Built in Guitar Headphone Amp
by Dean Barker in Living > Music
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Built in Guitar Headphone Amp
It's not exactly rock and roll but I like to take an electric guitar with me when I go away in my caravan, but dragging an amp around a caravan is a pain, so the obvious solution is a built in amp based on the popular lm386 power amplifier chip, they are very cheap on eBay (much less than one of the little plug in amps you can buy) and will happily run for ages on a 9v battery, and I found it can also be used as a monitor when plugged in to a "proper" amp
Building the Amp
parts list :-
1x 0.047 uf ceramic capacitor
1x 0.01 uf ceramic capacitor
1x 220 uf electrolytic capacitor
1x 100 uf electrolytic capacitor
2x 10 ohm resistor
1x 8 pin is chip holder
1x lm386 ic chip
1x headphone socket
1x switch
1x red led
1x 5v battery connector
1x perf circuit board
There are loads of different amps based on the lm386 chip I decided to use a modified version of runoffgroves little gem. I am gonna assume that you have some knowledge of electronics, how to read a curcit diagram and how to build a curcit if not there are lots of tutorials online done by people with more experience than me ; )
https://m.instructables.com/id/Instruction-of-Building-a-Simple-Circuit/step2/Find-out-components/ this is a good starting point
The modifications I made to the curcit layout are mainly to reduce the number of parts used to keep the amp as small as possible
1. leaving out the gain pot wired between pins 1 and 8, I didn't want an overdriven sound and wanted to keep the number of holes I had to drill in the scratch plate to a minimum
2. leaving out the output volume pot on pin 5 as you can control the volume and tone with the guitars controls
3. leaving the 110uf capacitor off pin 6 to gnd (+9v input) and using it between pin 7 ( bypass) and gnd I found this helped to stabilise the amp and stop any oscillation
tip; use a full strip on the perf board for the gnd connection (see pic2) as there are several wires that have to be connected to gnd
now you can build the wiring harness
1. solder one of the resistors to the + side of the led and attach this to the output of the switch and solder a wire to the - side of the led and attach this to gnd on the amp
2. solder the + lead of the battery connector to the input side of the switch and solder a wire from the output side of the switch to the 9v input on the amp (pin 6) solder the - side of the battery connector to gnd on the amp
3. solder 2 wires to to the headphone socket attach one to the amp output (pin 5) and the other to gnd
4. solder one wire to the amp audio imput (pin 2) and one to gnd these will be connected to the guitar jack socket
5. now is a good time to test the amp before you install it in the guitar so attach a battery and plug in some headphones, tape the imput wire to the tip of an amp cable and tape the gnd wire to the sleeve of the amp cable plug the cable into a guitar and if everything is working you should be able to play the guitar and hear it through the headphones
tip; after connecting the wires to the amp match them up in pairs ie power, input, output, and sandwich the paired wires between two pieces of electrical tape this makes it less likely that you snag or tangle the wires on anything inside the guitar (pic 8)
1x 0.047 uf ceramic capacitor
1x 0.01 uf ceramic capacitor
1x 220 uf electrolytic capacitor
1x 100 uf electrolytic capacitor
2x 10 ohm resistor
1x 8 pin is chip holder
1x lm386 ic chip
1x headphone socket
1x switch
1x red led
1x 5v battery connector
1x perf circuit board
There are loads of different amps based on the lm386 chip I decided to use a modified version of runoffgroves little gem. I am gonna assume that you have some knowledge of electronics, how to read a curcit diagram and how to build a curcit if not there are lots of tutorials online done by people with more experience than me ; )
https://m.instructables.com/id/Instruction-of-Building-a-Simple-Circuit/step2/Find-out-components/ this is a good starting point
The modifications I made to the curcit layout are mainly to reduce the number of parts used to keep the amp as small as possible
1. leaving out the gain pot wired between pins 1 and 8, I didn't want an overdriven sound and wanted to keep the number of holes I had to drill in the scratch plate to a minimum
2. leaving out the output volume pot on pin 5 as you can control the volume and tone with the guitars controls
3. leaving the 110uf capacitor off pin 6 to gnd (+9v input) and using it between pin 7 ( bypass) and gnd I found this helped to stabilise the amp and stop any oscillation
tip; use a full strip on the perf board for the gnd connection (see pic2) as there are several wires that have to be connected to gnd
now you can build the wiring harness
1. solder one of the resistors to the + side of the led and attach this to the output of the switch and solder a wire to the - side of the led and attach this to gnd on the amp
2. solder the + lead of the battery connector to the input side of the switch and solder a wire from the output side of the switch to the 9v input on the amp (pin 6) solder the - side of the battery connector to gnd on the amp
