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Old 22nd January 2009, 12:32 PM   #11
lineup is offline lineup  Sweden
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Yes, you are probably right.
Of course we should adjust the output voltage point.

This can be done by increasing R6 (27k). Could be replaced, temporary, with one 50-100 kohm trimpot.

My earlier post estimation of a good DC operation point for output,
would indicate something like 16-18.5 Volt.
A bit lower than 1/2 of supply voltage.

The Voltage Gain, Gv, is defined by R3/R5.
Which gives approximate ( 6k8/330 ) Gv = 20.
A bit lower gain than 20 can be expected..
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Old 22nd January 2009, 04:18 PM   #12
AndrewT is offline AndrewT  Scotland
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this is a single ended amplifier with a constant current load.
If the bias (and CCS) is set to 2A then the maximum current swing from the amplifier is 2Apk.
Don't even think about using less than 8ohms speakers.
Even these will require much more than 2Apk on short term transients.
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Old 22nd January 2009, 05:10 PM   #13
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True!
And as MOSFET will get hot (80 Watt idle)
it is questionable if IRF540 (TO-220) can make 40 Watt each.

IRF640 or even IRFP240 may be required ...?
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Old 23rd January 2009, 08:34 AM   #14
AndrewT is offline AndrewT  Scotland
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Hi,
SOAR of the 540 is not a problem.
You can run Tc<=100degC leaving an effective dissipation of 40W from the devices.

The problem is peak current on transients. This amp can only deliver 2A and will current clip.

A heatsink of about 0.6C/W to 0.7C/W for 40V & 2A will work at these very high temperatures. The amp can deliver about 15W to 16W into 8r0 but it cannot deliver into a reactive 8ohms.

However, it could make an excellent 9W into 16ohm amplifier.
or
reduce the voltage to 30V and increase the bias to 3.5A on a 0.3C/W sink for 8W into 8ohms speakers

I would be tempted to run with a pair of isolated heatsinks, with each FET direct coupled to it's sink.
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Old 23rd January 2009, 11:06 AM   #15
lineup is offline lineup  Sweden
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Okay, so the IRF540 may do it. This is good.
IRF540N variant is even better for higher heat.

When I design Class A, I use some 'normal worst case' numbers to calculate for current.

If make class A for 8 Ohm .. I use Bias Current for 5 Ohm (+ 10% extra)
for nominal output power level.
Same way, for 4 Ohms speakers, it is better to use like 2.5 - 3 Ohm value
for the calculations.

Yes, using like 30 VDC and 2.5-3.0 Ampere may be a first best try.
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