No, You Cannot Bi-Amp

Re Elizabeth's statement: Bombaywalla, I know that you have considerable technical expertise and relevant experience, but upon careful reading I don't see anything unreasonable or technically implausible about Elizabeth's statement. In its essence, at least, which as I read it is:

A few folks are able, with effort to make it work for themselves. In general it is a waste of time, if as much effort was made to have one amp work well. The worst side of it is all the folks who know nothing about it wanting to do it as if it is some easy magic panacea.

I have seen (and answered) more than a few posts here that would justify the last sentence of that statement. (In saying so, I am not implying that the OP in this thread is one of them).

Regards,
-- Al

Some good points have been made above. With respect to passive biamping, another commonly overlooked point is that in that arrangement both the high frequency amp and the low frequency amp have to output the voltage range of the full range signal. Which means that if a low powered tube amp is used for the highs and a much higher powered solid state amp is used for the lows, most of the power capability of the solid state amp will be unusable and wasted, because the maximum volume setting that can be utilized will be limited by the clipping point of the low powered amp. Also, of course, gain matching is likely to be an issue in that situation.

Regards,
-- Al

01-20-12: Magfan
Relative power needed is almost entirely governed by crossover point.
If you crossed over at 10khz, you could make due with a 30 watt tube amp on top of a 300 watt SS. Than, other issues would come to the fore.

Right! A major "other issue," if the two amps are being used in a passive biamp arrangement (i.e., without an active crossover ahead of the power amps) being that some number approaching 270 watts of the 300 watt capability of the SS amp could not be utilized, without clipping the 30W amp. Essentially the 300 watt amp would be reduced to not much more than a 30 watt amp in that scenario (the exact value depending on the voltage swing capability of the 30 watt amp). Which was exactly my point.

Best regards,
-- Al

01-21-12: Magfan
I'm sorry, but the power relative power needed IS governed by the crossover frequency.... The amp will only have to produce power for those frequencies for which the speaker has a need....

Hi Magfan,

Somehow my point isn't coming across.

Yes, the amp will only have to produce POWER for those frequencies for which the part of the speaker it is connected to has a need. However, in a passive biamp arrangement, where there is no active crossover ahead of the power amps, both the high frequency amp and the low frequency amp will need to produce the SAME output VOLTAGE, corresponding to ALL of the frequency components of the music.

And therefore if the low frequency amp has a rating of 300W and the high frequency amp has a rating of 30W, corresponding to the maximum power requirements above and below a crossover frequency that is very high, the 300W amp will not be able to produce 300W, or anything close to it (in a passive biamp arrangement), without the 30W amp (which presumably has a far smaller voltage swing capability) being driven into clipping. Therefore how far the volume control can be turned up without severe distortion occurring (and perhaps a blown tweeter as well) will be limited by the output voltage range of the 30W amp, and most of the power capability of the 300W amp will not be usable.

As I said earlier, that is a point that is commonly overlooked. I have often seen questions about passive biamping that refer to the possibility of purchasing amps such that the low frequency amp has a power rating several times greater than the rating of the high frequency amp, which makes no sense.

Best regards,
-- Al

JohnK, thanks for your comment.

To be sure it's clear to everyone, I'll repeat that the issue I cited, involving a large disparity in power capability between the high frequency amp and the low frequency amp, just applies to PASSIVE biamping, in which the only crossover that is present is the one in the speakers.

I'll add that my comment was intended to apply to the situation where a single pair of speakers is being passively biamped, with the single pair of speakers having been designed to provide reasonable results (in particular, some semblance of flat frequency response) when driven by a SINGLE amplifier.

John's comments about combining Class D and SET, etc., apply to a different situation altogether.

Regards,
-- Al