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Trevor Wilson[_3_] Trevor Wilson[_3_] is offline
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Default Motional feedback in speakers

On 5/11/2019 11:16 pm, Peter Wieck wrote:
No surprise at all


**Well, it was a surprise to me. In 45 years of servicing equipment,
I've never seen that technology is a valve based product. Perhaps it was
more common in the US.

, back in the day when tube-based amplifier damping factors were
somewhere between 10 and 15 on a good day with a tailwind, and acoustic
suspension was uncommon.

**The output impedance of modern valve amps is no better. In fact, the
real figures are frequently much poorer than the (inferred) figure of
0.8 ~ 0.5 Ohms you suggest (so-called 'Damping Factor' is a misleading
and rather poor specification to quote in technical matters). Here are
some figures from Stereophile:

https://www.stereophile.com/content/...r-measurements

https://www.stereophile.com/content/...r-measurements

https://www.stereophile.com/content/...r-measurements

Fundamentally, the output transformer is a major limitation in most
valve amplifiers. It leads to serious problems with frequency/phase
response figures for most loudspeakers.



The 'cure' for tubby bass would be some choice between a stiffer
spider, stiffer surround, smaller magnet, shorter excursion, any or all.

**Or, of course, a decent amplifier, with a sensible output impedance
figure (which excludes most valve amps).



Seems overly complex - and I can see where it did not last long, or penetrate past a very few makers. By the way, Philips speakers were "active" speakers, needing only a pre-amp input.


**Well, yes, but the Philips MFB speakers also employed a closed loop
system, which measured the output signal from the speakers, thus
negating much of the influence of issues with the amplifier used (and
the limitations of the enclosure). The Luxman/Panasonic system appears
to employ a similar, closed loop approach. And, as I stated before, I
was surprised to see such a system in a domestic system way back then,
due to the cost/complexity of such a system. I'd like to see one in the
flesh.


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Trevor Wilson
www.rageaudio.com.au

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