# Can I convert a mini guitar-amp into a portable auxiliary speaker?

Being a guitar player, my brother got me one of those guitar shirts for Christmas that has a board with pressure sensors attached to it and an actual miniature amplifier. When the sensors on the board are being pressed, a signal is sent to the circuit within the little amp and a guitar sound is played through the speaker. It also has a three-pin volume pot and apparently some kind of digital tone control. I've attached photos of the amp and the circuit down below.

Of course, the sensors don't really work perfectly and while it was a nice gift and we had some fun with it, I don't see an actual use for it. However, I do see a use for the little amp, since my laptop's speakers don't work anymore and I've considered buying a portable speaker of that size to play sound when I need to (which rarely happens, but it would be helpful for school presentations and such).

Would it be possible to either unsolder the front output jack and solder it back somewhere into the circuit as an auxiliary input to connect a laptop's headphone out to the amp (or connect an additional mini audio jack in that way)?

I would like to "tap" into the existing circuit to use it this way:

Audio Out - Audio Cable - Amp Input - Amplifier/Volume Control - Speaker


Unfortunately, I don't know where the amplifier circuit begins and where I would have to connect the tip and sleeve of an audio input jack.

Can someone here see if and how this could be possible?

Photos:

Front side: Power switch, power LED, output jack, tone pot (6-pin), volume pot (3-pin).

Circuit: Orange, green and blue cables connect to power switch, white to the speaker, red and black to the battery (4x AAA 1,5V in series), the 8 pins on the bottom left connect to the board with the sensors and would not be needed anymore.

Circuit, other side: Self-explanatory, front pots and output jack visible, the second pair of center red and black wires go to the LED.

UPDATE:

With Jasen's help, I attached a 3.5mm audio cable to the amp for testing. Unfortunately, I wasn't able to get any sound out of it, so far.

I've connected both signal wires with 10k Ohm resistors in between to the first pin of the volume pot and a ground wire to the output jack's sleeve connection on the amp. All connections make contact (I tested them with a multimeter). At first, I didn't want to cut the original trace on the PCB to the pot, so I just cut the corresponding pin on the pot itself and bent it forward, so that it didn't have a connection to the PCB anymore.

When I turned the amp on afterwards, I just heard a silent hissing sound and no input from my phone which I used for testing. I thought that I had to cut the old trace anyway and cut a small nick into the side of the PCB so that just the outer trace was damaged. Then, I got no sound from the speaker at all anymore. As far as I can tell by measuring the other connections, no other traces on the PCB were damaged.

I also tried to shorten the wires from the audio cable to the pot directly without the resistors, because I thought they may have been rated too high, but that resulted in no change.

Did I do something wrong or is there another way to get sound into the amp?

I've attached another photo of how the circuit looks now down below:

• it's really had to guess, but I suspectthat this "amp" also has the modulating circuitry that makes the electric guitar feedback sounds and that the shirt only acts as a kind of joystick thing., – Jasen Jan 3 '19 at 0:25
• This is exactly what I suspect as well, I may have formulated a bit unfortunate. To make it clear, the shirt is just a shirt. The "controller" board with the pressure sensors that looks a bit like a guitar fingerboard attaches to it with velcro. There are no chips on there in any way. The board is connected to the amp via that 8-pin connector and the whole signal must be processed on the amp's circuit. – Prototype700 Jan 3 '19 at 11:17
• Can you disconnect one of the white wires and measure what the ohms of the speaker are with a meter? – Voltage Spike Jan 7 '19 at 15:46
• Hi laptop2d, the back of it reads "8 Ohms 1 Watt", do you need the exact readings? Do you expect them to be different? – Prototype700 Jan 8 '19 at 21:38
• try injecting power line hum into the circuit using a long piece of wire .... use insulated wire with just a bit of bare wire at the very end ..... or use a disconnected voltmeter probe, if you have one ..... that may lead you to the point in the circuit where you could connect an audio jack – jsotola Jan 9 '19 at 2:05

A similar schematic I found is this one

Obviously is not this because the DSP chip has only 20 pins and the jack optput is placed in a different position

The amplifier chip is something similar to MC434119 , datasheet and looking into it I see the possible reason why you failed. The pot is in the feedback path not on the input path, leave the pot in place otherwise the gain is way to high and the circuit is unstable or oscillating. Jason's solution might work but it needs a decoupling capacitor to break the DC path through the external source.

The synth output goes through C4 (marked in red) to the jack output path (in purple) and to the amplifier path ( in yellow ).

What you can do is to unsolder C4 or cut the trace to it and use the Jack connector as input.

If the volume is to low then short the 10K resistor in the path (the one with 103 on it under the purple arrow).

To minimize the noise and spare battery power you can also cut the power to the synth chip by unsoldering the small 3 pin chip on the right of the yellow arrow or at least it's upper left pin. Just for the record, I don't understand why it has a permanent 4.7 ohm load that drains the battery with no obvious reason.

• Hi Dorian, thanks for your detailed explanation and the schematic. I've actually looked for that datasheet, but wasn't able to find it by the printed number. Just so I understand you correctly, you suggest that A: If I remove C4, I could simply plug an audio cable into the existing output jack on the board and it would basically be converted to an input, sending the signal from purple to yellow? And B: By "leave the pot in place" (for gain and circuit stability), do you mean I'd have to resolder the one disconnected leg of the pot to the trace below the "01v1"? Because I get less hum this way. – Prototype700 Jan 10 '19 at 13:17
• @Prototype700 Yes on both 1 and 2. If the sound is to weak try to short the resistor on the purple path. – Dorian Jan 10 '19 at 16:00
• On your board the jack plug is placed at the synth output, before the tone pot. – Dorian Jan 10 '19 at 16:06
• Hey Dorian, I've just had the chance to make the changes you suggested. That did the job - thanks a lot! You saved me 20 bucks and helped me get a little more insight into how small electronics work (and I gained some experience in desoldering SMDs without setting everthing on fire). I first desoldered the upper C4 and after this still resulted in no sound from the speaker, I reattached the leg of the volume pot and soldered a wire to the cut-off trace. After that, I got sound from my input device and even without audible noise. I also successfully cut the uppler left leg of the "Synth"-IC. – Prototype700 Jan 13 '19 at 16:20
• For the record: The volume control on the amp does not work anymore, but the volume of the input when using the output-jack is fine. It can be turned all the way up and it's not too lound (I would actually be concerned to remove the 103-resistor in the way). Volume can usually still be controlled on the input device, though. The tone control still works however, which is very nice. – Prototype700 Jan 13 '19 at 16:26

You could try cutting the bottom trace under the writing "C498" and then applying the input to the pin nearest "01v1" possibly through a 10K resistor. that will break the tone control, but the volume control should work.

This is the circuitry behind the volume control, so IC4 is probably the amplifier,

also there's a small capacitorin that trace so it's power or amplifier output trace - that makes it look like an input.

• Hey Jasen, thanks for taking a look at it. I don't care about the tone control very much. The volume control might be helpful though, because of how loud it can be. Could you elaborate just a little bit how you concluded that the connection must be made at this point? Just out of curiosity, since I couldn't make anything out of it. – Prototype700 Jan 3 '19 at 11:20
• Hi Jasen, I haven't been successful so far, unfortunately. I have edited my question to explain my progress. I suppose that this is what you suggested as well? – Prototype700 Jan 6 '19 at 15:14