[SOLVED] Buzzes squeaks and clicks through 3.5mm jack

[MattMorphett]

I'm experiencing some interesting noises when connecting to an external amplifier and speakers via the 3.5mm socket on the RaspberryJamma DAC.

They seem to be coming from the Pi itself as they fall in sync with the Pi doing something; i.e. moving the selection down in Attract Mode or when loading a game.

I've uploaded a video of them here...
https://youtu.be/Rjq1u4wSJYo

Here's what I've tried:
- 3 different audio cables.
- Connecting to a Lepai amp RCA inputs and 3.4mm inputs (this is my main intended setup).
- Connecting to a studio quality Mackie self-powered (i.e. own amp onboard) speaker.
- Unplugging the input to the amp to see if the sound is induced somewhere else (i.e. RF within the cab or by the Lepai amp sharing the 12v arcade power supply). No noises are present when I unplug the 3.5mm jack.

When a speaker is connected to the Jamma harness, I don't get the noises.
In other words, no noises at the speaker when the DAC is connected via Dylan's supplied 3.5mm cable going through the Jamma setup, but noises when I use the same cable to connect to an external amplifier and speakers.
This is interesting, I wonder what Dylan is doing to stop the noises propagating through his board. Dylan?

Can anyone help?

Thanks,
Matt.

[dee2eR]

I wonder if you can reduce or remove the noise by turning the software volume control down a bit. The hardware volume control onboard the RaspberryJAMMA for the mono audio is attenuating the signal from the DACs audio out prior to the amp chip, perhaps simulating this by simply turning down the sound will clear up your audio. Your stereo amp is probably much more powerful than the onboard RaspberryJAMMA amp so turning down the audio at the software level shouldn't leave it too quiet.

You can adjust the overall volume level from the service menu option "Adjust system volume" using keyboard up and down arrows, esc exits and it should save automatically.

Hopefully that helps and doesn't just adjust the volume of the unwanted noises with the rest.

If that doesn't work my next instinct would be to try something like this: https://www.fostexinternational.com/doc ... 1eBW.shtml (but that's mainly because I have one in a cupboard not currently being used, they're nice and solid and would work great as a cabside stereo volume knob) to attenuate the signal on the way to the amp in a similar way to what happens on board the RaspberryJAMMA. If you like I can try and replicate the problem here to see if the inline volume idea would work, first I'd try the software volume though.

[MattMorphett]

Hey Dylan,

Thanks for the snappy response!

I've just tried your suggestion and here are my observations:
- Reducing software volume didn't reduce the volume of the undesired noises
- Turning up the volume on the external amp (enough to bring back good levels of desired sound) bought up the volume of the undesired noises so that they were actually louder than before.
- Right now, software volume and max gives me the best signal-to-noise ratio, but the noises are still quite audible.

This suggests to me that those noises are being introduced late in the audio chain, after the software volume. Some forums - https://www.reddit.com/r/RetroPie/comme ... 5_mm_jack/ - suggest an update to the audio driver or the use of a ferrite choke around the audio cable. I'll see if I can find one of those. The other thing I'll try is a battery powered speaker to remove the possibility of any kind of ground loop or other shared power supply issues.

M.

[MattMorphett]

OK, I've gotten to the bottom of it and I have a workaround.

I just plugged in a battery powered speaker and the buzzes went away. This suggested there was an issue with my Lepai amp or possibly some kind of noise coming through via the shared power supply. I thought I'd tested for the latter but apparently not...

I plugged in a new power supply into the mains socket and plugged it in to Lepai amp and... NO NOISES!

So, it looks like there was some kind of noise being pushed back by the Pi through the Jamma 12v line to the power which was then pushing that noise back up to the power supply of my amp.

Here's a question... does the Pi or RaspberryJamma need the 12v line? Can I disconnect it? It didn't want to try in case it breaks something. It may be that the noise is travelling along the 5v ground which could be crossing over into the 12v in my arcade power supply (if they share a gnd) but I'm keen to try removing the 12v to the gamma harness.

At this stage my work around is to run a separate 240v plug pack power supply for the Lepai amp rather than taking 12v off the arcade power supply.

Thanks for the help so far D.

M.

[dee2eR]

In that case the good news is the only thing the RaspberryJAMMA uses 12v for is the audio amp so you can run without it in your setup.

[MattMorphett]

OK, I've gotten all the way to the bottom of the issue. It was a ground loop problem. I tried lifting the 12v lines (+ve and GND) and neither fixed the issue.

The solution is to place a ground loop isolator between the audio output and your amp. In Australia Jaycar sells one for $11 that does the job perfectly...
https://www.jaycar.com.au/ground-loop-n ... m/p/AA3086

I think you'll only encounter this problem if:
- You are using the 3.5mm socket going into an external amplifier, and
- You are trying to power that amplifier from the same arcade power supply that's powering your PI.

Anyway, just insert one of these units between the ArPiCade 3.5mm jack and your amp and all your audio should be squeaky clean.

GLIsolator.jpg (48.7 KiB) Viewed 7600 times