Guitar Preamp

The challenge – Take a friends childrens guitar (picture below) with a relatively quiet output and beef it up (in a veggie way) to create a neighbour annoying monster!

Jasons Childrens Bontempi Guitar be being Otalgia modifiedMy initial idea was to rip out the existing amp circuit and replace it with a new louder circuit.  This would be a lot of work, so plan 2 was to add a secondary amp that I could piggy back onto the existing circuit.  There was little free space in the back of the guitar to add new electronics, so the new amp circuit would have to be small …

I decided on using a LM386 amp chip (as used in the smokey amp and countless other mini amp circuits).  A simple amp circuit would only require 2 addtional 47uf 16v Capacitors and would be an ideal size.  The chip is also capable of running off 9v, so hopefully it could share the guitars existing battery power supply. I think this amp is a good choice, as if the circuit couldn’t be piggy backed on the existing circuit then it could also be used on its own.

Their are several variants of the LM386 on the market , all with varying wattage and power supply range (I plan to do an article on the 386 range at a later date).

For this project I chose a 1W version and decided I would incorporate a socket into my circuit so that the chip could be changed if required.

Here is the circuit showing the pinouts of the chip (quickly drawn for the purpose of this blog!) –

sketch of 386 amp

You can see from my illustration that pins number 1 and 8 have been connected together.  These 2 pins control gain stage, so connecting them together will ensure the chip has maximum output – lovely!

The circuit needs to be small so I have decided to use veroboard (strip board). Veroboard is board that has rows of holes and horizontal strips of copper and is ideal for quick circuit construction.  To design a compact circuit for the stripboard I have decided to use a great bit of freeware called DIY Layout Creator ( http://www.softpedia.com/get/Others/Home-Education/DIY-Layout-Creator.shtml ).

The result was this –

Layout for LM386 amp by Otalgia

 

 

 

 

 

 

 

 

 

On the layout you can see that there are red squares with circles inside.  These indicate places in the circuit where the copper tracks on the vero board must be removed.  This can be done with a dedicated bradle or by using the end of a drill bit and giving a few twists.

You can also see that a few items share holes in the vero (the capacitors in particular).  This placement is as a rule not recommended, however I did this so that I could keep the circuit size to a minimum.

With the copper track breaks completed, the veroboard is now ready for the components to be soldered to (see pic below).  The picture below shows the copper track side of the board and is a reverse layout to the one above (as the components are mounted on the plain non-copper tracked side of the veroboard.

Unpopulated veroboard for LM386 amp

 

 

 

About 30 minutes later, lots of cursing and a few solder burns and the circuit is built. Note the grey heat shrink tubing on either side, to give the wires a bit of extra support –

 

Assembled 386 amp built by Otalgia FX

 

 

With the circuit built, a quick test of the circuit was set up using lots of crocodile clips and a spare speaker – sounds good!

LM386 test circuit for Otalgia preamp project

 

 

Removing the back panel from the guitar revealed little space to add the new circuit so the old electronics were removed, leaving only the speaker wires, battery connection and pickup wires remaining.

 

Inside wiring of childrens guitar

 

The new amplifier was wired into place and the power turned on….. DISASTER!!!

A piercing shrill noise that sent the dogs running down the street and made grown men cry emitted from the speaker.  Suddenly it all made sense.. why had the guitar been so quiet in the first place … it was now obvious … if you put a pickup up next to a loud amplifier speaker it feeds back …. EPIC FAIL, project over!!!!!!

 


 

 

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