Input buffer and belamini powered from external power supply

quintosardo

This is something I really can't understand. Usually there is a very simple answer.

I built a basic input buffer for a bela mini, an op-amp in inverting configuration, 1MOhm resistor on input and on feedback: 1 gain, increasing the input impedance.
The op-amp is powered by a desktop dual power supply (+/-9V).
It is ac coupled through an input and an output capacitor.
Everything fine. I run the Delay example and the sound is clean.

Now my problem: as soon as I connect a third source from the same power supply (5V, its third output) to bela mini's voltage input (p 1.1) and a second cable from ground to bela mini's ground (p 1.16, but I've tested probably all the other options), I get lots of noise and whistles.

Remove the p 1.1 cable, everything fine (USB is still connected, so bela stays on), re-connect, crazy noise.

If I remove the USB cord from the computer I simply get a bit more noise.

Looks like a lot of pollution from bela's voltage input p 1.1? Converted into aliased audible noise in the A/D converter?

I'm trying with some filters at converter's input but I'm surprised there's no discussion about this, this is why I think I may be getting something heavily wrong.

I also tried to power bela from a phone charger through the micro-USB and looks quite fine, so is p 1.1 just bad?

Any tips?

Thanks! 😃

quintosardo

Thanks!
I really tried millions of combinations.
Now trying using an isolated supply just for the buffer.
Looks strange that nobody had such issues?
Bela forum: isn't this one? Am I missing something?

giuliomoro

quintosardo Bela forum: isn't thisnone

Yes. The answer you got looked like an AI-generated post which contained re-hashed and boilerplate text and a spammy link. User removed.

The issue you are having could be a ground loop. How many ground wires are you connecting to Bela? Try using only one.

Also, I think that the 5V line coming on via USB is fed to the power management IC on board the PocketBeagle via a ferrite bead, but this is not the case for the signal from P1.01 . So you may want to try to add a ferrite bead in series with your 5V line. I think that you could use the on-board ferrite bead if you disconnect the USB cable and plug the 5V into P1.07 (USB.VIN), so you can try that if you don't have a ferrite bead at hand. Note that this pin is shorted to the 5V input from the micro usb cable, so you shouldn't use both at the same time. As you won't be able to use the USB cable while powering the board this way make sure you have a test project running on boot.

giuliomoro

Or it could be down to an under-performing 5V power supply. What's its rated current? When scoping the 5V line, do you see meaningful differences between the two supplies? Are you using the analog ins in your project in such a way that noise on the analog ins would affect the resulting audio performance?

quintosardo

Really weird as I tried one million options.

This simplest circuit is on a breadboard.

The power supply is a laboratory one, with three outs.

I added a ferrite bead. Using the USB V pin instead of p 1.1 reduces the noise just a bit.

I tried all the possible GND pins.

Tried with several belamini units.

Nothing else is connected, no DI/DOs, no analog ins. They are both disabled.

Is using a power supply for feeding a buffered input and the bela unit something usual? I think many did it.

Not talking of background noise but something close to signal level, including z sttong whistle.

giuliomoro

quintosardo tried all the possible GND pins.

Did you try with a single ground wire for both audio and power?

quintosardo

giuliomoro I restarted this morning, as I realised I was keeping the audio-in ground cable connected.

First, I disconnected the ground cable for the audio input (being it connected to ground, too).
Then, connected the ground to all possible ground pins on bela mini, one by one: same result (strong noise, digital noise and whistling).

Then, I tried with floating supply for the audio buffer:
- audio in ground cable connected.
- ac-connected in and out for the buffer (by means of capacitors).
- FLOATING supply for the input buffer.

There is no interconnection between audio and bela's grounds, now, and looks like everything is working (besides normal noise, being all this on a breadboard).

So, looks like having an on-board floating supply for the audio buffer is the only way of using it?
I designed and added a switching flyback supply (+/-9V) and this is what I'm using now.

Not a common solution, though, that's why I wonder how is everybody doing this (Bela in a guitar pedal). Only powering from batteries?
This is why I suspect I'm making some trivial mistake with the non-floating supply.

giuliomoro

I have seen more than one issue with grounding on these SBCs, but I had not come across your specific one before.

Usually, battery power or transformer isolation on the audio inputs work fine, as you found out. Also, powering via ferrite bead had helped before. Now,cutting the trace connecting the analog and digital grounds on the Bela Mini cape worked very well in some cases, although this requires that an external ground connection is provided at all times. Careful with your sharp knife there so you don't damage the board by cutting nearby traces. For anyone else reading this: do this only if you are good at it, as it may void your warranty if this damages other parts of the PCB. Also, keep in mind that in order to "undo" this, you'll need to scrape off silkscreen and soldermask and join the traces back together with a bit of solder (well, or run another wire between exposed ground pads).

alt text

picture is for BelaMini Rev C1. Check design files for other versions: https://github.com/BelaPlatform/bela-hardware/tree/master/belamini.

quintosardo

giuliomoro Looks like this cut trace thing helped. I currently have the trace cut, the custom dual supply, the two stages filter on P1.1 voltage input and a ferrite bead there.

