Re: 4 speakers from a power amp instead of 2 (question)

On Dec 3, 12:21 am, Chris Hornbeck <chrishornbeckremovet...@xxxxxxx>
wrote:
On Tue, 2 Dec 2008 06:45:26 -0800 (PST), Mark <makol...@xxxxxxxxx>
wrote:





OK lets do the math... for a simple case of resistors...
assume two identical speakers in series, each speaker = 8 Ohms.
and assume the speakers behave as resistors
and assume the amp output Z   = 1 Ohm
assume all Z's are simply R

In this most simple case the math says each speaker sees a source Z of
the other speaker in series with the amp 8+1 = 9 Ohms...

I think your point was each speaker would see 1/2 Ohm, I don't see
that?

Yes, I see each speaker sees 1/2 the voltage, that is obvious and will
always be true even if the speaker Z varies as a function of frequency
or even if it is non-linear woth amplitude,  as long as the two
speakers are identical.

But I don't see the effective source Z being 1/2.

Whoa!, when did we change from four speakers per channel
to two? Somewhere upstream, fersure.

For four identical speakers in series-parallel, the load
to the amplifier is the same as that of one of the speakers,
and each speaker gets 1/2 the voltage and 1/2 the current
from the amplifier, so sees the same source impedance as
a single speaker.

Somewhere the discussion slipped from four speakers to two,
leading to this confusion.

Much thanks, as always,
Chris Hornbeck- Hide quoted text -


Chris,

the question is not about what load the AMP sees, you are correct what
you said about what load the amp sees but that is not in question, the
question I am discussing is not what load that the amp sees but rather
what does each speaker see as a source Z driving it...this is what
determines the damping factor for the speaker..

in the case of one speaker, the speaker obvisously sees the output Z
of the amp.

The question at hand is what does each speaker see as a driving Z when
two speakers are connected in series.. and it is not a simple
question as it first appears.....

Arny,

I think you are right..for the special case of two identical speakers,
the back EMF generated by each speaker is identical so you cannot use
the simple method of treating the speakers as seperate independent
loads. These are DEPENDENT sources. If you use the simple method of
indepenedent sources, you get the simple wrong answer of 8+1 = 9 Ohms
for the example case above. But I now think this is wrong and you are
right. To calculate the right answer you have to consider that the
two EMFs are not independent but identical so the back EMFs are
identical and you cannot analyze this in the simple way like resistors
but rather have to consider that you have DEPENDENT EMF sources.
Using symmetry as you suggested, I agree you are correct, each
speaker sees a source Z of 1/2 the amp source Z or 1/2 Ohm in this
example, a big difference!

Note this is true only for the special case of INDENTICAL speakers so
that the back EMF is identical and you have DEPENDENT sources. If you
connect two different speakers in series then they are no longer
exactly DEPENDENT sources and the problem gets much more complicated.
My orignal answer of 9 Ohms is correct for totally independent sources
like resistors.

thanks for the interesting discussion

Mark




.

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