Can bias transistor mount on top(body) of output transistor?? - diyAudio
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Old 12th February 2008, 02:34 PM   #1
Leolabs is offline Leolabs  Malaysia
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Default Can bias transistor mount on top(body) of output transistor??

Usually a bias transistor is mounted on the same heatsink together with output transistors.Wondering can it be "stacked" on the body of the output transistor instead on the heatsink???
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Old 12th February 2008, 02:49 PM   #2
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Default Re: Can bias transistor mount on top(body) of output transistor??

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Originally posted by Leolabs
Usually a bias transistor is mounted on the same heatsink together with output transistors.Wondering can it be "stacked" on the body of the output transistor instead on the heatsink???
Rotel use to do so in some of theirs power amps, namely in the RB-980 BX ( a tipple follower with 3 parallel output devices per rail )...
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Old 12th February 2008, 02:54 PM   #3
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I have seen it done in many amps, so I see no problem in that
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Old 12th February 2008, 05:25 PM   #4
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Yes.
There even transistors which have the bias (diode in this case) inside the transistor case.
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Old 12th February 2008, 05:39 PM   #5
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Old 12th February 2008, 07:17 PM   #6
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Douglas Self, in his writings on amplifiers in Electronics World ten years ago has discussed this in some detail. He has found that one gets a quciker, and more dependable response, mounting the bias transistor one of the output transistors, than on the heat sink, as the heat sink has a temperature lag compared to the transistor itself. I have tried it on his amp, and it works extremely well. I would think that commercial builders would stay away from doing it because it involves hand work, rather than the quicker and more economical robotic assembly. Try it--you will like it.
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Old 14th February 2008, 06:04 PM   #7
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Default to my understanding

this is done from some companies when the cooling is not enough and or the design of the amp is case sensitive to thermal run away .....

like if it gets warm any way and this tents to multiply after some point of heating then they need bias to go down quickly to avoid thermal run away ......

lets suppose that you do this and attach the bias transistor over the power transistor ...... you sould try that once with a very efficient heatsink .....and then a gain with a less or terribly eficient hetasink .....

i thing that if you do this and the heatsink you use is terrible your amp will work safelly but sound is going to be crappy since bias might get too low .....

the best will be to have some way to make amplifiers operate in a standard temperature like cars .... the best of performance for my car is 85 degrees celsious ..... ( since the car is old and suffers from various things when the weather is cold this cannot be reached and car works in a lower temperature..... this increases consumption at least 20%.....)

simullar way should apply to amps .....
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Old 14th February 2008, 10:54 PM   #8
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Default Re: to my understanding

Sakis, I have to diagree - your justification is wrong!

The winner is Cornelis Spronk, because the reason for mounting the sensing element directly to the sensed element is that the bias stability (junction temperature / idle current) is better in this case.
See AKSA's amplifier KIT for reference - Hugh Dean might visit the thread and tell us his own justification (namely he using the same approach related to the sensing element).
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Old 15th February 2008, 06:08 AM   #9
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Best discrete solution I hav e found is still to use a small SMD transistor and mount it close to the collector (assuming TO247 packages) lead on the PCB. This gives fastest response and involves no 'hand work'. I use it am am very pleased with the results - its also stable once set up.

This subject comes up every few months . . . would b e nice if we could link all the threads on it.
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Old 15th February 2008, 07:08 AM   #10
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Hi Bonsai,

That sounds really cool. Would you happen to have a pic of this?

Thanks,
David.
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