Debugging tool idea / advice needed

[utarefson]

First of all, although I've been lurking here for a while, this is my first post so hello everybody

I've had this idea for a simple pedal debugging tool that contains a lm386 amp, a speaker, some kind of sample player and all necessery connetions all in one box powered via usb for convenience. At first I was planning to make a simple two-button arduino platform, but since arduino does not have a built in dac and the TMRpcm was noisy and glitchy, the next idea was to use an old and trusty mp3 player. I've put everything together which looks like this:



As you see theres a mini usb output to charge the mp3 player during playback and it's connected to the usb power rail along with the lm386 amp. But the thing is that when it's charging, it kills the audio output to the amp. When the mini usb plug is not connected, and the player runs from the battery, the playback is fine and clear. But the moment I connect the power cable it gets muted and some quiet cracks and buzz occur. What am I doing wrong here?

[11-90-an]

I believe we need a schematic to see if anything gone wrong... 

[utarefson]

It goes like that:



As I said, when the power plug is not connected to the mp3 player, the playback is loud and clear. But as soon as I plug the mini usb in and the charging begins, the audio goes away and some digital noses occur.

[11-90-an]

Try this...

[amptramp]

You might need a higher value of input impedance for a signal tracer.  The 10K pot is what you need for an LM386 but you may need an emitter follower / buffer ahead of it.  A lot of circuitry would not be able to drive 10K.

[antonis]

Does USB port also feed other consumptions..??

[utarefson]

Try this...

Did not work unfortunately

You might need a higher value of input impedance for a signal tracer.  The 10K pot is what you need for an LM386 but you may need an emitter follower / buffer ahead of it.  A lot of circuitry would not be able to drive 10K.

Will try that later on, thanks. Although when running from the battery everything works pretty well, output is loud and clear.

Does USB port also feed other consumptions..??

Straight from power adapter to the device.

[Marcos - Munky]

Are you getting power from a pc usb port or from a wall socket usb adapter? Sounds like whatever you're using to power the whole thing is having trouble to power the mp3 player and the lm386 at the same time. The lm386 have a kinda high current draw.

[utarefson]

I used a 18W cellphone adapter, so current draw should not be an issue. I'm going out of town for a few days, but when I come back I'll post a video of this behavior to show you guys better what's going on. Thanks for the feedback so far, the community here is outstanding and I hope to contribute myself too as I'll learn more about electronics and whatnot!

[utarefson]

Sorry for the long absence, here's a video picturing this behaviour:

[Marcos - Munky]

Maybe the mp3 player have a circuit that makes it pause/turn off while it's charging. Does this also happens when you're using a headphone and without powering the LM386 amp?

If the power supply can handle both the mp3 player and the LM386, then there's nothing wrong at all with your setup.

[utarefson]

The mp3 player plays fine through headphones while charging, so it's not it. Maybe it's some sort of a grounding issue?

[FiveseveN]

Do you have a battery pack (or at least a different charger) to test it with instead of the 18W cellphone adapter? USB chargers vary wildly in quality and I suspect yours is injecting noise.

[utarefson]

Tested with 3 chargers and pc USB port..

[PRR]

The MP3 player may not be negative ground. It is very possible the audio plug sleeve is at half of the 5V.

[utarefson]

Well I measured the voltage on the sleeve of the mp3 output jack and it showed 1.3V (compared to the power supply ground). Could that be the problem? And how to separete the grounds if that's the case?

[PRR]

You need to float the audio from the shared DC power. Fortunately the LM386 has differential inputs. The MP3 player output is surely so hot you do not need the cap pin 1 to pin 8.

[kraal]

You need to float the audio from the shared DC power. Fortunately the LM386 has differential inputs. The MP3 player output is surely so hot you do not need the cap pin 1 to pin 8.

Hi,

I have two questions about the modifications you made.

- The modification you made makes use of the ground of the probed circuit as the "-" input of the LM386 (in order to avoid unwanted offsets and other issues when probing for instance reverse polarity circuits). Is my understanding correct?
- Why have you added a 10k resistor to both inputs ? (why are they required ?)

Thank you in advance !

M.

[jfrabat]

Why not ditch the MP3 player and just use a simple sine wave tone generator?  I made one with a TL071 for troubleshooting, and it is a very simple circuit (you WILL need a 2 gang pot if you want to make the frequency selectable with a Wein bridge, but it is simple to get one).  Now you can run the whole thing from a regular 9VDC brick...  Just a thought.  Also, for trouble shooting, I prefer a single frequency than music, but that is just me...

[ElectricDruid]

Why not ditch the MP3 player and just use a simple sine wave tone generator?

Why not just stick an MP3 of a sine wave in what he's got now? Much easier, no?