"Clifford Heath"
Phil Allison wrote:
You keep saying this, and it's true it was based on a false
assumption (that the graphs had anything to do with average
dissipation), but it wasn't a nonsense question.
** Fraid it was - in relation to your concerns.
If you'd told me to read up on secondary breakdown, I think I
would have got it sooner. For thirty years I've thought that
"thermal runaway" was a process that took minutes, not ms.
** You were right the first time.
Thermal runaway and second breakdown are two quite different things.
When I suggested integrating the instanteous power formula over
a half-cycle, it wasn't nonsense - but in the sinewave case with
a single reactance it turns out to be unnecessary.
** As I told you - several times.
I don't think
it deserved to be described as "drivel". Graham's answered my
question though by saying that real engineers use Mathcad or the
like for a numerical solution to such problems.
** But Graham is not a real engineer - he is an ex DJ posing as one.
In fact, the task is easily done with a pocket calculator or even a slide
rule. You just need to realise that amp dissipation is always simply the DC
supply watts MINUS load watts taken over one cycle.
If you calculate the peak load current then that is *exactly* the same as
the peak DC supply current - the waveform is half sine in each DC rail and
the *average* is 0.32 of the peak. Just multiply the average rail current
by the rail voltage to get DC power in watts. The load power is easy -
just the rms current squared times the resistance.
For your case with a 37 volt rail and 100 watts at 4 ohms - peak current is
7.07 amps so the DC power is 84 watts per rail or 168 watts total. The
output is 100 watts so the dissipation is 68 watts, divided by four makes
it 17 watts per device. You should repeat this calculation for a variety of
power levels and plot the results to find the power level that creates the
most amp dissipation.
With a resistive load, maximum amplifier dissipation happens at about 42 %
of max output power. However, if reactance is added in series with the load
creating a 45 degree phase angle, the maximum dissipation shifts up to
full power and is just a tad less than before.
Sorry for being so thick. I just wanted to know how to calculate
whether the amp would be safe. You didn't seem to want to help
with that part, but I shouldn't have abused you for it.
** I did try to help - but you did not listen.
When you ask a technical question, on a non simple matter, you MUST not
specify the kind of answer you want - that often precludes supplying the
right answer.
Whether a pair of ETI 480s operating * IN BRIDGE MODE * is safe from
failure depends totally on the particular speaker load it has to drive, the
way the amp is to be very operated and what the chance of ever suffering a
low impedance or short circuit is. The latter may be sudden death of the
fuses are not fast acting enough.
BTW In the ETI 480 - the BD139/140 driver transistors are MORE likely to
fail due to over current or SOA limits being exceeded than the outputs are.
The SC 480 is the better amp.
Your other comments were useful and mostly to-the-point. So yes, take that
as an apology.
** Hooray !
BTW, I returned the Jaycar speaker (the box was unopened). They
were pretty reluctant (past their 7-day policy), being quite
convinced that the particular speaker would be fine, possibly
more robust that many instrument speakers (they sell the same unit
in 4 ohms for car subs which take a bit of punishment).
** Those dudes would not have the slightest idea.
Do any of *them* design or even repair guitar amps for a living ??
But they
did it - after I mentioned "statutory warrantee regarding
fitness-for-purpose". I have yet to choose another, though WES
have a 12" in the same range as your suggested 15" (which doesn't
suit the cab).
** For bass guitar, the more cone area the better the result. Thrashing
one 12 inch driver to within an inch of its life is not the best way to go -
very few commercial amp use that idea. Using two or even four 12 inch
drivers is far better, and actually increases the efficiency ( in dBs per
watt) so far more SPL is achieved than with one higher powered driver.
As you have seen, decent instrument speakers have around 100 dB/watt
sensitivity, while hi-fi woofers are some 8 to 10 dB less efficient. That
is the same as going from a 10 watt amp to a 100 watt one !!
Using four 100 dB /watt speakers will give 106 dB /watt efficiency (
anywhere on axis) - equal to four times amp power on top - so equal to
a massive forty times amp power.
There are several other reasons to prefer instrument speakers, plus they are
designed for the job.
............... Phil