Originally Posted by DonH50
I am not sure (both) the individual amps see higher impedance; that depends upon the speaker's impedance profile.
All that matters is the impedance load (and phase angle) the amp sees at the frequency it is currently being tasked to reproduce.
This is how FTC data is generated under the 1974 guidelines we all use and advertisers are held to in magazine ads in the US. [Japan is different.]
As I have already shown you previously, with this animation I prepared of a Kef speaker I once sold [measurements by AJ in FL], the 4-ohm impedance load the amp sees in the most critical, power sapping bass region [in this example 200-350Hz] does not change when passively bi-amping, therefore the amp's ability to reproduce a loud note in this same region also
does not increase simply because we have disconnected the tweeter load from its "burden" (passive bi-amping, that is):
[The vertical scale is impedance, measured in ohms, from 0 to 100. The horizontal scale is frequency, from 20Hz to 20kHz.] In both scenarios, conventional and passive bi-amping, the impedance from 200-350Hz stays exactly the same: 4 ohms.
Let's pretend for the moment that the tweeter amp works completely differently and suddenly has, I don't know, let's say 6000% more output capability over all frequencies when used in this passive bi-amp scenario I'm showing [unlike the above woofer measurement curve, this "6,000%" is just a pretend value]. It doesn't matter to the FTC spec, which quite fairly asserts that when we talk about the output of an amplifier we must specify its capability over the full bandwidth
, not just some cherry-picked frequencies it happens to be good at. [THANK YOU
, FTC.] The bottle neck in maximum music output is nearly always due to bass frequencies unless one cheats and cherry picks some wacky music like solo flute recordings, where there is
never any deep bass.
When we talk about a conventional 200 watt amplifier having the capability to play 3dB higher in maximum clean output than a 100w amplifier, under the same conditions, are we talking about cherry-picked frequencies only? NO. We mean all
of them over that stated bandwidth, usually 20-20kHz. Using this exact same and fair methodology the increase in effective output when passive bi-amping is 0 dB,
none, nada, zilch, because as I've demonstrated in the measurement above, if the most power sapping part of the music's range does not increase in power output, at all, then we also can't fairly say the system's overall 20-20kHz output has increased either, since 200-350Hz is a subset of that full range.