C
Clifford Heath
- Jan 1, 1970
- 0
Phil said:** You were right the first time.
Thermal runaway and second breakdown are two quite different things.
Same basic problem, just occurring on the chip - and much faster.
You just need to realise that amp dissipation is always simply the DC
supply watts MINUS load watts taken over one cycle.
Yes, that's the easy part. It's getting the peak load current into a
reactive load that I wasn't sure of. I think it's just V/|Z| isn't it?
The load power is easy - just the rms current squared times the resistance.
Ok, that's nice, because it also works with a reactive load without
needing to worry about the inductance - only the resistive part
can dissipate the input power.
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.
I've derived an expression for it, instead of a numerical search.
Shouldn't be too hard to simplify and plot now.
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.
Yes, I worried about that - the Hfe of the 2n3055 is so low.
I'm guessing that with MJ15003/4 that one could reduce the bias
current (increase the 470 Ohm resisters) through the BD139/140
to reduce the load on them, while still providing a enough drive
current.
The SC 480 is the better amp.
What transisters does SC480 use instead of BD139/140? Or does it
just do what I've suggested above? It's rated only 70W into 4 Ohms,
from a +-40V supply - this seems unnecessarily wasteful since you
only need a +-24V swing for 70W.
** Those dudes would not have the slightest idea.
Do any of *them* design or even repair guitar amps for a living ??
The bloke who actually sold me the speaker is a pro bass player,
and I believe he uses one of those speakers in his kit. But that
doesn't mean he has experience with more than this one setup.
He also wasn't there when I returned it.
** For bass guitar, the more cone area the better the result.
Ok, I'll see if I can adapt the cab.
Using four 100 dB /watt speakers will give 106 dB /watt efficiency
(anywhere on axis)
Bass is not really directional like this is it? The sonic wavelength
is just too long (13+metres at 25Hz).
Clifford Heath.