Diy mini amplifier for guitar

miniamp with othersThe easiest way to build a mini amplifier is to modify a powered PC speaker by adding a simple preamplifier. And you can do this very cheaply. I bought my speaker set from a flea market for two euros!

Small size was my main priority but the box should have room for the preamplifier. Itīs also important that the box contains all electronics (mains transformer and amplifier board). Some sets have the transformer in the other speaker box.

I wanted an amplifier at my workbench. It would be handy when working with guitar electronics and this little box seems to do the job better than I expected. And it doesnīt take much space on the shelf.

schematic diagramThis preamp circuit is probably the simpliest you can find. Transistor 2N3904 is very common and easily available but many others would do as well (just check the pinout).

Emitter resistor R3 controls the gain, smaller value would give you more gain but I was happy with 270 ohms.

Smoothing capacitor C3 is here because it is a good practice to have one. You could probably leave that out to save space if thatīs an issue.

A small circuit like this is easy to assemble on a piece of veroboard (stripboard). I didnīt have any so I placed all components on a piece of plastic and made the connections backside "point to point" style.

veroboard layoutI planned this layout anyway just in case someone might need one. Note that I havenīt actually tried this - hopefully I didnīt make any mistakes!

preamp boardYou have to attach the preamp somewhere. I simply bolted the board to input jack terminal. Stiff ground wire supports the board at other corner.

I used speakerīs fixed input cable to connect my preamp to the main board (shielded grey cable at right). Since the preamp gets ground connection via that cable I only needed a positive supply voltage somewhere (red wire).

Disconnect the speaker from mains before opening the box! I have done that mistake a couple of times in the past - itīs not nice to stick your fingers to 230 volts AC, believe me!

main circuit boardThe best place to find supply voltage is somewhere near the power on led at the main circuit board. Normally one led terminal is at ground and other is connected to supply voltage via a series resistor. That resistor should be easy to find if you follow the traces from the led.

Easy job in this case. Perhaps you can also see how the supply voltage goes to power switch terminals - another good hint.

To be sure I wanted to measure the voltage before making any connections. Mains transformer and other dangerous things were in a safe place inside the box so it was ok to power up the system in this condition. I measured the voltage between the "upper" end of the resistor marked in the picture and the - terminal of the speaker (it is usually connected to ground). + 12,6 V, perfect!. Anything from 6 to 15 volts should be ok.

Next I pulled off the mains plug (!) and started to put the thing together.

finished amplifierI installed the input jack (with the preamp) on top for two reasons. First, there was room for the preamp and second, it is possible to connect the guitar cable with one hand without turning the amp over.

Finally I enhanced the looks by tearing off ugly foam that was hiding a very professional looking speaker grille. I also added a Yamaha logo that was laying around.

So, how does it sound? Well, not bad at all considering the starting point. When connected straight to a guitar it certainly works for my original purpose. To find out how it would do as a practice amp I put it behind my pedalboard. I started to go through factory presets of my old Digitech RP50 multieffect. So far Iīve found those to be mostly useless with a "real" amp but this time most presets sounded pretty decent, especially more acoustic/ambient effects. Overdrive sounds were mostly ok but anything beyond AC/DC started to sound crappy. You donīt want to play death metal with this amp.

The quality of the original speaker defines the end result, of course. A preamp doesnīt make it sound any better. This little thing cannot replace a real guitar amplifier but I think it would be good enough for low volume practicing without disturbing other family members. A headphones output might be useful to somebody, mine does not have one.

Value for money is hard to beat! Thereīs a lot of amp for two euros!

ĐJukka Korppi 2015
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