Is there a way I can power this circuit without a +- 15V power supply?

Question

Simple Amplifier Kit

I built this audio amplifier for my circuits lab course. In the lab I had access to a benchtop power supply for the necessary +- 15V. Now I'd like to give the completed amp and speaker to a friend as a Christmas gift, but I need some way of powering it without the power supplies we had in school.

I intend to build it inside a tin lunchbox and I'm considering just using the necessary batteries. However, I'd also like to know if there is a simple way to get the +- 15V I need through a standard 120V wall outlet.

Thanks!

Answer 1

If you can find a 10VAC mains powered transformer, then you can make a simple power supply like this:

^{simulate this circuit – Schematic created using CircuitLab}

Failing that, two 12VDC-output switching adapters will do a fine job, at the cost of some convenience (two plugs have to be put in the wall). Put a diode (eg. 1N4004) across the output of each adapter (reverse biased) so that if just one is plugged in the other one does not see much reverse voltage at the output.

The power supply voltage for your amplifier is not critical, a bit lower does no harm other than lowering the maximum power output (for a given speaker) a bit.

Laptop supplies may not work because some of them have the minus lead common with earth, so connecting them as required will short the outputs.

Answer 2

Get a centre-tapped transformer that gives 30V (or more) between the ends. With the secondary side's centre-tap as your 0V, you'll have +15V and -15V which you can rectify with diodes and smooth with a big capacitor:

^{simulate this circuit – Schematic created using CircuitLab}

You don't NEED anything past the diodes and capacitors, but...

Here, the transistors Q1 and Q3 work with Z1 and Z2, respectively, to regulate output voltage (choose zener voltage about 1V higher than you want your output).

Q2 and Q4 limit the maximum current that you can draw (set by R2 and R4, respectively).

[0.7V ÷ (10Ω) = 70mA]

Other options:

• Use single-sided op-amps (such as the LM386N), so you only need ONE positive power supply.

• Use an AC/DC wall adapter you have laying around. Just make sure it can draw enough current for your circuit, and that the DC voltage that it supplies isn't too much for your op-amps.

• Create a virtual ground using one extra op-amp, like this:

^{simulate this circuit}

Hope this helps.