Overview

The Pocket Amp is a small guitar amplifier built into an Altoids tin. It is powered by a 9V alkaline battery and uses a basic LM386 audio amplifier with a small 50 mm speaker. The goal was to create an affordable holiday gift that could also be refined into a small production or wholesale product.

Version 1

This version used a unique amplifier design. It used two LM386 amplifiers in a push-pull configuration with the goal of increasing wattage and volume. However, the 9V battery did not provide enough headroom for that approach to be worthwhile.

A rough schematic of the Version 1 amplifier.

The speaker was a high-end 50 mm subwoofer designed for use in headsets and other small electronics. It was a good speaker for this application. The speaker and the rest of the amplifier stayed consistent through the later versions and have not needed to be changed.

Version 2

The second version of the amplifier removed the second LM386. It seemed unnecessary for the goals of the project and caused more issues than it solved. For this version, I followed the LM386 documentation more closely. The biggest change in this version was the addition of a static equalizer.

The equalizer is made from a combination of a low-pass and high-pass filter to scoop out the mids and create a more pleasing tone. Below are some frequency response charts created through a SPICE simulation.

This amplifier also included gain adjustment. Using a trim pot between the two gain pins on the LM386, I was able to set the gain to the desired level. Below are some simulations showing how the potentiometer behaved.

The main flaw with this version was the removal of the buffer circuit. This made the input impedance too low, which negatively affected the guitar’s frequency response and caused poor audio quality. The third version is planned to solve these issues and add more useful features.

An inside view of the second version amplifier.

Here is a short audio demo of the Version 2 Pocket Amp being played.

Version 3

This version is still a work in progress. The new amplifier is similar to Version 2, but it adds the buffer circuit that was missing from the previous version. It also includes an adjustable voicing circuit in place of the static equalizer. In addition, it adds a bright switch in the tone circuit for even more tone control.

Here are some more photos of the latest version. As of writing this, it is close to being completed and finalized as the finished design.

Here is a frequency sweep simulation of the voicing circuit at different knob positions. You can see the peak frequency move depending on where the knob is positioned.

Here is the PCB layout. I had to do a lot to take advantage of all the space available, which meant making a uniquely shaped PCB.

Here is a 3D model of the PCB.