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Old 17th August 2015, 05:38 AM   #1
Batson is offline Batson  Turkmenistan
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Default Need advice

Hi guys
I built stereo amplifier.Output transistors each channel are 2x5200 and 2x1943.Rail voltage is +/- 50volts.Works fine.
Then i added simple tone control bass and treble.Now sounds even better it is what i wanted.More bass and treble i can hear.But im having problem in output transistors they getting extremely hot.
Should i add some more transistors or add fan?or is there any problem?
Thank you.
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Old 17th August 2015, 06:00 AM   #2
Mooly is offline Mooly  United Kingdom
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What is the quiescent current of the output stage ? Is this stable with temperature or is there a tendency to thermal runaway. Instability can also be a cause of excess heat generation.
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Old 17th August 2015, 06:13 AM   #3
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Originally Posted by Batson View Post
....But im having problem in output transistors they getting extremely hot.....Should i add some more transistors or add fan?or is there any problem?
You need to specify:
- Class A, AB etc and how much bias current.
- What rail voltages?
- How long does it take to get them so hot?
- Do they get hot with no output or only at high power output?

The transistors are normally capable of quite high dissipation but most heat with class AB is due to bias current. A steady 100 mA or so bias current for EF output stages develops quite a lot of heat when there are 4 power transistors dissipating it from say, +/-56V power rails. If you have at least an estimate of your heatsink ratings, you should be able to calculate the approximate temperature rise to see whether you need more or if there is another problem causing this excess heating, such as oscillation.

As an example, a single pair of output transistors with 100mA bias as above, will dissipate 112*0.1 = 11.2W and 2 pairs 22.4W. Thus a stereo pair of these will pump out 45W just idling! A heatsink rated at say, O.5C/Watt would then rise 22C above ambient and in warm weather, that could well seem very hot.
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Old 17th August 2015, 07:00 AM   #4
Batson is offline Batson  Turkmenistan
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Mooly I dont know the current of the output stage.
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Old 17th August 2015, 07:04 AM   #5
Batson is offline Batson  Turkmenistan
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Ian Finch
It is AB class
Rail voltage is 50volts
It gets after 5-10minutes
No it doesn get hot with no output
It gets hot only i connect tone control circuit.
Bias on 5200 little higher than 1943
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Old 17th August 2015, 07:28 AM   #6
Mooly is offline Mooly  United Kingdom
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You need to monitor the current across the output stage emitter resistors to see if it increases as the amp warms up and when you connect the tone stage. It could be that the tone stage is oscillating and feeding a lot of HF into the amp.
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Old 17th August 2015, 07:51 AM   #7
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Quote:
Originally Posted by Batson View Post
Ian Finch
It is AB class
Rail voltage is 50volts
It gets after 5-10minutes
No it doesn get hot with no output
It gets hot only i connect tone control circuit.
Bias on 5200 little higher than 1943
If it only gets hot with tone controls, do you have DC blocking cap at input of amplifier? Or...maybe you like bass a lot and its simply producing more power.
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Old 17th August 2015, 09:59 AM   #8
Batson is offline Batson  Turkmenistan
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Mooly.Yeah i have seen changing all the time current across that resistor.ofcourse it is increases when i turn more volume or when it hits bass.is it oscillating?But i dont hear nothing or is it frequency beyond my ear capability
Triplej yes there is cap at input the amp.Yes i like bass alot .
It is ok if i turn on the fan and heatsink gets normal temp (warm)but without fan i dont know.I dont like using fans because of noise.it doesnt get so hot that you can burn your hand but hot enough
Thank you guy for your answers.You always great
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Old 17th August 2015, 10:09 AM   #9
Mooly is offline Mooly  United Kingdom
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Oscillation is often in the 1 to 10 Mhz range was way over what you can hear. If it is oscillating then it would probably do it with no signal present but leaving the tone and volume controls set as they are. It should run at the same temperature in that condition as it would without the tone control.

Its very hard to advise without knowing a lot more about the set up and how loud you are running it.
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Old 17th August 2015, 10:20 AM   #10
Batson is offline Batson  Turkmenistan
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Ok Mooly i will try tonight leave it open without a signal on high volume and see if it gets hot or not,Sounds good idea to know oscillation without osciloscop.
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