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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 3 responses by magfan

magfan
1,583 posts
 
With a 4khz crossover, at least 80% of the power required for 'normal' music is below that frequency.
So, you are still working the lowfrequency amp pretty hard, but not quite as hard as before. You have the addtional beneift of the hi frequency amp simply loafing along. Nothing you could do would tap THAT part of the amp out, so your distortion at higher frequencies, where fundamentals leave off and the overtones predominate, is pretty much history. I suspect that is the source of the help you perceive.
The low impedance dip is a little bothersome, especially when driven by HT amplifiers, which are notoriously unfriendly to such specs.
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.
magfan
1,583 posts
 
Yes, indeed, Al, I was ref'ing the amount of POWER needed by frequency band. The 50:50 point is around 350hz while 4khz means you need only a fraction of the power total at and above thate frequency.
That's why people who have a pair of identical mono's set as biamp with the crossover at higher frequencies are wasting the potential of the HF amp.
In my fantasy world, I'd want BOTH amps to redline at the same point.

Here is link to a fine bi-amping article, which if I failed to link is my bad.

http://sound.westhost.com/bi-amp.htm

My panels cross at 600hz, so while the ratio is not quite 50:50, I wouldn't feel too bad getting another duplicate amp for the bi-amp attempt. I'm guessing that a kilowatt@4 per speaker should about settle the 'enough power' question for Magnepan 1.6s or maybe not!

I'd like to see measured data for the B&W685. If it follows the pattern of many other B&W systems, I woudn't attempt the Tube Solution.
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