In a recent post about heatsinks and fans the topic of
ME Techologies ME850 amp came up as a side issue.
Trevor Wilson, Sydney's ME "expert" could not seem to tell us exactly
how much class A
was produced in this SS amp, and expected me to inform him fully,
something I was unable to do
because I've never measured the bias current in an ME850 amp.
So lets forget about ME amps completely; If Trevor wants to talk about
the details of the ME 850, he's welcome,
but he better get his facts all straight and clear, before he makes a
monkey out of himself again.
Consider generic facts about SS amps configured with complementary pnp
and npn
output devices in series for the output stage.
Consider rails are +50V and -50V.
Assuming the peak load voltage travel is +/- 48V,
you can get 33Vrms, giving 136Watts into 8 ohms, and 272 watts into 4
ohms and 544 watts into 2 ohms
but ONLY IF the PSU is stiff enough and there are enough output devices
to ensure all remain
in their SOA.
But even with a 200 watt PSU, if the power supply caps have enough
capacitance,
and there are enough devices, power peaks with music can be handled up
to the theoretical
figure of 544 watts if the load is as low as 2 ohms. 48pk V into 2 ohms
is only 24 amps peak.
With only two output bjts or mosfets, T03 packaged, one would have
current limiting
to prevent more than say 100 watts of PO regardless of the load,
as well as have rail fuses, and some well placed thermal sensors mounted
on the TO3 hold down bolts.
All this power in most SS amps is mainly in class AB with very low bias
currents at idle of perhaps
typically 30mA per output device.
So say you had a total of 30mA rail to rail, idle Pd = 100V x 0.03A = 3
watts,
and the the amount of class A PO available depends on the idle current.
The maximum peak Iswing available in the load during class A operation =
2 x idle current.
So say Iq dc = 30mA, then max peak class A load current = +/- 60mA,
or 0.042Arms, and if the load is 8 ohms, class A PO = Irms squared x R =
0.042 x 0.042 x 8
= 14 milliwatts, which is hardly any class A at all so its why most SS
amps
are regarded as virtual class B amps.
But suppose we raise IQdc to 1.0 Amps, so Pdq = 100 watts.
Max load current for class A = +/- 2peak amps,
or 1.414Arms, and if the max V swing is 33Vrms, then the load for
maximum possible pure class A = 33 / 1.414 = 23.3 ohms.
One would get 46.7 watts max of pure class A PO,
and efficiency is 46.7%, since the power drawn from the PSU reamins at a
constant 100W
when the amp remains in the pure class A operating region.
But if we had 8 ohms, the PO is I squared x R = 1.414 x 1.414 x 8 = 16
watts of class A.
The output voltage could still reach 33vrms, and PO reaches a max of
136W but its mainly class AB.
With 4 ohms, only 8 watts of class A is made, with 100watts of idle
power.
If you read Douglas Self, the high SS class A isn't worth the trouble
because the power supply
works hard and noise artifacts muddy the signal more than when there is
low bias AB.
The jury is still out of course on all this, personally
I prefer tubes.....
Douglas is very interesting on steam engines BTW....
Patrick Turner.
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