I recently bought a little electric guitar for my daughter. It had a few issues (more on that in a future post), but all in all it’s a decent enough instrument. I wanted to build her a small portable amp to go with it, and this one, based on a TDA7052 audio power amplifier, came recommended: http://www.redcircuits.com/Page96.htm
It’s a very simple, compact circuit, and theoretically you can fit it into a cigarette packet. However, I was happy enough to fit it into a plastic box, which allowed me to use a slightly larger speaker and also not have to worry so much about cramming everything into a tiny space. I also left out J2 (the output jack), since I have no intention of connecting this to a larger amplifier. Here’s the box, speaker, speaker grill (actually, it’s a fan grill from a computer, since I couldn’t find a suitable speaker cover), jack and veroboard.
First we drill (well, bore) a large hole for the speaker in the lid.
Centring the hole:
Boring:
This is how it turned out, still requiring a little work with a file to smooth the edges.
A few more holes for the nuts and bolts, and here is the speaker in place:
Then in the box itself, we need an 11mm hole for the jack socket. For this I use one of my favourite tools – the stepping drill bit. I don’t know how I ever lived without one of these. They’re especially handy for drilling holes in stomp boxes.
OK, now that the speaker and jack socket are fitted, we need to think about the placement of the circuit itself. In this case I’m going to cut the circuit board to fit the box, since there are some very nice little slots to hold it in place already there.
I also cut a section out so that it doesn’t bang up against the speaker when the box is closed.
Test fit:
Now on to the circuit. Here’s the veroboard layout I drew up from the schematic itself (as with all images here, click for a larger version). Note that you have to cut the tracks between pins 1 and 8, pins 2 and 7, and pins 4 and 5 of the op amp, but you should NOT cut the track between pins 3 and 6 (I did originally, just out of habit, and then had to put a jumper wire in to correct the oversight). I've marked where the tracks should be cut with three "x"s.
Here’s a top view of the completed circuit:
And a bottom view (remember, unlike what you see here, you shouldn’t cut the track between pins 3 and 6):
All back in the box:
The battery, incidentally, was held in place with double-sided sticky sponge tape and wedged under the speaker. And here it is all closed up and ready to go:
It’s about as simple as you can get. No volume control (volume is controlled by the guitar volume itself) and no switch (the amp switches on automatically when the jack plug is inserted).
The sound quality is about what one might expect out of such a small speaker (tinny and easily distorted), but for a simple, highly-portable amplifier, it does its job. Battery life is pretty short too, so remember to unplug the guitar cable when not in use (you might want to consider wiring it up to accept a stomp box power supply, as shown here: http://diystrat.blogspot.com/2009/06/buhow-to-wire-up-stomp-box-effects.html). If you'd like to make something with a bit more oomph, while still being very portable, have a look at this post: http://diystrat.blogspot.com/2008/12/chiquita-amp.html.
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