basic DIY amp for pc speakers (no ICs)

[spotty5000]

Hi, I'm looking to rig up a circuit to connect a little 4 ohm, 3W speaker to my pc. I've looked at various schematics online but they all involve IC amps. Is it possible to power this speaker with a circuit composed of just resistors, caps, and transistors?
My parts supply is limited (or I'd just go buy a cheap pair of speakers) but I do have the following:
all kinds of resistors
capacitors ranging from 1uf to 1000uf
a handful of NPN and PNP transistors
the aforementioned speakers.

basically I salvaged all the parts from a couple clock/radios and an old keyboard. There was an LA4550 in the keyboard to power the speakers but I failed to desolder it and now it's probably junk. So, can I replace it with some combination of discrete components?

[dangus]

You could build a discrete transistor amplifier, but designing one from scratch will be very educational.

However, it's easy enough to salvage an amp from a scrapped TV, and CRT TVs are not worth much these days. Which is presumably why I keep seeing the crushed or shot-up remains of them dumped in the woods or patches of urban wasteland. Look for a peculiar chip mounted on edge, near where the speaker wires plug onto the board (or headphone jack). When I found the remains of a Sony TV out in the woods, I was able to break off the section of board that had the audio amp with my bare hands (phenolic pcb stock is not very strong). Googling the chip number and board designator turned up the Sony service manual with board overlay and schematic, although it wouldn't be hard to trace the wiring without it. Just find a data sheet for the amp chip. The speakers may be worth salvaging, if they're not too badly damaged by rain (and in a moderately expensive TV like a 27" from the 1990s). I suspect that in a proper box (not something made of flimsy plastic) they may sound quite tolerable, and they're shielded.

[jaycee]

You could probably build a circuit like the following: Simple 60 Watt Power Amplifier

This is for 60W but you could easily scale it down. You could remove Q4, Q6, R10 and R11, and move R9 to the emitter side of Q5. Q2 could become a BC556 or similar. You might be able to use BC639/640 or similar for Q3/Q5. 1000uF should do for the output capacitor C5. You may need to change other resistor values to suit a much lower voltage.

However this gets a lot easier if you can get a dual opamp. Even a 1458 or 4558 would work if quality doesn't matter too much. All you need to add then is a pair of transistors such as BC639/640 or BD139/140 to act as current gain. This is quite often done for headphone use. See this for example: Headphone Amplifier

[Bigun]

It sounds fun trying to make something with what you have and this forum is full of folks who like to make things out of discrete parts instead of ic's.

You don't need much power, 60W would be rediculous in my opinion.

So, what kind of power supply do you have ? - is it single rail, i.e. only a positive voltage and ground ?

What kind of transistors do you have - they are probably rather low power so we may need to augment your junk box ?

[paulb]

Maybe something like this:
Mini-box 2W Amplifier - RED - Page33
There are others at the same site