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Old 26th January 2009, 08:24 PM   #1
Mark245 is offline Mark245  United States
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Default Testing Class B Output stage

Hey,

I have a question cencerning the testing of my class B output stage. I want to test this circuit on a breadboard and I want to know if it is ok to run this circuit with no heatsinks installed. I will be running the test with no load connected. I just want to make sure the voltage output is good. Is it ok to do this? I just want to test the circuit by itself before connecting it to the previous stages. It seems that it should be fine since there is no speaker connected that will draw a lot of current. I guess I should simulate this in spice as well.

Thanks in advance!

Mark
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Old 26th January 2009, 09:41 PM   #2
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I would use heatsinks the first time, so that I could adjust the bias without worrying about blowing up my precious transistors and breadboard. O_O

It's probably best if you leave the heatsinks on. When you make a change to the circuit, you might have to change the bias again and if something's screwed up and there's no heatsink, BOOM.

So I would go through the trouble.

Might want to wait for a second opinion, more useful information could be on the way.

- keantoken
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Old 26th January 2009, 09:50 PM   #3
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Hi Mark

If it is a real class B stage, then, in theory, you do not have any quiescent current, so it should be OK to test it without mounting the transistors on a heatsink.

BUT.....
If it is class AB (most is) you have a quiescent current of some value, and then it is depending of the supply voltage how much power there is dissipated in the transistors, and you have a great risk of smoking the transistors.
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Old 26th January 2009, 10:14 PM   #4
wg_ski is offline wg_ski  United States
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Breadboards are a very good and reliable method for getting things to accidentally short or open. Push pull output stages of any kind don't like for that to happen. If you have to breadboard, use a lab power supply with a user-settable current limit.
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Old 26th January 2009, 10:58 PM   #5
Mark245 is offline Mark245  United States
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Thanks everyone. Yes maybe I should use a lab power supply. I was thinking about using the unregulated raw dc power supply that I will use in the final version of this amplifier. However I have not installed the fuses yet.

Also, when I install the heatsinks, is it totally neccessary that I mount all the output transistors on the same heatsink? What about the bias transistor can I leave it off or should it be mounted too? I am trying to make this as easy as possible for a quick test but I don't want to risk blowing something up.

Thanks for all the replies so far!

Mark
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Old 26th January 2009, 11:47 PM   #6
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To be safe, it is best to mount everything as you would in the actual amp. You don't need a big heatsink but I would recommend that you attach all of the output devices and very importantly the bias transistor to the same piece of metal.

Don't forget the lightbulb in series with the mains trick for when you first power it up.
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Old 27th January 2009, 12:27 AM   #7
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I never connect the output transistors on first power up.
I run the output of the VAS back into the LTP for feedback.
At this point i would turn the bias volts down.

Only when I am convinced the driver is working fine do I consider connecting the output transistors.
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Old 27th January 2009, 10:23 AM   #8
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Default SZIKLAI AND CLASS B

is something that exites me very very much ....can i get a schematic please ??????

thank you
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