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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
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almarg
9,670 posts
 
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
tonyangel OP
309 posts
 
Al, I really appreciate your taking the time. Like an idiot, I completely forgot about the crossover and the affect that it would have on the load, or its distribution. You really cleared things up for me and now I almost regret having posted. I could have answered my own question (or gotten pretty close to it) with a little more digging.

As for the volume level, it could be all in my head. That tends to happen to me sometimes.

In the end, I do know that MY system sounds better in this configuration and would have accepted the trade off in power (had there been one) for the better sound.
hifihvn
943 posts
 
If I'm looking at the right amp on the net, the PA-5800 is a 5 channel. If your using 4 of these channels, I can see the volume and wattage drop. This would be due to a lighter load on each channel of the amp. I believe each channel will be putting out less in this case. Just guessing, each channel may be seeing 16 ohms nominal, and causing less power due to the lesser load on them. Anywhere from 40 to less than 80 watts per channel, except when your speakers drop to that lower impedance at certain frequencies. The sound can be different from this lighter load. [http://www.harmankardon.com/resources/Brands/harmankardon/Products/ProductRelatedDocuments/en-US/OwnersManual/PA5800%20om.pdf]
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