As the name suggests, an amplifier is a device that increases the amplitude (the power) of an electrical signal. Amplifiers can be used for any kind of electrical signal, but audio amplifiers are the most well-known. As an example, the audio output from an electric guitar is very faint, because that signal is produced entirely passively by the electromagnetic pickups. To push that up to a respectable volume, an audio amplifier is necessary. If your headphone volume coming from your smartphone is too quiet, GreatScott! explains how you do the same thing by building a tiny amplifier for headphones.
There is already a small amplifier built into your smartphone to drive headphones, but that can be fairly weak on some models. If you’re listening to an audio file that isn’t very loud, like an audio book, then you may have trouble hearing it clearly. This mini amplifier solves that problem by further boosting the amplitude of a smartphone’s audio output before passing it along to the headphones. There are many types of amplifiers out there and the most common is the Class D, which is popular because it is affordable and very efficient. GreatScott!’s design is a Class A amplifier, which is extremely inefficient but which offers great sound quality. It can also be built with just a few common components.
Those components include two BJTs (Bipolar Junction Transistors), resistors, capacitors, a USB-C breakout board, an audio jack, an audio plug, and a perfboard for connecting everything. The transistors are the components that are actually boosting the audio signal’s amplitude. The signal is fed into a transistor, which is increased in power by 5V going into the collector. Because your headphones are stereo, you need two transistors: one for the left channel and a second for the right channel. That 5V comes from your smartphone itself from the USB-C connector. If your phone uses another type of port, like Lightning or micro USB, then you’ll need to use the appropriate plug to pull the 5V. Those components are then soldered onto a perfboard that can be housed within a simple 3D-printed enclosure. Class A amplifiers may have very poor efficiency, but that isn’t a big deal at this small of a scale. As GreatScott! demonstrates, this does a great job of increasing the headphone volume.