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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!

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  • \$\begingroup\$ Simple as in something you can buy and just have it work or simple as in something you can design and build? I don't recommend messing with mains power as an early project, and the first part is more a shopping question which is frowned upon on Stack Exchange. \$\endgroup\$ – NathanielJPerkins Dec 16 '15 at 8:16
  • \$\begingroup\$ If you can scavenge some laptop power supplies, they are usually around 20V. If you can find 2, maybe the circuit would work OK at +/- 20 V. \$\endgroup\$ – mkeith Dec 16 '15 at 8:31
  • \$\begingroup\$ I wouldn't mind having to build it from a reliable design. I wouldn't know where to begin the design myself. @NathanielJPerkins \$\endgroup\$ – BobbyPhysics Dec 16 '15 at 8:32
  • \$\begingroup\$ @mkeith If I had two laptop power supplies would it be possible to then splice them to require just one outlet? \$\endgroup\$ – BobbyPhysics Dec 16 '15 at 8:35
  • \$\begingroup\$ Electrically possible. Sure. But there is the safety question. Once you cut off plugs and splice wires, you introduce a greater possibility of the user getting shocked (if a bare wire touches the chassis for some reason). \$\endgroup\$ – mkeith Dec 16 '15 at 8:43
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If you can find a 10VAC mains powered transformer, then you can make a simple power supply like this:

schematic

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.

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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:

schematic

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:

schematic

simulate this circuit

Hope this helps.

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