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Old 2nd March 2011, 01:46 PM   #1
diyAudio Member
Join Date: Nov 2010
Location: Devonport, Tasmania
Default Technical question about heat.

Hello from the bottom of Australia. ( I mean Tassie, not the bum-cheeks)!

When I use my amplifier to listen to a movie, most of the 'source input levels' are very low, for example the sound of a breeze rustling some leaves or the sound of running water in the background. Or there may be silence for some time. As opposed to bullets whizzing and bombs blasting.

Let me elaborate a little more. My volume knob - for arguments sake, may be in the 12 O Clock position, but the sound of silence is being fed into my pre-amp. Very little sound is being reproduced in my speakers. But does the amp (an 'A' class, I think), still have to work hard to reproduce the sound of silence?

Or put another way: During these 'quiet' times will my amp be 'resting' or generating heat?

I know this question sounds a bit silly, but it's something that puzzles me.

Bye for now, Phil Elliott
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Old 2nd March 2011, 02:02 PM   #2
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Join Date: Apr 2009
Location: Farmington Hills, MI USA
If it is a Class A amplifier it will dissipate significant power as heat even when the amplifier output is zero. This is because Class A amplifiers run high levels of bias current to improve the linearity of the output.
Kevin(ahcc20)...I'd rather have a bottle in front of me than a frontal lobotomy!
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Old 2nd March 2011, 02:26 PM   #3
diyAudio Member
Join Date: Nov 2010
Location: Devonport, Tasmania
Cheers Kev. I guessed that this was the case. Now I will go back to the basic electronics book that I have been reading for some time, and look up the section on transistors and biasing - no trivial matter, could take some time and a few beers, so Cheers and thankyou very much.

Sincerely, Phil Elliott
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Old 2nd March 2011, 07:02 PM   #4
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Join Date: Nov 2005
Location: East Coast of South Africa
Hi Phil,
With most push/pull class A designs you can bias them into class AB without any real loss of sound quality, thereby dropping the heat as well as your electricity bill significantly and the need not make Wallaby any warmer than it is. Have you called on Sheila to bring some spanners, blow the froth of your beer and take a peek inside?

We can talk about what you see. Inside the box I mean.


Last edited by Nico Ras; 2nd March 2011 at 07:07 PM.
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Old 2nd March 2011, 11:14 PM   #5
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Join Date: Nov 2010
Location: Devonport, Tasmania
Hi Nico, nice to meet you cobber - most of my mates dont have two phd's so I'll put you down on my list of friends! Most of my acquaintances (like me) have trouble reading the newspaper every morning!

Mate, the shielas are great in here in Tasmania where I live, but I don't get a look in, because I'm polite and a decent bloke. They seem to go for the bald, tatooed, chaps with the hot cars and loud mouths. I have never understood that phenomenon.

This is the home of the Bennetts Wallaby, (red neck) he is a little fella with a happy smiling face. He hops around & hides in the scrub and eats grass and shrubbery. I actually used to shoot them for my dog years ago, until one fixed an eye on me as I was looking down the barrel of my 12 gauge. I have since repented and would rather feed the dog some chicken; because chickens are stupid!

I think there is also a rugby team named The Wallabys here in Au. but unlike the Bennetts red neck. I would have no qualms about shooting one of them; as they are large (often bald) tatooed men, the likes of which steal all the women around this joint.

Cheers Nico

Tassie is a little island off the South South East coast of Australia. It is much more mountainous than most places on the mainland and we have a long cold winter. Trout fishing is a popular passtime here and we have an inland fisheries department that stock all the lakes up in the midlands, where the water is clear. This water is then run through turbines to give us 240VAC at 50Hz at the wall socket outlet.
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Old 3rd March 2011, 09:44 AM   #6
AndrewT is offline AndrewT  Scotland
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Join Date: Jul 2004
Location: Scottish Borders
the amplifier (every amplifier) consumes some power when it is doing "nothing".

If you feel competent you can measure the quiescent current and from there the Power that is being dissipated.
Some of the switching amps consume quite low levels of quiescent power Pq.
ClassAB consume more.
ClassA consume a lot of power and thus need very large dissipation capacity to keep the semiconductors from burning up.
regards Andrew T.
Sent from my desktop computer using a keyboard
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Old 3rd March 2011, 10:03 AM   #7
DF96 is offline DF96  England
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Join Date: May 2007
A genuine Class A amp will be cooler when outputting sound. This is because it always draws the same power from the power supply, and then turns it either into heat or sound. In reality most of the 'sound' going to the loudspeakers will be turned into heat too (in the voice coils), most of the rest will end up as heat as it bounces around your room. So, as thermodynamics says, it all ends up as heat eventually!
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Old 3rd March 2011, 10:22 AM   #8
Mooly is offline Mooly  United Kingdom
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If you tell us the make and model of amp we might be able to tell you what class it runs in. If it is true class A you'll know... it will run HOT HOT HOT all the time.
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Old 6th March 2011, 12:48 AM   #9
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Join Date: Nov 2010
Location: Devonport, Tasmania
Hi there Mooley, I just took a glance at your public profile and couldn't see your occupation, I'm guessing you are a Dairy Farmer? Any way, it's good to make your aquaintance.

The question about heat and power was a bit of a hypo'.

I am a real peasant so my equiptment is very mundane;

Favourite (half of) amp: Denon avr1000 - most surround boards are shot and tuner doesn't work but it has an awesome front end that delivers the best 85w that my old ears could wish for. I got this beast from the tip, and it has +-56V rails, a beautiful tone control preamp section, and not a scratch on the real black anodised aluminium fascia!

My other workhorse is a Sony STRDB2000 surround system and It would be nice if a sony technician could come into this thread and help me to understand the output section of this amp. The specs for the Aussie model say 120w into 8r or 100w into 4..... This is contrary to most other amps that logically deliver more power into lower impedances. But I guess that's another kettle of fish.

I live in Tasmania where it is getting on towards the end of summer. The sun is shining so I think I will go for a walk Good to talk Moo, see yoo. Phil E.
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Old 6th March 2011, 01:19 AM   #10
diyAudio Member
Join Date: Nov 2010
Location: Devonport, Tasmania
Just wanted to say hello to Andy "T",,,,,,, Hello Andy! I didn't recognise your flag, so looked up your location I and can see that you live on the opposite side of the globe to me, If you stand upright and drop a soldering iron into a well,,,,,,, I will catch it a couple of days later!

I reckon I will have a poke around in one of my old amps and see if I can measure the voltage drop across a resistor, just got to take a look to see if there is one!

I don't usually probe too often as it is very easy to make sparks that frighten the clappers out of me and this in turn puts unnecessary stress on the laundry maid, who is usually very understanding.

I hope to talk again soon and I will try to be sensible next time; although I find it very difficult. Cheers.
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