies, and it will see a nominally 8 ohm load at high frequencies, supplying whatever amount of current and power the signal calls for at high frequencies.

The actual values of the 8 ohm nominal loads will vary depending on how the speaker's impedance varies as a function of frequency. I note that the impedance of the 685 is specified as 8 ohms nominal, 3.7 ohms minimum. I would suspect that the 3.7 ohms occurs at frequencies that are handled by the woofer, which are also the frequencies that almost always require the most power.

I don't know why you might be finding that you have to turn the volume control to higher settings in the biamp configuration, unless the gain of the amplifier somehow changes when it is set up for four channel operation.

Regards,
-- Al
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almarg
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11-17-11: Hifihvn
One odd thing that I can never figure out is, a knowledgeable person that may be a manufacturer (speaker?) that posts (or used to) here sometimes said that when you bi-amp, the load is divided equally on the speaker inputs when their post separated. This was in the past few years. I don't recall who it was, but that never quite made sense to me. I looked at some old forums and can't find it, and may be gone. Oh well. I do think the impedance can be divided more closely between the highs and lows on a close to linear speaker like a Magneplanar though.
I suspect that he would have been referring to an approximately equal division of the amount of POWER that would have to be delivered by the two amplifiers (or amplifier sections), which I would expect to occur with typical music if the speaker had a crossover point between the two sections somewhere in the low 100's of Hz. I would not expect that to be applicable to speakers that don't have a separate mid-range driver (and even to many that do), because in most or all of those cases the crossover point would be much too high to result in an equal division of power.

Best regards,
-- Al
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