The custom dual supply works better if not left floating.

Looks like P1.1 is quite delicate and noisy and needs careful treatment, not just a wire going into it from a breadboard. There is a lot of radiated noise while using it (the breadboard has long wires, etc.)

quintosardo

Thanks Giulio! I'm testing this.
Currently, I obtained quite good results with this:
- Custom isolated supply for audio input buffer.
- Two stages LC filter at V+ input (quite big values, like two 2200uF capacitors).
Works quite well but looks overkill and still leaves the feeling of putting something on top of something wrong.
I don't think I had any result with a ferrite bead at V+ input. Looks like the noise is at high frequencies but also going down into audio because of modulations.
Testing the trace cut thing!

May be useful: there is no way of accessing the differential audio inputs of the codec instead of the single-ended ones, right?

Oh, I also saw this: Bela is not visible through USB (no IDE) while it is being powered like above.
(USB cable only between belamini and Mac, 9Vdc Boss supply feeding the breadboard, switching +5V regulator feeding Bela, with ground common to the 9V supply it is fed from, floating custom +/-9V supply, fed by the 9V supply, feeding the audio input buffer, guitar connected to buffer input and to floating ground).

giuliomoro

quintosardo Currently, I obtained quite good results with this:
- Custom isolated supply for audio input buffer.
- Two stages LC filter at V+ input (quite big values, like two 2200uF capacitors).

Are both or these needed together? Does the LC filter make a difference while using the isolated power supply on the input?

quintosardo May be useful: there is no way of accessing the differential audio inputs of the codec instead of the single-ended ones, right?

No, they are not broken out. Hard to fit everything on a tiny design like this.

However, if you are using only one input channel, you could feed a differential signal to left and right inputs and subtract those in software (assuming they have the same gain). While not exactly a fully balanced input, I have seen snr improve drastically with this trick.

quintosardo Bela is not visible through USB (no IDE) while it is being powered like above.

Hmm that's weird. USB it should be working all the time unless you are messing with the USB pins on P1.

quintosardo

giuliomoro Yes, I only use the isolated supply for the audio buffer, now. The two stages LC filter is needed. The most annoying part of the noise is a steady freq like 5kHz. One stage is not enough, the noise is still hearable.

Great, I'll try with the 3.3V part.

USB and IDE: no, just the normal connection.
Like if the grounds are moving too much?

giuliomoro

Btw see if you have any luck placing caps in parallel on the 3.3V line instead .

giuliomoro

Is the 5khz the switching frequency of your power supply?

quintosardo

giuliomoro No, it is 200KHz.
I also tried all this with the voltages all coming from a desktop power supply.
This whistling has always been there, since I connected to P1.1 for the first time, in many different configurations.

If anybody reading is interested to try, it is just an inverting opamp with two resistors.

quintosardo

alt text
Current situation.

quintosardo

The first problem in this thread looks like this?

https://forum.bela.io/d/1244-digital-noise-on-audio-out

... but there's no follow up by the user...

giuliomoro

With the trace cut, can you go back to having a common ground for the 5V and ±9V rails ? I checked a custom design we made in the past and that had the equivalent of that cut in the ground track, then a common ground for the 5V power supply for the PB (fed through a ferrite bead) and for the ±5V power supply for the THAT1570 preamp. The results were pretty good; both ferrite bead and track cut were needed, but we didn't need additional LC filtering. The 5V regulator for the PB was the Recom R-785.0-1.0, while a LM317/LM337 pair was used for the bipolar rail (we had an 18V AC input). All of this on a dedicated PCB. Did you have a chance to test the noise performance of your power supply?
Also, are you currently using the analog inputs for anything? If you have pots there that are used to e.g.: control the volume or other parameters in your test code, then any noise on that ADC would be coupled into the audio path by the software without appropriate treatment in the DSP.

Btw, why are you going for ±9V in the first place when the input into the codec is about 1.8Vpp?

quintosardo

giuliomoro All the grounds are together (including the floating one, not floating anymore).
I don't think that power supply quality is an issue as everything was tested with all the sources from a bench supply, too.
The +5V regulator is Recom R-785.0-1.0 (but I also tested it simpler, no regulator, 5V coming from the bench supply).
The analog inputs are not used, not connected to anything.

+/-9V are there because we want to prevent overload on input and to have good voltage on balanced output.

What sends me crazy is that without a buffer, everything is straightforward: connect guitar to audio input, power supply to bela mini, everything is clean.
As soon as I put the audio buffer in the middle, everything goes crazy (not simply noise but very strong noise).
This happens in all simplest configurations, including this one:

bela mini besides a breadboard, an op-amp with two resistors on the breadboard, guitar to the op-amp. Nothing else. Guitar to bela: perfect. Guitar to buffer (unity gain): disaster.

Same situation, guitar to buffer, bela mini powered only from the USB connection: everything fine.

External supply to bela mini and audio buffer can't go together.

giuliomoro

You could also try an isolation transformer on your input. Assuming no pathological condition with your 5V supply, that has solved similar issues, according to other reports on the forum.