3. solder 2 wires to to the headphone socket attach one to the amp output (pin 5) and the other to gnd
4. solder one wire to the amp audio imput (pin 2) and one to gnd these will be connected to the guitar jack socket
5. now is a good time to test the amp before you install it in the guitar so attach a battery and plug in some headphones, tape the imput wire to the tip of an amp cable and tape the gnd wire to the sleeve of the amp cable plug the cable into a guitar and if everything is working you should be able to play the guitar and hear it through the headphones
tip; after connecting the wires to the amp match them up in pairs ie power, input, output, and sandwich the paired wires between two pieces of electrical tape this makes it less likely that you snag or tangle the wires on anything inside the guitar (pic 8)
Making a Space for the Battery
this modification is for a guitar with a hole on the back of the guitar for the tremolo mechanism
first turn the guitar over and take off the tremolo cover plate the one with the little holes where you poke the strings through then cover the outside edge of the hole with a good amount of masking tape (pick 2)
next draw around the tremolo cover to make sure that you don't cut outside this Line so everything will be neatly covered then Mark out where you can cut out a recess for the battery and little amplifier ( pic 3 and 4)
next score around the area you are going to remove to avoid cracking the
paintwork and carefully start to remove the wood with a chisel (pic 5 and 6) carefully cut down until the hole is deep enough to accommodate the battery and amp
first turn the guitar over and take off the tremolo cover plate the one with the little holes where you poke the strings through then cover the outside edge of the hole with a good amount of masking tape (pick 2)
next draw around the tremolo cover to make sure that you don't cut outside this Line so everything will be neatly covered then Mark out where you can cut out a recess for the battery and little amplifier ( pic 3 and 4)
next score around the area you are going to remove to avoid cracking the
paintwork and carefully start to remove the wood with a chisel (pic 5 and 6) carefully cut down until the hole is deep enough to accommodate the battery and amp
The Scratch Plate Modifications
this will work on any guitar with a loaded scratch plate like a strat or telecaster I don't own a guitar that doesn't have a scratch plate so I can't say the best place to drill holes for the switch Jack plug ect so I'll have to leave that to your own creativity
you will need to drill holes for the jack plug and for the led and 2 holes that you can cut into a slot for the switch
1. glue the led into the appropriate hole In the scratch plate
2. then glue the switch into place on the scratch plate
3. then fasten headphone socket in the appropriate hole
4. next remove the guitar jack socket (if its not located on the scratch plate pic 2) pass 2 wires through the hole between the the scratch plate and the guitar jack socket cavity in the guitar, solder one to the tip contact and the other to the sleeve contact (pic 3), attach the tip wire to the amp imput (pin 2), and the sleeve wire to ground
5. replace the Jack socket then you can pass the amp and battery connector through the gap between the tremolo mechanism and the body of the to the cavity you have made on the back, carefully replace scratchplate then turn the guitar over and gently pull the amp and battery connector through.
6. now you can fix the amp in the space you cut out on the back of the guitar ( the reason for locating the amp on the rear of the guitar is to keep it away from the magnets in the pickups) I used hot melt glue to hold it in place and fastened a piece of Velcro to the guitar and another to the battery connect the battery press it into place replace the tremolo cover and your good to go :-)
you will need to drill holes for the jack plug and for the led and 2 holes that you can cut into a slot for the switch
1. glue the led into the appropriate hole In the scratch plate
2. then glue the switch into place on the scratch plate
3. then fasten headphone socket in the appropriate hole
4. next remove the guitar jack socket (if its not located on the scratch plate pic 2) pass 2 wires through the hole between the the scratch plate and the guitar jack socket cavity in the guitar, solder one to the tip contact and the other to the sleeve contact (pic 3), attach the tip wire to the amp imput (pin 2), and the sleeve wire to ground
5. replace the Jack socket then you can pass the amp and battery connector through the gap between the tremolo mechanism and the body of the to the cavity you have made on the back, carefully replace scratchplate then turn the guitar over and gently pull the amp and battery connector through.
6. now you can fix the amp in the space you cut out on the back of the guitar ( the reason for locating the amp on the rear of the guitar is to keep it away from the magnets in the pickups) I used hot melt glue to hold it in place and fastened a piece of Velcro to the guitar and another to the battery connect the battery press it into place replace the tremolo cover and your good to go :-)