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Why is it so hard to answer this question. (Which transistor to select).

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chinoy:
Im a bit of a two stroke engine nut and run a forum where I answer peoples complex questions in simple english.

Ive had this one question nagging me ever since I took up audio. Ive tried it on various forums maybe even here. But never got a simple answer.
I know the answer is not simple. But if you understand a subject part of that is being able to explain complex questions with simple english.

Here is my question.
I have a bag full for transistors. Or the person who has designed the amplifier has given me a list of transistors that can be used.
How do I select the best one.

My application is music.
my objective is to have the best power output efficiency and most of all lowest noise and distortion.
How do I select from a bunch of options.
Say circuit calls for TO92 NPN
I have 10 options with me. Which datasheet parameter should I be looking at. Or what can I measure other than THD and Noise to see which one works better. ?.
Or like some say it really does not matter.

peter-h:
It does matter, within a certain broad band, but you need to know quite a bit about electronics, so a quick forum answer is not possible. If you post a circuit diagram, someone may be able to help.

Wallace Gasiewicz:
Make sure that the transistor has the appropriate voltage rating and current rating for the job you want it to do. There are big variations in the current handling and voltage of TO92 package transistors., some applications even require a heat sink cap.

You did not say if you are replacing a transistor on a board or developing a new circuit. If you are replacing a transistor the pinout should be the same, it will be easier to put it in.If you are replacing a transistor you could put in a transistor socket and try different transistors easily.Chose a transistor made for audio, Usually low freq transistors will work better in audio since this is what they are used for.
There are plenty of comments about this topic on audio forums.  This NPN  BJT :  2SC1815SL   is an example of low noise audio transistor.
https://html.alldatasheet.com/html-pdf/110924/TOSHIBA/2SC1815L/295/1/2SC1815L.html    You can look at the noise figure in the data sheet.
The complementary PNP is 2SA1015L
The   "L"   is for low noise.
The transistors I mentioned are old transistors and maybe some newer surface mount transistors would be better. Another problem is that there are a lot of purposely falsely labeled transistor out there, That is another subject.
 

coromonadalix:
Transistors are more than 3 legs

Silllicuim, Gemanium, Mosfets, bjt ujt ...
gains / HFE
voltages
capacitances in some cases
leakage
avalanche 

Simple ones, darlington  etc ...  Matching pairs

List goes on and on,  in some projects, simple substitution can work, in other cases they uses caracteristics ...

I have some projects who can only use one transistor kind, no substitutions is found at the moment


You will never get an simple answer

If it was so simple,  you would not need tons of brands, tons of part id's  etc ... 2sa 2sb 2sc 2sd  2n  mje 2sk 2sj  ......

You try to find the part who will make everything possible, NOT 

Designs take many factors into consideration, calculations, tryouts and errors

You change one part and everything may become messier, bad,  less good in specs, noisier etc ...


Audio is not Video, Data is not this or that    etc ...  applications depends of the needed use,  vice versa   ...



Zero999:
Your question is vague and ill-defined, so you won't get a good answer.

There are three different types of transistor normally used in audio amplifiers:
BJT
J-FET
MOSFET

These can then be split up into NPN and PNP and N-channel/P-channel for J-FETs and MOSFETs.

Then there are the different voltage, current and power ratings. J-FETs are purely used for low level signals, BJTs exist in both power and signal types and MOSFETs do come in low power variants, but are mostly high power devices, used in class-D amplifiers nowadays.

You won't get any better answers, without a schematic or photograph of the device in question.

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