Lepai LP-V3 Killed my Tablet!

hax

Member
2013-02-27 12:09 pm
Right, I got one of those "700w" amps ages ago with a view to making a portable system for flat parties/outdoors, recently I got it out, hooked up two Creative satellites from a surround system.

Worked fine with a wall plug but then I plugged it into an AT psu to check if the plug was limiting it due to lack of current.

Then I plugged in the tablet; loud distorted static started coming out of the tablet speaker instantly, rebooted it and the left channel only outputed a few of the peaks but extremely distorted and now it won't turn on. The battery and ARM chip get warm if plugged in but other than that it's useless. Not too bothered as it was a £33 one from china, but how can I stop this from happening again or with other peoples devices? Was hoping to use it for a party system where anyone can plug in their device, but I really don't want to be risking frying an iPhone or similar expensive audio source.

Should the input and output grounds be isolated/seperated? I did do a little mod to it in order to use the RCA jacks as outputs and didn't bother rewiring the ground as they seemed to have continuity between them when I checked. Obviously I disconnected the signal posts from the board though, then wired them to the back of the real output's cable clamps.

I suspect it was something to do with grounding/floating issues. Quite worrying as I use that AT PSU to supply power to all my USB devices!

I was thinking some sort of pre/opamp to re-isolate the aux in jack?

If I did this, would there be a way to also normalise the input signals coming in so that messing with the gain pot on the front doesn't/cannot lead to distortion?


Also, how would I go about linking the channels to be used as a mono output? I've found the best placement of the speakers to be on top of each other, angled at about 10 degrees vertically.
 
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hax

Member
2013-02-27 12:09 pm
Ah crap, but there is direct connectivity on the board between the two?!

This particular model I've since found out is an ultracheap knockoff (haha as if such a thing as possible) and uses a TDA8566 chip instead.

Basically I just wanted to use the RCA jacks as out, and aux in for in. I left the rca ground connected cause there is a direct trace between that and the speaker ground posts.

All was fine till I used the bloody AT psu thinking it would be more reliable/safe than a 2A 12v adapter which came with a external drive case.
 

hax

Member
2013-02-27 12:09 pm
Ill have to open it again and fix it up tomorrow, I thought I'd properly checked but could well have messed up.

Does that mean that the ground connection presented to my tablet's audio out would have been a polarity reversed signal of the audio at a few amps? Ouch.

I still can't see why this didn't happen with the wall wart though. Had stress tested it for 3 hours or so right before this.


Also, about the limiting/normalising op amp, any hints on what kind of circuit I'd need to build.

As for the mono linking, I guess that wouldn't be possible with a bridged amp.
edit: actually I guess i could just build a summing op amp to process it to mono before the input.
 
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hax

Member
2013-02-27 12:09 pm
It did struggle along for an hour or so before it refused to turn back on lol, ah well lesson learnt not to mess with circuits I don't have full knowledge of. Only 30 quid, just gotta wait another month for a new one to be shipped from china!

Was using an led, resistor and battery as a continuity tester which obviously gave a false positive -.-
 

hax

Member
2013-02-27 12:09 pm
EDIT: Oh. It didn't. Ok, common sense restored, there are after all no such thing as miracles.

I'm intrigued now, even if it is impossible!

This thing gets ridiculously hot though, might have to get longer bolts and add an old PSU heatsink to the case.

is this what I want?
[IMGDEAD]https://www.rane.com/n109fig2.gif[/IMGDEAD]
On the rane why-not-wye page, which seems pretty helpful for this http://www.rane.com/note109.html

What sort of resistors would I be looking at using?

Can't work it out cause I'd be going from stereo headphone level to two mono at headphone/aux or whatever the ideal for this amp is.


Still gonna use the RCA's for output, once I've properly isolated and wired them that is!


You can probably tell I'm quite new to the DIY aspect of this. XD
 
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