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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
hifihvn
943 posts
 
Also, with the load shifting toward the low end, the OP said his "bass tightened up and went lower".
magfan
1,583 posts
 
IF the '685s follow normal B&W practice, they are a real handful in the lowest couple octaves and may even dip to 3 ohms with a largish phase angle. This is amp killer territory. With a crossover at 4khz, you could do with an amp of 1/4th the 'wattage' above crossover than below....easily, and never clip the HF amp.
http://sound.westhost.com/bi-amp.htm
It wouldn't surprise me even a little that the combination of being able to turn it up, slightly, and the reduction in HF distortion allowed a higher playback level with the same perceived loudness as 'before', while the bass balance shifted lower. Same SPL to within a few db. Even on my system with LOTS of power, the bass opens up when I play at a radioshack analogue meter'd 80db. I believe ear sensitivity plays a large part in this...at lower levels, especially.

The impedance curve of the speaker? almost irrelevant when dealing with a SS amp. Note from the Stereophile test of the Harbeth you link that potential probems with rising response were with a Tube amp...not SS. Stereophile is generally more concerned with wacky phase data when it occurs WITH impedance minima.
hifihvn
943 posts
 
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.
tonyangel OP
309 posts
 
Of course, a lot of this is over my head, but I really can't see (due to ignorance?) how the load can be divided equally. I only did that in my model for simplicity's sake.

What leads me to believe that the loads are not equal is my one experience with active speakers. In my case, it was a pair of Paradigm Studio 20s. If I remember right, the highs had a 50 or 75W amp on them and the lows had a 100 or 125W amp on them.

What I've gathered from the information that you guys have provided so far is that getting the better sound is about the lack of distortion that is brought about by not having to stress the amp.

What I'm hoping all of this is telling me is that if I get an amp with some nuts, I won't have to worry about bi-amping in the future.
almarg's avatar
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almarg
9,670 posts
 
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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