Potential Ground Loop Problem

[james2k2]

Hi All,

Got (hopefully) a simple problem, and trying to find a good solution before I buy parts. I've got one of those pre-made amplifier PCB's from ebay (this one: http://www.ebay.co.uk/itm/231179156525), and also one of those 1A Car USB chargers, which I've dismantled to get just the PCB to convert the 12V to a 5V USB so that I can either charge a phone, or power in this case my Chromecast Audio.

With the Chromecast Audio in place, it has an annoying inconsistent buzz. With the Chromecast on it's own PSU, it works fine. Now my feelings are there's an Audio Ground Loop. The attached diagram is not very good (accuracy with symbols, or correct use), but demonstrates what I mean. I've shown it with the physical connections as I have it wired.

As I've ruled out the amp on its own, as well as the Chromecast, it's either a poor DC-DC converter (quite likely for £1), or there's the loop I mentioned. Any thoughts? I haven't a lot of room in the box I've used, so those tiny USB Car charger PCB's are kind of my maximum, and I'm not sure, but I think a 7805 might generate too much heat in such a small space with a 1A load.

Thanks for any insight anybody has.

[samnmax]

I would place my bet on a noisy DC-DC, as £1 car chargers must be quite shitty.
You could try to power the amp and the charger with different isolated 12V supplies to rule out a ground loop. If the buzz is still there it means the problem is the DC-DC.

If that's the case, you may be able to get away with a 7805 as the Chromecast audio seems to use 1.5W max:
http://forums.slimdevices.com/showthread.php?104383-Chromecast-Audio-ideal-for-Squeezebox-replacement-hardware&s=546a0aa7b8ed9a308765fb6be6099432&p=832361&viewfull=1#post832361
https://productforums.google.com/d/msg/chromecast/61Hpnta5yC8/2xJN-vpxBwAJ
https://productforums.google.com/forum/#!topic/chromecast/NTC-VRU9IeU

So that's 0.3A at 5V: 12V-5V=7V, 7V*0.3A=2.1W to be dissipated at the 7805.

[james2k2]

Thanks for the info. That's a great help. I didn't realize how little power the CC Audio used!

I have ordered a better DC-DC block (very similar in appearance, but features a LOT more protection and features): http://www.ebay.co.uk/itm/252110025993

But if that still fails, then I'll do what you suggested of testing the DC-DC unit on a separate supply. The current unit is actually still in place, but just doesn't power anything. However, I have now noticed that there is a slight hiss coming from the amp where there wasn't before, so my gut is telling me the converter is junk.

As another test I used my phone connected up in place of the CC Audio and found the noise didn't change - so the CC Audio obviously has something to do with it slightly too.