The open-source restim project's Wiki provides extensive technical documentation for building these boxes. For instance, one stimbox detailed on the Wiki uses a TPA3116d2 stereo amplifier, elma TT IZ1892 transformers, and current-limiting resistors, and was thoroughly characterized and measured for optimal performance. These DIY devices are often considered superior to commercial ones because they are purely analog—the output is a direct, amplified copy of the input audio, giving the user total control over the sensation's waveform.
Open Audacity and generate a constant tone (e.g., a sine wave at 1,500 Hz) to act as your carrier wave.
Websites dedicated to hardware estim often host user-generated library sections where hobbyists share custom-made WAV and MP3 tracks.
Disclaimer: This article is for informational purposes only. Always consult a healthcare professional or a licensed physical therapist before beginning any new form of electrical stimulation.