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 5 responses by hifihvn

Also, with the load shifting toward the low end, the OP said his "bass tightened up and went lower".
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]

11-17-11: Magfan
With a 4khz crossover, at least 80% of the power required for 'normal' music is below that frequency.

I think it varies a lot depending on the speakers impedance curve, besides the music, and crossover point. Every speaker amp and speaker combination may have different results. That's why I can see what the OP results were. It would be nice to have this type of test results for all speakers. Some examples from Sterephile. [http://www.stereophile.com/content/magnepan-magneplanar-mg36r-loudspeaker-measurements] [http://www.stereophile.com/content/atc-scm-11-loudspeaker-measurements] [http://www.stereophile.com/content/harbeth-p3esr-loudspeaker-measurements]
Magfan, I haven't owned a B&W since they went toward metal dome tweeters, and I don't keep up with them to much anymore. I'm just thinking it is hard to put a set figure with too many variables. These three speakers will give totally different results on a tube amp vs. a solid state in my opinion. The OP's amp is a 5 channel, and probably can't give a linear output across all frequencies like a potent solid state one or two channel can. I do remember the killer Apogees and Infinity Kappas.

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.
each channel may be seeing 16 ohms nominal

This was in my first post and in error. I was thinking the highs are easier to drive and may see less power when on a separate amp.