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freo 8th January 2006 05:01 AM

class a running hot
My first post on this site, the site looks great so much information, I would still like to ask a question, my diy solid state 15w class a power amp is sounding fantastic, but it is running very hot you can keep your hand on the heatsinks but only just, the seperate regulated power supply does not get as hot, is this temp normal, and should I not worry about it or is there anyway to disipate the heat further. I am using the amp with a bottlehead pre that has quite a lot of gain would this make any difference, also the weather has been very hot for the last couple of days that may have made a difference. Thank you in advance
Regards from Western Australia

Greg Erskine 8th January 2006 05:35 AM

Hi Rojer,

What amp is it?

Your description sounds about right to me. If the temperature remains stable after an extended period of use, especailly in hot weather, then it should be OK.

You can increase the size of your heatsink or add a fan if you are worried about temperature of the transistors. The cooler they are, the longer they will last.

Did you follow the advice of the amp designer?

amplifierguru 8th January 2006 05:52 AM

Hi Roger,

Yes Australia's having it's hottest weather on record and it's catching up with Perth ( and freo). Could be all those Class A amps?

Have to ask the same question as Greg #1. What is it?

Possibly, if it's unusually hot over the heatsink, that the temp comp is not adequate so when the ambient temp rises, so does the standing current in the output stage, compounding the rise.

Need to see a schema...


lineup 8th January 2006 06:00 AM

As Greg say
a Class A can get very hot.

Same amount of current runs in supply regulator as in amplifier.
But heat = power.
And power is Current x Voltage.

Across regulator maybe 6+6 Volt.
Across amplifier maybe 24+24 Volt.

This makes power ( and heat ) 4 times more in amp than in regulator heatsink.

Power = Current x Voltage
Watt = Ampere x Volt
P = I x U

Class A often requires large heatsinks.
This lowers the temperature.
Nelson Pass extreme Class A amplifiers, uses enourmously overdimensioned heatsinks.
This gives much lower temperature. Maybe only 40-60 degrees Celsius.
And he says transistors, MOSFETS, will work better.

This picture shows a Pass DIY, Zen Amplifier, version 2

lineup 8th January 2006 06:18 AM

You should find out how much current is flowing in output.
You can measure across a know resistor in output stage.

To produce 15 Watt Class A into 4 ohm you need:
2.74 A for single end (constant current source)
1.37 A for push-push output stage

adding 20% gives:
1.6 Ampere and 3.2 Ampere

For 15 Watt into 8 Ohm
1.2 Ampere and 2.4 Ampere

Setting bias to more current than this, is a waste of current and heat.

freo 8th January 2006 12:28 PM

Wow what great replies
Thanks for all the advice, the amp is an altronics kit, I think it is from silicone chip and it is a really sweet sounding amp it can be found on the altronics web site under kits, the instructions are pretty vague but it was worth it. I can send photos of internals by email if anyone is interested, the amp does reach a stable temp but it has been hotter the last few days, following the air temp I hope. Picked up a great cd Harry Manx Mantras for madmen really worth a listen (blues with a little eastern flavour)
Regards Roger

amplifierguru 9th January 2006 02:06 AM

Hi freo,

You might find that's all it is - just the heatsinks tracking the rise in ambient temp. Just check the small black transistor (going from the catalogue pic) between the large flat output devices to make sure it is in intimate contact with the heatsink to sense variations.


Greg Erskine 9th January 2006 06:22 AM

Hi freo,

I think this amp is known as the SC ULD Class-A (or the Silicon Chip Ultra Low Distortion Class-A in long hand). I believe I have heard a slightly modified one of these a few years back and you are right it was a sweet little amp. :D

Did you use the regulated power supply kit? I have one of these amps in my "to be completed in 2006" list.

I guess a picture is worth a thousand words so I would appreciate if you could send me a PM.

Hey 'guru,

If we are looking at the same amp, I noticed it is a CFP amp (I have the schematics) so I was wondering why they mount the Vbias transistor with the output devices and not the drivers?


amplifierguru 9th January 2006 06:39 AM

Hi Greg #1,

You have the schema... i was just going from the pic. And you're absolutely right, a CFP amp does not need the bias sense BJT in intimate contact with the heatsink. If the drivers (can't spot those unless they're TO92's) don't have a heatsink, it's quite customary to arrange the bias BJT in 'semi-intimate' contact so it doesn't over-compensate and back the bias off too much. Of course with a Class A, it could be a extra measure against severe overheating to overcompensate as it's really only going to cut back at the full power end.


Greg Erskine 9th January 2006 10:24 AM

Hi freo,

I accidently deleted your email instead of downloading it. :o Do you mind resending it. :D


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