In article , GB
wrote:
alexander.keys1 wrote:
On Mar 4, 11:09 am, "GB" wrote:
The Natural Philosopher wrote:
Dave Plowman (News) wrote:

Just solder a new 80v cap of almost any flavour over 1000uf across
whats there.

Why would these need to be 80v out of interest? What sort of DC
voltages do these amps typically operate on? I know there's a margin
for peaks, but 80v across a nominal 8 ohm resistance would provide
800w. This is an amp, not a room heater, surely?

80 volts is the *peak* voltage, at the top of the waveform. The
wattage delivered to the speakers depends on the 'RMS' (root-mean-
square, a sort of average), which is 0.707, (for design purposes; it
varies with the waveform) of the peak.

Actually for a sine wave the mean power is half the peak power. It is the
voltages that are in the ratio of 1/root(2).

That's still 400w average power.

Not seen the initial postings in this thread as it has only just started
being xposted to uk.rec.audio. However, the above depends on details like
if the amp is single-rail or dual-rail.

For example, for 100wpc (mean for sinewave) into 8 Ohms you'd need +/- 40V
rails as absolute minimum in a dual-rail design. If it were a single rail
then you'd need 0V and a 80V rail as absolute minimum. In practice a
real-world design would need headroom to drive the devices, so 0 - 80V
single rail would get you somewhat less than 100 wpc (sinewave mean). Then
if you also take into account that the PSU will probably 'sag' under
sustained load you'd find that 80V single rail would be much less when you
are trying to get sustained sinewave power from both channels of a stereo
amp.

So in practice you might find that 0 - 80V single rail ended up giving you
far less than 100wpc mean sinewave stereo. (Although peak levels for music
could be somewhat higher.)

And as I see someone else has pointed out. You also would be safest
choosing a cap whose spec voltage was well above the actual rail voltage
when the amp isn't sagging the voltage. Hence an 80V cap would be sensible
for use with a somewhat lower rail voltage.

Slainte,

Jim

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