Re: LCD refresh rates - don't understand

The message <VA.000004d1.008eb0e9@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>
from Roderick Stewart <rjfs@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>
contains these words:

In article <313030303737303648B5FDEA02@xxxxxxxxxxxxxxxx>, Johnny B Good
wrote:
I'm not decrying the use of a "Rock Solid" power supply, hell. I've
designed my 50w per channel mini stereo PA amp with a 35v regulated PSU
(just below the max voltage rating of the op-amp chips used in that
design :-). I'm just pointing out that there is no advantage to the
manufacturers of consumer grade Hi-Fi amps to add the extra complexity.

They can 'legitimately' claim significantly higher headline "Total
PMPO" figures for their product when they forego such a nicety and use
devices that can handle the higher transient voltages involved. Provided
the "smoothing caps" are selected to handle the lowest audio frequency
rather than for 100Hz ripple suppression, this can be a quite effective
tactic.

I'm sure it is, but the difference in running an amplifier from a 35V
regulated power supply and running it from the slightly higher voltage of
the unregulated source that would have fed the regulator if you'd used one
can only be a dB or two.

You're rather underestimating the benefit. I'd say the transient peak
handling limit would be raised by a good 4 to 6 db in the absence of
power supply regulation (and this assumes a pretty 'stiff' unregulated
psu). In more cost cutting designs, the transient power limit might be
as much as 10db higher than the steady state rms per channel (both
channels driven) limit.

What complicates the comparison is the fact that the rms figure is an
average rms power rating and the PMPO figure is the instantaneous power
at the crest of the waveform, which, for a pure sinewave, is 3db higher
than the average rms value. Straight away, you have a doubling of the
per channel wattage figure before you even take the effect of an
unregulated power supply into account. When you double the rms figure
yet again on account it's the _total_ PMPO of both channels
simultaneously, you have a 6db higher figure still without taking into
account the effect of an unregulated supply. Once you've added the extra
4 db or more due to the unregulated PSU characteristics, the Total PMPO
rating figure can easily be 10 times or more greater than the rms per
channel rating.

Whilst you and I know that the only meaningful figure is the lower 4 to
6 db boost due entirely to the voltage swings of the unregulated PSU,
the admen will gaily seize upon the 10 db figure offered by the Total
PMPO rating and use it as a headline figure (100W PMPO sounds so much
more impressive than 10w per channel :-)

So I'll grant you it's a real improvement and it enables you to make quite
legitimate claims, but I think the increase in the numbers in the spec
sheet must be more impressive than any audible increase in loudness.

Yes, such PMPO figures tend to be inflated by 6db over the real gain
from the use of an unregulated PSU due to the "apples and pears" nature
of the specification figures.

Of course we shouldn't underestimate the importance of those numbers if
you're designing those amplifiers to sell them....

Sometimes the advertisers will simply make assumptions that a factor of
20 times the rms per channel power rating legitimately applied to one
product's PMPO figure can be directly applied to their own product.

A classic example of this was the 100W PMPO rating on a USB powered PC
speaker set I bought a few years back. Since there is only a 5 volt
supply to play with, even a low voltage bridged output amp driving into
4 ohm speakers is unlikely to exceed an output of 2 watt rms per channel
(or a peak power of 4 watts). Since the 5 volt supply is reasonably
regulated, the total PMPO figure could only have been 8 watts at best.

In this case, I think the guy writing the ad copy must have scanned the
Hi-Fi mag adverts for inspiration to work out a likely PMPO 'fudge
factor' and then rounded up. In any case, the power rating figures were
a total nonsense designed to impress their target audience.

--
Regards, John.

Please remove the "ohggcyht" before replying.
The address has been munged to reject Spam-bots.

.

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