Noisy output running Bela Mini via LM2696 Step Down Converter to TPA3118 Amps

ThomasEg

I'm trying to remove some noise from a system. The noise consist of a high pitched tone around 5500Hz and a saw-like tone around 640Hz. Once every minut or so their is a brief pause in the noise (200ms ish).

I'm powering my Bela Mini Multichannel Expander through a LM2596 Step Down Converter Module from a 12V source (to 5.05V). Meanwhile the same 12V source is powering 2xTPA3118 Mono Amp Modules. The Bela output 1-2 is connected to these. The audio GND cable split to both amps.

The system works noiselessly and well when I power the Bela from a USB outlet (computer/adapter). But when I power it with the step-down converter I get low bleeps and blops when the Bela is booting. As it starts playback on boot the noise starts. The high tone not always at the beginning but coming it later - for then to be consistent.

I'm suspecting ground loop or inductor-caused noise? Am I on the right track? Would it be better to use a LM7805 Voltage Regulator than the step-down? I have tried to add electrolytic (1000uF) and ceramic (100nF) caps in parallel at before and after the step down module.

What can I do to remove the noise?

I have uploaded the parts and setup + datasheets:

LM2596: https://www.ti.com/lit/ds/symlink/lm2596.pdf
TPA3118: https://www.ti.com/lit/ds/symlink/tpa3118d2.pdf?ts=1779191444546

Bonus info:
I'm going to run the system on a 12V battery with an Arduino controlling a push-button power latch. The Bela sending HIGH from D0 to Arduino while playing and LOW to shut of the power. Here I'm going to use 2xXY-MOS Power Mosfet Modules to control the power going to the Bela Board and the power going to the amps.

giuliomoro

Are you actually using the multichannel expander or is it just a Bela Mini? You only mention two channels.

You most definitely don't want to use a linear regulator because that would heat up a lot, 200mA * 7V = 1.4W. And that's unlikely to fix anything, as the issue seems to be about sharing ground between the two.

One option is to try using the audio ground for powering it: plug the - from the power supply into one of the GND pins on the audio side: in the audio input connector, in the headphone connector or in the line out connector. See these three pins highlighted below, pick one of them:

when I power it with the step-down converter I get low bleeps and blops when the Bela is booting.

Before the Bela program starts the codec, you should have the amp muted. Your breakout should have pins for that: use a 10k resistor to set it to muted then connect the mute pin to a GPIO on Bela and unmute it when the program starts.

PS: for the next time, links and product datasheets are more useful than pictures

ThomasEg

giuliomoro I'm using the multichannel expander since I'm going to have 4 outputs. Right now I'm just testing with two.

I have now tried to power through the GND PIN of the audio in and outs, but the noise is still occuring during playback.. 🥲 Do you think it can be the step-down converter causing the noise?

I have added datasheets to the first post for TPA3118 and LM2596.

giuliomoro

Try without the expander, just the mini cape and one pair of outputs, with power ground provided on the audio input or line out pins.

ThomasEg

giuliomoro

Tried both options (line out and audio input pins). The noise is a bit lower when using the audio compared to the others GND. But it is still very prominent.

Tried powering Bela Mini from another power source again. Quiet as a mouse and clean as a whistle.

Is it possible to power the Bela and the amps via the same power source without getting noise?

giuliomoro

Have you got the links to the breakouts? Those may be more relevant than the chip's datasheets. For instance, is the TPA3118 module's input fully differential? If so, as you have spare channels you may generate pseudo-differential outputs (i.e.: where the R output is the opposite (* -1) of the L output) from a stereo pair and then connect L out to the + input and R out to - input and see if that works. You can also try to connect nothing to the - input, or connect it to the on-board ground instead of bringing the ground across from the Bela Mini.

See if any of these works, otherwise I have another more convoluted plan in mind.

ThomasEg

giuliomoro

I have searched for the breakout datasheet but sadly I'm not able to find it.. Only the chip: https://www.ti.com/lit/ds/slos708e/slos708e.pdf

Pseudo-differential output

giuliomoro is the TPA3118 module's input fully differential? If so, as you have spare channels you may generate pseudo-differential outputs (i.e.: where the R output is the opposite (* -1) of the L output) from a stereo pair and then connect L out to the + input and R out to - input and see if that works.

I tried connecting the signal wires (red and white) to INPUT+ and INPUT- and sending the same output to both, but phase inverting one of them. I tried with ground going from the step-down converter to GND pins and audio grounds. In either case this produced a lot of noise until I touched the INPUT- with the Audio GND wire, which muted all sound. So I guess my breakout board isn't setup to use the differential option and either the RINN or LINN on the chip isn't used for input (see section from datasheet below).

FROM CHIP DATASHEET:

PATCH FOR DIFFERENTIAL TEST:

Nothing to INPUT- AND INPUT- to Power grounds

giuliomoro You can also try to connect nothing to the - input, or connect it to the on-board ground instead of bringing the ground across from the Bela Mini.

This produced a lot of really loud noise! 😃

giuliomoro

ThomasEg So I guess my breakout board isn't setup to use the differential option and either the RINN or LINN on the chip isn't used for input (see section from datasheet below).

I guess you could verify that by checking with a multimeter (with the module unpowered) whether the - input is shorted to ground?

