The Smokey Headphone Amp

I'll start by admitting that this is not my original design, but it's actually based on CMoy's Headphone Amp, as well asNwAvGuy's O2 Headphone Amp. It's a bit of a mix, taking the output stage of the O2 Amp with the sheer simplicity of the CMoy Amp, but adding an extra 6 dB amplification.

Goes like this. I wanted to try a headphone amp. My colleague at work eats and shares his mints with me, so I thought, "Give me one of the empty boxes, I'll build an amp out of it". So the box lay on my desk for ages, and I realised

a) it was too small

b) it would fit only one 9V battery

c) I'd have to do a surface mount design

Long time did the box lay on my desk without me doing anything about it, reminding me, reminding me of my unfinished project, until one day, I got as a present a cigar metal box from my sister. After I moved the cigars to another box, I suddenly had an empty metal box, bigger than the mints box, and good enough to fit two 9V batteries and some electronics. I could finally begin.

First I went and got the peripheral parts. On/off switch, volume pot (didn't use it in the end), two stereo headphone jacks and an LED. I put all inside the box, batteries, peripherals, and measured how much space left I had for the circuit board (3cm x 6cm).

I designed the circuit board as usual with Kicad, a very simple two channel amp using the (in?)famous OPA2134, a split power supply and a status LED (on/off). The power supply are two 9V batteries after a switch (not seen on the schematic diagram) and the OPA2134 could be exchanged for other pin compatible models (e.g.: NJM4556, as per the original O2 Amp).

Below is a view of the front (left) and the back (right) of the circuit board layout:

The connection diagrams, without and with potentiometer option.

And the finished circuit board with components soldered.

Some more views during the construction process. All components are there, all peripherals soldered to the board. You'll see there's a pot between the input jack and the input. In the end I did not use the pot, after a few tests I noticed it was crackling a bit and it was noisy when it was rotated.

Working on the case. It was soft metal, it took me about an hour or so to finish all the holes.

The board is attached to the case using double sided tape. Another thing, to avoid any unwanted shorts, the inside of the case is padded with transparent sticky tape.

The first test with the board ready. No smoke coming out! And the temperature on the op-amps is not even warm.

The final headphone amp in it's enclosure.

The sound of this little device is great! I find it very transparent, and as a plus, it's really really silent (that is, when there's nothing connected to it or the input is nothing, you really hear nothing on the headphones). It does not generate any noticeable heat. There is a slight pop when you turn on the amp, as well as when you turn it off. As it does not have a volume control, you have to be very careful when you plug in your source and let it rip, always remember to check the volume on your source.

Links

The O2 Headphone Amp by NwAvGuy here.

The CMoy Headphone Amp here.
The complete Kicad project here.