When I connect my cell phone to my DIY guitar amplifier, I get a noticeable buzzing noise when I touch the phone's touchscreen. The phone is connected via a fairly good aux cable to transport the mp3 signal. The buzzing noise seems to be at a low frequency, and it disappears when I touch the PCB's ground or the guitar strings.
Why does this phenomenon occur and how can I solve it if it is solvable?
Even without the cell phone connected, touching the ground with my finger reduces the faint background noise. Why does this happen?
Why does this phenomenon occur and how can I solve it if it is solvable?
Even without the cell phone connected, touching the ground with my finger reduces the faint background noise. Why does this happen?
Sounds like your amplifier ground is floating with a ~50/60Hz voltage on it. The amp should have a 3 wire AC power cable that grounds the amp chassis and circuit ground and be sure your AC source has a good safety ground. SMPS are bad for ground noise because they have a RF pi filter on the line that returns to the safety ground, so if the safety ground is open, you get half the line voltage on the system ground.
The amplifier is battery-operated and without any connection to the power grid. I have also tried disconnecting the general power supply to my piano but the problem remains
I assume you have this typical "mobile noise". In that case we are talking of radio frequency interference, a general problem.
The simplest solution: keep mobile at a distance >2m / 1 fathom far away from any audio electronic.
Specially the vintage tube amps are very sensible.
Otherwise you may check rf filtering of all incoming cables, or revisit your hole circuitry under the aspect of rf immunity.
That was the way I tamed my DIY stuff - a complete re-design anyway.
The simplest solution: keep mobile at a distance >2m / 1 fathom far away from any audio electronic.
Specially the vintage tube amps are very sensible.
Otherwise you may check rf filtering of all incoming cables, or revisit your hole circuitry under the aspect of rf immunity.
That was the way I tamed my DIY stuff - a complete re-design anyway.
I increased the gain in order to record it better.
- The intensity of the interference is proportional to the skin/touch contact surface, and it gradually increases during recording;
- even if the mp3 signal does not pass through the preamplifier, it is influenced by the gain and the volume of the guitar;
- if the guitar is not connected, no interference occurs; the guitar is an Eko S300, an economical model for beginners, and I do not have any other guitars or
- higher quality guitar jack cables for a comparison test;
- it also occurs with only the jack cable connected; Could it be a ground loop issue?
You could try running a wire from the signal ground of your guitar output jack to a good earth.
I used a radiator (central heating) as ground on a similarly powered guitar amp once.
It led the noise away.
If you have no radiators, but access to a garden, a copper pipe driven 1m into the ground with the wire attached would work.
If it's mobile signal interference, try shielding the phone with aluminium foil or get the phone farther away.
You can test this by going near a standard radio. With / without shield.
If the phone is shooting the interference down a cable, that's trickier.
Can you use airplane mode?
I used a radiator (central heating) as ground on a similarly powered guitar amp once.
It led the noise away.
If you have no radiators, but access to a garden, a copper pipe driven 1m into the ground with the wire attached would work.
If it's mobile signal interference, try shielding the phone with aluminium foil or get the phone farther away.
You can test this by going near a standard radio. With / without shield.
If the phone is shooting the interference down a cable, that's trickier.
Can you use airplane mode?
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I will try to use airplane mode. I tried with a different cell phone model and the problem still occurs, but with lower intensity and the sound tone has also changed. If the problem were the cell phone, then I should experience the same issue even when listening to music with headphones, but that's not the case.
Just the jack inserted is enough to cause the buzzing (so the input amp is not connected to ground) with touch, and if I touch the ground with one finger and the touchscreen with another finger, the buzzing stops.
Furthermore, I just discovered that it also occurs even if I don't touch the cell phone, just by bringing the palm of my hand about ~1-2cm away.
I don't have the possibility to bury cables in the ground, can I use the grounding of the house, but should I connect it to the battery's ground, won't I risk damaging something?
Just the jack inserted is enough to cause the buzzing (so the input amp is not connected to ground) with touch, and if I touch the ground with one finger and the touchscreen with another finger, the buzzing stops.
Furthermore, I just discovered that it also occurs even if I don't touch the cell phone, just by bringing the palm of my hand about ~1-2cm away.
The distance of the phone does not help in any way.
I don't have the possibility to bury cables in the ground, can I use the grounding of the house, but should I connect it to the battery's ground, won't I risk damaging something?
Signal ground to earth won't risk anything.
Sounds like you created a crude Theremin🙂
Capacitive touch screen theremin
Cell phones create lots of RF signals for tx/rx data, clocks, screen and touch scan, etc, so it is itself a good noise generator. Suffices to put it close to an AM receiver.
I tried connecting only the GND of the phone to the GND of the board and it is enough to transmit the buzzing. I also tried with a TDA2030 module and even in this case the buzzing can be heard, so it is not just a problem with my board. Even with a 1:1 audio isolator, the buzzing can be heard.
What else could I try to solve, or at least understand how to limit the problem? Headphones and the guitar amplifier do not have this issue, so it is certainly solvable, any ideas or suggestions?
What else could I try to solve, or at least understand how to limit the problem? Headphones and the guitar amplifier do not have this issue, so it is certainly solvable, any ideas or suggestions?