DIY Portable Headphone Amp / Line Mixer

Headphone Amp / Line Mixer

Completed August 31, 2008

Main features:

Single 1/4" stereo input, presumably for headphones but could be something else
Single 1/4" mono input, presumably line out from instrument amplifier but could be something else
Summing amplifier with fixed gain ~= 3
Stereo input has level control (attenuator)
Mono input has level, gain, and pan controls
Low output impedance, for headphone speakers
Miniplug output jack
Single 9V battery powers a MAX1044 charge pump, which provides +9V and -9V for op amps

I made this because I couldn't find a commercially-manufactured product that did what this one does for under 50 bucks and runs on a battery. The total cost of materials for this was about $30.

Schematic and board layout in Eagle CAD. I got the ideas for the circuit from various internet and textbook sources, then adapted it to fit my needs. Eagle provides a board layout based on the schematic, but the positioning has to be done manually. Eagle has an 'Autorouter' feature that I've never used so I can't say much about that.

Circuit board pattern printed to Press-'n-Peel Blue paper on a laser printer.

My trusty Shark, which I used to iron the toner from the paper to a copper board.

Ironing complete. Toner is now on the copper board. Board is ready for etching.

In the bowl is ferric chloride, which will eat away the copper on the board that isn't covered by toner. Ferric chloride stains just about everything, and is poisonous so don't drink it or get it in your eyes. Notice I've got some on my hand, which I shouldn't - the stuff I get is usually heavily diluted, but I should be wearning gloves.

The board is now immersed in ferric chloride. I usually agitate the board every few minutes and just let it sit inbetween (copper face-down).

Etching done. The only copper left on the board is underneath the toner.

After rubbing the toner off with paint thinner.

Board sawed, edge-sanded, and ready for drilling. I use a Dremel mounted on a Dremel drill press. Since these boards are fiberglass, I wear a respirator and goggles when sawing, sanding, or drilling them.

To saw the boards, I usually use a ryoba or dozuki saw.

Board populated and soldered, with test output jack (final version will a stero miniplug).

Closeup of the board. The chips are four TL072 op amps (for processing audio signals) and a Maxim-IC MAX1044 charge pump (for providing a negative voltage bias to the op amps). Audio connectors are Neutrik PC-mount. Resistors are 1% tolerance, except for those brown ones which I think are 5%.

Closeup of the underside of the board. My soldering technique is improving. The solder here is lead-free. Later, I covered the board with conformal coating, which prevents the bare copper from oxidizing.

Circuit board, pots, switch, LED, and battery mounted inside a Hammond 1590BB enclosure. I probably should have mounted the board trace side up to provide more room for the pots, but in this case, I wanted to be able to see the whole mousetrap in case I had to tweak something. I drilled the holes in the box with my trusty Irwin multiple-gauge drill bit, which fits a standard power drill.

Completed prototype of my first portable headphone amp / line mixer. The photo doesn't show this, but the enclosure is a sort of metal-flake green. The lettering is this metal-paint ink stuff I got at an art store. The on-off switch is kind of retro but the unit does what it's supposed to and sounds decent. So far, my only beefs with my little creation are:

The pan control isn't sensitive enough - it pretty much gives you left, center, or right. I'll fix that in a future revision.
The knobs don't feel right. I'll replace them with narrower ones when I can get some.
I'm wondering if the output jack should be mounted on the top instead of on the side.

Otherwise, design and construction seem to be solid.

Another view of the completed prototype.