ThomasEg

giuliomoro

Yup. I can confirm that. There’s connection between INPUT- and POWER-.

Are there other ways to “clean” the signal from noise even though it is single ended? Is it necessary to get other equipment or is there a way?

giuliomoro

You should be able to do that by cutting the trace that joins the 'digital' ground and analog ground islands on the top layer of BelaMini, see the illustration below:

You'll need a good knife (stanley knife?) and firm hand. Start with the tip of the knife immediately above the trace to cut and cut towards the bottom, so you won't risk damaging the traces that are above it. If you ever need to "cure" this cut, you'll need a jumper wire between the audio grounds and one of the other grounds, or scratch a bit more of the soldermask around the cut and put some solder on it to bridge it.

ThomasEg

Can't seem to see the clear GND trace you are talking about on the front of the Bela Mini? I have a rev. C2.

Splitting digital and analog grounds on Multichannel Expander
Also, is there a way to do the same on the Multichannel Expander? Otherwise it is not going to work I guess - when I'm going to use 4 channels? Since the analog and digital grounds will bridge through the Multichannel Expander?

Plan B?
If you can confirm the location of the traces I will go with that solution! Thank you! But if it is not possible will it be worth considering powering the Bela via an Isolated DC-DC Converter? Like a TDK-Lambda CCG3-12-05SR DC-DC Converter? Will such a component be able to feed the Bela with 5V DC that doesn't spread into the grounds of the amps?

giuliomoro

ThomasEg Can't seem to see the clear GND trace you are talking about on the front of the Bela Mini? I have a rev. C2.

It's the horizontal trace that joins the two islands, see the annotation below:

It doesn't look like the multichannel expander has a similar single point to cut. Did you try connecting the multichannel expander to the amp without connecting the Mini? Does it have the same problem?

The solution could be to use audio isolation transformers for all the signals going to the amplifier. Search online for "HiLetgo-EI-14-Coupling-Isolation-Transformer" . They are pretty cheap e.g.: on Amazon.

ThomasEg

giuliomoro

Okay. I can't get hold of amazon goods before I need to use the setup 🥸 But I can get hold of 2 Monacor Ground isolator. Don't you think they will do something similar as the HiLetGo transformers? Based on this video with a similar product.

If I get hold of these I guess I won't need to cut into the Bela Mini ground trace?

ThomasEg

giuliomoro

giuliomoro It doesn't look like the multichannel expander has a similar single point to cut. Did you try connecting the multichannel expander to the amp without connecting the Mini? Does it have the same problem?

First I just checked with a multimeter between audio gnd and digital gnd on the expander. Since I there is a connection guessing I would run into the same problems when adding the new channels to the mini?

I'm not sure I get the question a 100%, but I tried plugging in the expander without the Bela Mini. Since no digital stuff was going on there was no noise at all.

ThomasEg

giuliomoro

Aah. Of course. Yes this setup is noisy too..

ThomasEg But I can get hold of 2 Monacor Ground isolator. Don't you think they will do something similar as the HiLetGo transformers? Based on this video with a similar product.

If I get hold of these I guess I won't need to cut into the Bela Mini ground trace?

I've ordered some HiLetGo transformers and I'm getting hold of a Monacor Ground isolator for testing. So putting them between the audio outs and ins.

Hope that solves it. If not is it possible to separate the digital and analog grounds with the multichannel expander mounted?

giuliomoro

ThomasEg I'm not sure I get the question a 100%, but I tried plugging in the expander without the Bela Mini. Since no digital stuff was going on there was no noise at all.

I was suggesting:

full PB2 + Mini + Multichannel stackup
power from the +12V -> +5V switching regulator that shares power with the amplifier
connect audio signal from the multichannel expander to the amplifier
Is it noisy?

If not, then it seems that you can just use extra channels from the multichannel expander and not use any from the Mini cape itself, right?

giuliomoro

ThomasEg If not is it possible to separate the digital and analog grounds with the multichannel expander mounted?

Looking at it, it doesn't look like that's the case, but maybe I am overlooking something.

ThomasEg

giuliomoro

Okay. I will try with the isolating transformers. Thank you very much for your help!!

By the way, I had a spare amp and I tried removing the R2 resistor in the voltage divider setting the gain of the chip (mentioned in the datasheet). To achieve a lower noise floor. Then turned up output from Bela digitally. Works quite fine and might be my plan B if the transformers fail..

giuliomoro

well if you have that type of reworking skills, you may want to look into reworking the amp board for differential input?

giuliomoro

see https://www.ti.com/lit/ds/symlink/tpa3118d2.pdf?ts=1779191444546 for an example of a single-ended input vs differential (JP1 and JP2 need to be grounded for single-ended and open for differential I think, though it doesn't say it anywhere). Also I doubt pin 3 (the normalled connection) exists on an RCA socket ... I'd assume they used the wrong symbol.

ThomasEg

giuliomoro

Well I'm not that experienced and found the resistor I had to remove via a forum post og two.

But I would like to try and might be able to succeed with some guidance. As I understand I have to open the connection between INNR and GND. I could do this by removing the SMD ceramic cap. Then soldering a wire to the pad (marked with a blue dot) going to a new 1.0uf ceramic cap? How important is the cap? To this cap/pad I connect a phase inverted signal? *~ -1 from Bela?