How can you tell when you pushed a heatsink far enough? - Page 3 - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 26th January 2013, 08:29 PM   #21
sreten is offline sreten  United Kingdom
diyAudio Member
 
Join Date: Nov 2003
Location: Brighton UK
Quote:
Originally Posted by counter culture View Post
A class A amp runs cooler when there is music playing.
Hi, No it doesn't, significantly, for decent quality music programme, rgds, sreten.
__________________
There is nothing so practical as a really good theory - Ludwig Boltzmann
When your only tool is a hammer, every problem looks like a nail - Abraham Maslow
  Reply With Quote
Old 26th January 2013, 08:52 PM   #22
diyAudio Member
 
Join Date: Jun 2011
Quote:
Originally Posted by sreten View Post
Hi, No it doesn't, significantly, for decent quality music programme, rgds, sreten.
Are you sure you really want to argue this point sreten? The energy dissipated as sound is no longer dissipated in the devices.

With zero signal applied at the input of the class A power amplifier, ac power developed across the load reduces to zero and therefore all the power fed to the transistor is wasted in the form of heat. Thus, a transistor dissipates maximum power under zero-signal condition. Thus the device is cooler when delivering power to a load than with zero-signal condition.

Now, you can argue it's significance if you like, but it certainly doesn't run hotter.

This is absolutely basic.
  Reply With Quote
Old 26th January 2013, 09:08 PM   #23
dmills is offline dmills  United Kingdom
diyAudio Member
 
Join Date: Aug 2008
Location: High Wycombe
The key work is 'significantly', consider program audio with say a mere 10dB peak/average ratio (very low for anything exept highly limited pop), turned up so that the peaks are just below clipping, then the average power delivered to the load is 10dB down on the input power to the class A stage....

10dB down is 10% power, so the power stage is still dissipating 90% of the input power under these conditions.
Even if you go for a mere 6dB peak/average ratio, you still get 75% of the no signal dissipation, and that is a completely unrealistic condition.
By the time you have average level 20dB below clipping, you have 99% of the no signal power dissipated in the power device, and I suspect that is close to the usual case.

It does not appear that there is a significant reduction in heat from a class A stage when audio is applied under realistic conditions for music reproduction.

Regards, Dan.
  Reply With Quote
Old 26th January 2013, 09:17 PM   #24
diyAudio Member
 
Join Date: Jun 2011
Quote:
Originally Posted by counter culture View Post
A class A amp runs cooler when there is music playing.
Quote:
Originally Posted by dmills View Post
The key work is 'significantly'
So the statement I made is true?
  Reply With Quote
Old 26th January 2013, 09:26 PM   #25
dmills is offline dmills  United Kingdom
diyAudio Member
 
Join Date: Aug 2008
Location: High Wycombe
Yes, as far as it goes, but this statement points out that the effect is trivial in magnitude, and can be ignored for all practical purposes.
Quote:
Hi, No it doesn't, significantly, for decent quality music programme, rgds, sreten.
73 Dan.

Last edited by dmills; 26th January 2013 at 09:30 PM.
  Reply With Quote
Old 26th January 2013, 09:37 PM   #26
diyAudio Member
 
Join Date: Jun 2011
Well, a politer way of putting it would be to say, 'IMO the reduction is trivial', but to jump in with both feet, 'Hi, No it doesn't', I'm afraid, calls for a refutation, no matter how it's subsequently qualified.

A class A amp runs cooler when there is music playing.

Anybody else?
  Reply With Quote
Old 26th January 2013, 10:35 PM   #27
diyAudio Member
 
Join Date: Jun 2011
There's an absolutely serious point to this.

The way to practically evaluate the heatsinking on a class A amplifier is to leave it switched on with no signal source, as this is the condition in which it will get hottest, regardless of whether the difference is large or small, and this is also a likely condition.

Unlike a class A/B (depending on the exact bias conditions) or class B amplifier, a class A amplifier does not cool down when the operator forgets about it and walks away, leaving it switched on. It gets hotter, until the unit achieves equilibrium with the ambient. It's not a question of 'withstand operation at full volume for 3 hours'. It's 'withstand operation at zero volume in perpetuity.'

Amplifiers are frequently switched on and left idle. Bias may not be thermally stable. Obviously we try to ensure that thermal runaway is obviated, but a good heat-soak in worst-case conditions is a sensible precaution, because it's in the extreme conditions that a problem is likely to reveal itself.

Plenty of pieces of kit I've tested went in an oven.
  Reply With Quote
Old 26th January 2013, 10:55 PM   #28
dmills is offline dmills  United Kingdom
diyAudio Member
 
Join Date: Aug 2008
Location: High Wycombe
I figure everyone knows worst case class A is zero output, but that putting audio through it does not reduce the dissipation in a meaningful way is possibly somewhat less obvious.

I have had a class AB biased RF deck get into trouble when switched on at very low ambient temperature as the FET die heated faster then the heatsink and the bias compensation thermistor failed to track fast enough, it tripped the overcurrent hard fault shutdown when Id reached 5A @ 300V up from a designed 0.1A standing bias....

The fix was to reduce the drain voltage to reduce the slope enough that the short term thermal behaviour became reasonably stable at a sane dissipation.

You always test at BOTH extremes of the designed temperature range if you have any sense, environmental chambers exist for good reason.

Regards, Dan.
  Reply With Quote
Old 26th January 2013, 11:30 PM   #29
diyAudio Member
 
Join Date: Jun 2011
Quote:
Originally Posted by dmills View Post
I figure everyone knows worst case class A is zero output
Some things are best not left to chance.
  Reply With Quote
Old 27th January 2013, 04:26 PM   #30
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
Quote:
Originally Posted by dmills
I figure everyone knows worst case class A is zero output
I doubt it. If facts were well known, many of the arguments on here would never occur. There are facts where the converse appears to be better known than the fact. I won't mention any, as I don't want to divert the thread!
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
a 2 yr old pushed in my friend's FF85K fishski13 Full Range 25 12th June 2010 07:37 PM
How far can a pair of SAP15 being pushed into a 4 ohm??? Leolabs Solid State 5 16th January 2007 04:42 PM
Help me please 12 inch pushed in centre dodgy99uk Multi-Way 7 16th February 2004 10:54 PM
Pushed In Cones Bider Multi-Way 11 27th January 2004 06:30 AM
Help: Soft dome tweeter pushed in Al Garay Multi-Way 46 8th October 2003 06:45 PM


New To Site? Need Help?

All times are GMT. The time now is 05:47 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2