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Simple Symetrical Amplifier
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Old 9th February 2012, 09:56 AM   #1901
guitar89 is offline guitar89  Malaysia
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LC, I think I will get those when i get a chance, but i have some bad news of mine.

Maybe is due to poor electricity, when I close one of my house ballast light, the voltage went somewhere else, and my fuse trip. well... i thought it protected my device, but no.... something went burn and I don't know where.... (replaced fuse and still trip, only happen to one channel)

So now i would upgrade my power section first before i proceed.... don't want the same thing happens (I just use unregulated dual supply consist of rectifier and capacitor)
Any suggestion for me regarding this ? Have not much knowledge on power supply.... especially destroy my another amplifier..... (protection against small main voltage fluctuation)
About the amplifier, i can't find the fault so I will desolder it and examine later....

Extra question : I have some spare BD139/BD140 but they are from different manufacturer so their attribute could be very different. So I would ike to know what should do to determine is it "complementary" enough for use in amplifier ?
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Old 9th February 2012, 03:46 PM   #1902
Salas is offline Salas  Greece
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I found original Philips ones (grey body) matching well between NPN/PNP and faster than ST and ON.
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Old 10th February 2012, 03:12 AM   #1903
guitar89 is offline guitar89  Malaysia
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My combination is quite weird.....
BD139 : BD140 : I was wondering Could they really "complement"
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Old 10th February 2012, 03:26 AM   #1904
Salas is offline Salas  Greece
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The multicomp one looks like Philips to me, the ST has a bigger die hence lower Ft. To really know two BJTs are following is curve tracing them but no NPN and PNP are ever perfect mirror images anyway. See to have a pair of near hfe and Cob from the same maker is the practical thing. If you can get the multicomp BD139 also it will be OK. What variety is it? -16? In Philips I got many near hfe ones from both sexes in -16.
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Old 10th February 2012, 03:37 AM   #1905
guitar89 is offline guitar89  Malaysia
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no idea.... The web-dealer i normally brought from don't have other high FT (>100MHz) that is more than 50 Vce... with suitable for heatsink places... any idea how to know those 2 transistor's ft ?
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Old 10th February 2012, 03:50 AM   #1906
Salas is offline Salas  Greece
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The easiest way I ever seen, don't know if best.
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File Type: gif MILSPEC_Ft.gif (16.8 KB, 562 views)
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Old 10th February 2012, 07:20 AM   #1907
Esperado is offline Esperado  France
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Quote:
Originally Posted by mikelm View Post
Could you tell us what the current is above which you cannot notice improvement - thx
The answer had still be given previously in this thread: ~ 150mA per device for FETs, 300 for BJT (but i have less experienced those concerning this point and it depend there more on the open loop bandwidth of the amp).
Of course, assuming you feed them with a GOOD PSU.
The best way is to make (objectively) your own testings.
Concerning temp, the best seems, on my experience, around 40 -> 50, not less.
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Old 10th February 2012, 07:39 AM   #1908
guitar89 is offline guitar89  Malaysia
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LC, regarding VAS, i think i couldn't get those good BJT needed from my dealer. So I think another alternative is using MOSFET, BIBGT, etc. I personally think MOSFET is good for me, since i frequently getting VAS thermal problem, so i think it's thermal property would suitable for me (increase in resistant when heated)

Need alot of advise on this since i don't know them at all...... need information such as criteria of choosing for VAS, modifying circuit connection, precaution using them, etc.

As in 2SA1707/4487, i'm thinking of using them as driver/predriver, is it appropriate ? or excessive ?

Last edited by guitar89; 10th February 2012 at 07:42 AM.
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Old 10th February 2012, 09:14 AM   #1909
dadod is online now dadod  Croatia
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Originally Posted by dadod View Post
It is not quiescent current what is important in case of BJT OPS but the voltage between emitter resistors. Look in Audio Power Amplifier Design Hanbook by D. Self.
Maybe that was not clear enough. Barny Oliver optimum point is 26mV across one emittor resistor(plus re of the transistor) to set quiescent current. If set to high you get more A class power but after that there is more distortion. 300mA is way to much.
dado
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Old 10th February 2012, 10:05 AM   #1910
Nico Ras is offline Nico Ras  South Africa
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I read some years ago that John Curl also recommends 25mV across Re.
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