In article , Bob Latham
wrote:
In article , Chris Morriss
wrote:

Why do you need 150V parts? Are they kilowatt speakers?

I don't know, KEF designed them and obviously considered it important.
There are 2 x 30uF one is 50v the other 150v and the PCB is designed for
the size difference so it was clearly intentional.

I am not sure about this as I don't know the details of the caps and
loudspeaker in question. However one thing that occurs to me is that IIRC
it is not unusual for the 'd.c.' voltage rating of a capacitor to be
somewhat different to the 'a.c.' rating. Hence a spec of 150V might be to
ensure it is safe for the a.c. levels of voltage/current KEF assume may
sometimes be applied.

If you don't actually play the loudspeakers very loud, then a lower voltage
rating might be fine. What peak voltage levels can your amplifier produce,
and how load do you play music? :-)

Slainte,

Jim

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In article , Bob Latham
wrote:
In article , Jim Lesurf
wrote:
[snip]

If you don't actually play the loudspeakers very loud,

I'm afraid I do though.

OK. :-) Worth checking with a 'scope, though, as the actual levels might
be lower than you think. The ratings given for caps are usually on an rms
(a.c.) or steady (d.c.) basis. Music tends to be occasional peaks
accompanied by a much lower steady level. Hence the rms voltage level when
you're not clipping your amp will often be a tenth of the peak levels.

then a lower voltage rating might be fine. What peak voltage levels
can your amplifier produce,

Well as a rough calculation P=V2/R or V=square root of PxR so assume
nominal 8 ohms and 180 watts gives 37.9 volts rms. The DC rating tends
to be 70% or even 80% higher than the AC so I don't think 50v caps are
good enough.

Bear in mind the peak/mean form-factor I mention above for real music. You
may be peaking at over 40 Volts, but the rms level might still be well
below 10 Volts. I'm not sure of the way reversable caps handle waveforms
like this, but I suspect they'd survive, albiet with higher distortion on
peaks.

But what about resonance? Can you not get a much higher voltage than the
source voltage due to the interaction of inductors and capacitors in AC
conditions?

To some extent, yes. This will depend upon the details of the design.

and how load do you play music? :-)

As a general rule of thumb, just below any audible or visible trace of
distress from the system. :-)

In that case you may have to disregard the above comments about
form-factor, depending upon your hearing acuity, etc. :-) Sometimes
people clip peaks without being aware of it. This will reduce the
form-factor. That said, if you have been doing this to a severed degree for
many years with the KEF's I would have expected them to have failed long
ago. Hence the above comments re form-factor may well be relevant.

Slainte,

Jim

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html
Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html
Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html