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Class A Amplifier Heat Sink Requirements!?
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soldar:

--- Quote from: Refrigerator on March 14, 2019, 10:17:12 pm --- Surely it produces "warmer" music :-DD
--- End quote ---

Best post / poster combination :)
Audioguru:
This 40W amplifier produces 100W of heat all the time and should be banned for wasting so much electricity and air-conditioning.
Class-AB amplifiers perform fine without the waste.
David Hess:

--- Quote from: soldar on March 14, 2019, 06:35:30 pm ---
--- Quote from: Robbie010 on March 14, 2019, 12:29:17 pm --- This is my first DIY build, the amp is actually a Nelson Pass A40 design.
--- End quote ---
I believe that is not a Class A but a complimentary pair class B amplifier.
--- End quote ---

I wondered what he was talking about with class-a and complementary.  That is class-ab where 40 watts in a small enclosure is very feasible.
soldar:
I had another quick look at the article and I find it unconvincing in several ways.

It uses a center tap 44 + 44 Vac transformer, rectified, and later says it is using 32 + 32V. That does not add up. 60 ~64 Vdc rail to rail sounds about right so I would say the transformer spec is mistaken.

The high quiescent current means the need for four power transistors on a huge heat-sink. This is unnecessary and the article says you can decrease the quiescent current and the amp will perform well. So why not do it? That removes the need for four power transistors and only two will be needed and much less heat sink.

If I looked at it slowly I am sure I would find more problems and inconsistencies. Not to mention a few typos. (If the amplifier can be "fumed on" instead or "turned on".)

That project does not inspire confidence in me.

These complementary pair amplifiers are classics and there are a million variations out there but they are basically the same idea. I just can't see any benefits to unnecessarily increasing the quiescent current unless it is heating the room. 

Look at the similar design I posted upthread; it says to adjust quiescent current to 40 mA which at 60 volts is 2.4 W quiescent power.  That makes much more sense.


--- Quote from: Audioguru on March 15, 2019, 12:22:37 am ---This 40W amplifier produces 100W of heat all the time and should be banned for wasting so much electricity and air-conditioning.
Class-AB amplifiers perform fine without the waste.

--- End quote ---

What he said. Bad design.
Gary.M:

--- Quote from: Benta on March 14, 2019, 09:21:07 pm ---
--- Quote from: soldar on March 14, 2019, 08:14:51 pm ---
--- Quote from: Benta on March 14, 2019, 07:35:21 pm --- The Nelson Pass A40 is indeed a class A design, and from pictures it has enormous "cooling towers" with fans:
--- End quote ---
I stand corrected. It seems a bit crazy though unless you want it to double as a space heater.

--- End quote ---

No, you don't stand corrected :)
I've now read the Nelson Pass article on the A40, and I conclude that it's a high quiescent current class AB amplifier. He presents some waffle about "push-pull class A", which is nonsense in this context.

A pure 40 W class A will dissipate at least 210 W per channel, 250 W is more likely.

--- End quote ---
Nothing Nelson Pass has to say is waffle. Class A operation is solely determined by the quiescent current vs load demand, not the topology. In this amp the peak output current at which the amplifier comes out of Class A will be approx 2 x the idle (bias) current. This is where one side of the output stage cuts off. You could bias it into Class B, Class AB, or Class A on a continuum.

Sent from my MI 8 using Tapatalk

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