I am not sure I understand the question here. My impression
of why power doubles into 4 ohms compared to 8 ohms is
that most amps are voltage limited, not current limired
(until the impedance gets super-low).
And for a given voltage limit V , the power(which is nominally
VI= V(V/R)= Vsquared /R) will double when R is halved.
Of course, it depends on what makes the amp run out of steam!
Low current amps like the Quad current dumpers do not double into
half impedance loads very far down--they start to current limit.
But big current amps do do this,double into hald impedance, down to
fairly far down.
I am not an electronics designer but I think the causes of
current limiting and voltage limiting are in effect different.
Of course the one actually happens when the other happens:
an amp cannot maintain voltage without maintaining current too
(and vice versa). But I think the causes of limiting into high
impedance and low impedance are different.
Current strength(which is what gives out in this informal sense into low
impedances) is attached to big power supply storage and (I think) lots
of output devices paralleled , and voltage(which is
what limits when the speaker impedance is high, again in this informal
sense) is attached to the rail levels or the voltage limits of the output
devices.
Anyway, the little Quad 306 say , which has a dumpy(no pun intended)
little power supply,
works really well into high impedance, but ask it to put out much current
on account of low speaker impedance, and it cries uncle. Power rating is
50 watts into 8 ohms, only 70 into 4 ohms. (Quad was and is very up front
about such things)
Please forgive, you electronics designers out there, this no doubt
over-simplified view.
Am I missing the point here? I do not see how feedback could fix these
limitations, though feedback surely does lower the output impedance--
one of the things it is for!
Robert
On Fri, 29 Jul 2011, Sampo Syreeni wrote:
On 2011-07-27, J?rn Nettingsmeier wrote:
power transmission impedance matchin>
That I've understood for the longest time. But then, when you learn it from a
power electronics book -- which only talks about power transmission
efficiency at a single frequency -- and then somebody tells you that's all
bunk with audio circuits since there the ideal is to have total input-output
control by always going from minimum output impedance to maximum input
impedance... You can sort of lose track.
When I reread Robert's idiot-guide, I literally blushed.
if you look at the spec sheet of a commercial p.a. amplifier, 9 times out
of 10 you will see twice the power rating for 4 ohm loads than for 8 ohms.
How is this possible, given that the intrinsic ohmic load of the source ought
to stay the same? Yielding more current, and as such not just more (linear)
distortion from the reactive components, but also more power lost within the
amp instead of within the wiring and/or the speaker?
Then the real question is, why can't this sort of a problem be corrected
across the board with adaptive feedback? Even in the endstage? I mean, it
shouldn't be too difficult to measure both the current and the voltage going
out, and to feed them back via some mock-reactances over to the input side of
the final amp. Over a narrow band that should be doable even in analog
hardware. And over the whole audio band, why not just go with adaptive zero
latency convolution and be done with the speaker impedance matching problem,
once and for all? I.e. even if you don't have real variable reactances on the
amp side which could be brought to bear, you could always go with the
smallest common denominator you *can* handle by brute force, achieving a flat
matching to an arbitrary speaker.
Or is it that this is patent protected already? Somehow I seem to remember
reading a research paper or somesuch from Meridian, which could have
suggested they did something like this at least for their flag-line DSP
speakers' bass section, in order to get optimum power transfer, and as such
high SPL's. Do we perchance have Bob Stuart online to verify or dispute my
hunch?
--
Sampo Syreeni, aka decoy - de...@iki.fi, http://decoy.iki.fi/front
+358-50-5756111, 025E D175 ABE5 027C 9494 EEB0 E090 8BA9 0509 85C2
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