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[Reply #15 on:]

That's a rather Lo-Fi approch, but with square wave input who cares, it will make noise. 

Transistor is good noise filter.

[Reply #16 on:]

Not really.  In fact it is more often the opposite.

In order to switch a bipolar transistor, some amount of current required, isn't it?

[Reply #17 on:]

Yes. And every active component (and many passive components) are all sources of noise and/or distortion.

Sounds a bit paranoidal. 
"This is my HiFiduino, baby." (c)

[Reply #18 on:]

Okay I built the circuit, and the wonderful smell of magic smoke started to come out both bjts after decreasing resistance in the pot to 0 (I had to replace both). By the way, I kept the 4k and 1k voltage divider in the circuit. How do I calculate the power dissaption of the bjts in this type of circuit?

[Reply #19 on:]

Okay, here is a more appropriate circuit.

The MOSFET I used is IRF3707Z, any higher current logic level MOSFTETwill work, such as the sparkfun sells.
The speaker is from mpa.com
http://www.mpja.com/prodinfo.asp?number=14618+SP
90dB. I use it in a fencing club to make a warble when a touch is scored, can be heard very well.

The current thru your BJT is (Vsource - Vce)/(sum of resistance in the rest of the path)

So (5V-0.5v)/(8ohm+pot)

[Reply #20 on:]

I think I figured it out, since with 0 ohms at the pot, the voltage output of the opamp would be 1V, which means 12 - 1 = 11V must be dropped across the bjt. 1V/8ohms = 0.125 A ----> power disipation of bjt = 11V*0.125A = 1.375W > 0.6 W max of the transistor. Is this correct?

[Reply #21 on:]

Which circuit do you have wired up? There have been several posted.

[Reply #22 on:]

This one:

[Reply #23 on:]

Yes, for full on (12V - Vce)/8 = ~1.44 amp.
P=I^2R = 16W goin thru the speaker
P= IV = ~0.5*1.44 = 720mW dissipating in the transistor.

2N4401 absolute max 600mA - 1.44A would smoke 'em!