Hello!
I have drawn a schematic of 2N3904 2N3904. Here is a simple audio amplifier with an npn transistor. What should be the value of voltage source V2? How can I calculte it? output connects to a speaker and input can be pc or mp3 player.
Could you teach me ? I'm new to audio and electronics.

Dude, you're doing it all wrong!

2x6 will be around here soon to tell you 2N3904 should be a 12AE10. In that case, V2=160V.

OK, let's see... (it's been a lot of years so take this with a grain of salt):

1. I don't think this is going to be sufficiently powerful to drive a speaker, at least not with any noticeable volume. The transistor is a low power one, rated for well under a watt. Looks like more of a preamp stage than a speOther limits for the 2N3904 are 200mA current and 625mW power, so let's see...aker amp.

2. The transistor is rated for ~40V max, so that is the absolute highest, but this seems more like a circuit that would run on 12V or less to me.

3. R1 seems to be in an odd place, normally input resistors like that are connected to ground not to a power rail.

EDIT - might have found the original source, or at least another instance of it. In the following link the author is using a 3v cell, which seems reasonable.

http://hackaweek.com/hacks/?p=327

In this article r1 is there because the circuit is being used as a microphone preamp. The author mentions an electret microphone but I didn't think electrets needed a bias voltage from the preamp. Oh well...

Hope this helps. Good luck...

Looks like a cat walked across the keyboard while I was copying and pasting. I don't have a cat though, so must be my fault. Should read:

Looks like more of a preamp stage than a speaker amp.

Another argument for this being a microphone preamp are the values of R3 and C2. R3 is going to limit current to the point where driving a speaker isn't likely to work, and same for C2 at audio frequencies.

If I was going to try to drive a speaker with this kind of circuit I would make the following changes:

- use a battery between 3 and 9V

- remove R1 or (better yet) connect other end to ground instead of power

- change R3 to lower value, between 1K (9V) and 220 ohms (3V)... that's probably still higher than needed but trying to be safe here

- I think R2 needs to be scaled down accordingly, so keep it ~10x the value of R3

- replace C2 with something ~10x larger

- if you don't already have the 2N3904, consider getting a higher power transistor instead

Note that the real answer here would be to go with at least a two transistor circuit but assuming that experimentation & learning is one of the goals here (in which case starting with 1 transistor is good).

Circuit ignition, enabled by V2 connected to 120VAC, will certainly provide more enjoyment than listening to any audio through this amp.