ISU Audio and Arduino Clubs

Altoids amp project

The Altoids amp is the Audio-Club version of a headphone amplifier. Inserted between the output of an audio source — a smart phone or a computer — and the input to a set of wired headphones, the amp boosts the signal to the headphones The extra amplification will certainly make your headphones louder at a given input level. The extra loudness may make the audio "sound better". It also reduces the power drawn from a smartphone, which might extend the phone's battery life a bit. The circuit is quite small and tucks comfortably into an Altoids mint tin.

Do you need a headphone amp?

Probably not. The output amplifiers in smart phones and computers provide decent gain, are fairly efficient, and can drive standard wired headphones well enough. A headphone amp will certainly increase the voltage applied to the headphones. This may be useful if your headphones are not very sensitive and require higher input voltages to achieve reasonable audio output levels. However, most people use wireless headphones these days, and a wired headphone amp has no purpose in a wireless application.

But you should build a headphone amp, anyway.

The reason is simple — if you are interested in learning to design and build electronics, particularly audio electronics, the Altoids amp is an excellent way to start. The amplifier circuitry is simple — two non-inverting amplifiers right out of EE 201. Add input protection filters, a couple of batteries with smoothing capacitors, and you've got a functioning stereo amp suitable for use with headphones. The low component count and the simple PCB make this a great project for learning how to solder. The circuit is simple enough that if something does go wrong, it should be easy to debug. And debugging is an essential electronics skill.

Features/characteristics of the Altoids amp

• It is a stereo amp — there are two identical circuits providing amplification to the left and right channels.
• Power is provided by a pair of 9-V batteries.
• There is an on-off switch to connect/disconnect the battery power to the amps.
• Two light-emitting diodes indicate that the batteries are providing power.
• Audio input and output connections are made via 3.5-mm TRS connectors.
• There is no volume control on the amp. We assume that the amp will be connected to a smart phone or computer, and those provide perfectly adequate controls for adjusting volume.

More to read

• Circuit schematics & description.
• KiCad schematic & layout.
• Bill of Materials
• Step-by-step build instructions (PDF).
• Some Questions and answers about the Altoids amp.
• Test results.

External links & info

• Headphone amplifier on Wikipedia.
• Original CMOY design. A memorial to Chu Moy, who died in 2016. (This is a partial archive of Moy's Headwize web site.) If you search for CMOY amp, you will find dozens of different version of the little amp.
• TangentSoft's version of the CMOY. Lots of good discussion here.
• Headphone amps from Crutchfield. Some outrageous prices here.
• Headphone amps available on Amazon.
• Here's a build-it-yourself kit for $379. It certainly looks very nice with a volume control and all. It might sound better than our Altoids amp. Or it might not.