copyright 2001, D. Glenn Arthur Jr.
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[photo of my battery-powered guitar amp]

Design for a portable guitar amplifier

Comments about my amp

Over the years I've had lots of people ask me about the battery-powered guitar amp I carry. It's a homebrew conglomeration of off-the-shelf parts -- you can't buy one like it already assembled, but you won't need a soldering iron to put one together. I'm talking about simply buying the right parts and plugging them together here. Easy project. When I get a chance to put some graphics up I'll include a schematic and a drawing. In the meantime, there's a description and instructions below.

There are complete amplifiers available, of course. There's the Pignose, which I hadn't heard of when I put mine together; and tiny little Marshall and Fender amps (designed to look like cute minitatures of their real ones), which only came out recently. The Pignose is a useful little amp. The Marshall is useless for anything except practicing alone. I haven't tried the Fender, but it looks like the same sort of thing as the Marshall.

My amp is far from perfect, but it's loud enough for jamming along with other instruments except for drums, accompanying an unamplified singer, or playing as I walk down the street.

The Parts

As I mentioned above, you won't need a soldering iron for this project. The basic parts list is: Additional parts, if you want to build a slightly more powerful version are:

How to wire it up

The simple version

[hookup diagram]
  1. Take the 1/8-inch stereo phone plug attatched to the extension speakers, and plug it into the stereo-to-mono adaptor.
  2. Take the mono end of the adaptor and plug it into the "Ext. Spkr." jack on the small, grey amplifier.
  3. Take one end of the 1/4-inch patch cord and plug it into the 1/4-inch to 1/8-inch adaptor.
  4. Plug the small end of the 1/4-1/8 adaptor into the "Input" jack on the small, grey amplifier.
  5. Install batteries. Stuff everything into its carrying case, leaving the end of the patch cord hanging out (so you can plug it into your guitar), and you're all set.

The fancier version

  1. Plug the extension speakers into the "gender changer".
  2. Plug the two-mono-to-one-stereo adaptor into the other end of the gender changer.
  3. Connect the "Ext. Spkr." jack on each project amplifier to one of the jacks on the adaptor in the previous step, using the 1/8-inch patch cords.
  4. Plug the male ends of the 1/8-inch Y-adaptor into the "Input" jacks of the project amplifiers.
  5. Plug the 1/4-to-1/8 adaptor into the Y-adaptor.
  6. Plug one end of the 1/4-inch patch cord into the 1/4-to-1/8 adaptor.
  7. Install batteries and stuff everything into its carrying case.
[photo of amp open to show the components]


Turn on the small, grey amp (which I'll henceforth call the "pre-amp") all the way. If you built the dual-pre-amp version, turn on either or both of the pre-amps. The volume control on the pre-amp will be a little awkward to reach when everything's in place, so it's best not to have to fiddle with it, and I seem to have more control over distortion if it's all the way up. Use the volume knob on your guitar for your main volume control.

You'll notice that as you get louder, you start to get distortion. If you want to play louder with a clean tone, turn on the extension speakers and turn down the volume on the guitar. If you want to play at maximum volume (extension speakers on and guitar all the way up) you'll only have the distorted tone available, but indoors, in a quiet or moderately noisy room, you should be able to play at a useful volume either clean or distorted. On a city street, you'll probably have to turn everything up all the way.

As the 9V battery in the pre-amp starts to fade, distortion will show up at lower and lower volumes, so if you're trying for a clean sound and can't get it, change the battery in the pre-amp.

If you built the dual-pre-amp version, you can start by turning on only one pre-amp, saving the other for when the battery dies in the first; or you can turn on both pre-amps for a littel more volume. Each pre-amp goes to a different speaker, by the way.

Technical bits

The pre-amps are needed because the extension speakers are expecting headphone-level signal going into them, and an electric guitar doesn't produce enough power. So the pre-amps amplify it to speaker level, but by themselves they sound pretty crappy. Using the extension speakers simply as better speakers does a lot to improve the sound, and using powered extension speakers means that when we want a little more volume we can turn them on for a second stage of amplification.

The Radio Shack pre-amps use an LM386 op-amp chip. It's not quite hi-fidelity, but for a basic instrument pre-amp it's not bad. The distortion comes from overdriving the pre-amp itself, by the way, not from the pre-amp overdriving the extension speakers.

With such tiny speakers, you won't get a heck of a lot of bass, unfortunately. That's the price of portabiity. This design works pretty well for a guitar, but plugging a bass into it results in more mud than music.

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