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In Reply to: Unless you use an electronic crossover, you do not have true bi-amping... posted by C.B. on February 21, 2007 at 06:46:54:
When using two amps to drive a speaker that is bi-wireable is that each amp will only see half of the crossover and drivers that are behind that crossover, and not the two halves in parallel. For example my soundlabs are passive biamped, which eliminates the problem of the amp seeing a rather low 2-3 ohm load at the crossover point. The individual transformers and crossover of the speaker present a more realistic 4-6 ohm load on each amp. Trust me amps are a lot more happy driving that, than a low impedance.dee
;-D
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Follow Ups:
You are right in the sense that each amp indeed works better because it sees easier load, true. But what also is true that even though the mid-hi amp doesn't have to drive the woofer, it still sees the whole signal, with large low frequency signal present.That signal can clip the amp even though there are not enough mid-hi content to do so. It can also inter-modulate with the other frequencies.
"But what also is true that even though the mid-hi amp doesn't have to drive the woofer, it still sees the whole signal, with large low frequency signal present.
That signal can clip the amp even though there are not enough mid-hi content to do so. It can also inter-modulate with the other frequencies."How can that signal clip the amp when the mid-high crossover caps prevent the mid-high amp from being loaded with bass frequencies?
Does low frequency signal present, have the exact same effect on an amplifier as low frequency load?
Cheers
You can clip the amp without any load, simply when the signal swing exceeds the power supply rails. The point here is that when large low frequency signal is present, it sets the limit on signal magnitude, even though the mid-hi frequencies signal by itself is much lower, and could have been amplified further, if low frequencies were removed by an active crossover.Having the load for those low frequencies only make it even worse - heavily loaded amp will usually clip earlier.
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The amp that just drives the high (or low) half of the speaker sees the whole signal but how much does that matter?Assuming that neither the highs nor the lows run full range, I presume that the impedance of each gets quite high as you get well past the crossover frequency. If, for example, the impedance of the tweeter is roughly 8 ohms within its operating range, then it's not hard to imagine the impedance being a large number by the time you get to 3 or 4 octaves from the crossover point.
Presumably little power or current would be drawn by the amp at frquencies well beyond the crossover point. This would of course not eliminate any distortion that results from hitting the output voltage limits of the amp but seems likely to help a lot if you are approaching the amps current limits.
I know that you understand this far better than I do and I look forward to your response.
It all of course depends on the music, but if you look at it with an oscilloscope, you will often see bass notes modulating the rest - you can clearly see high frequencies sitting on the bass sinusoid. This modulation will rob the mid-hi amp of its available headroom.Say, a particular piece of music has mid frequencies that are 20% of strong bass notes. That means that if you removed the bass frequencies, your amp now would be able to develop much more power.
Hand in hand with this is another observation - that the bass amp should be stronger, or else the total output will be limited by it - you will not be able to play the mids louder, because the bass amp will be clipping. I would say it should be 3 to 10 times more powerful than the mids amp.
In addition the inter-modulation distortions that I mentioned before, and also the operating point shift - both might damage the sound.
On the flip side is the quality of the electronic crossover... which often is not quite on par with the rest of the system.
I think that I understand the point you are making about superimposing high and low frequency signals. This seems to be the voltage issue I touched on but did not explain as clearly as you have.With regard to the relative power of the bass and treble amp with active bi-amping:
(1) Are you assuming that the existing speaker crossovers remain in place or that they are bypassed?
(2) If they remain in place will crossover losses limit the extent to which you can reduce the power of the treble amp compared to the bass amp?
you rip out (or bypass) the passive XO that came with the speakers.The whole point of active bi/tri/quad amping is that the amps are directly connected to the drivers, with no coils or caps "in the way".
Regards,
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