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hax 27th February 2013 11:19 AM

Lepai LP-V3 Killed my Tablet!
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.

theAnonymous1 27th February 2013 04:28 PM

You connected the speaker grounds and input grounds together? Well there's your problem then.

The TA2020 amplifier IC has bridged outputs, which means the negative speaker output is not at ground potential. You should never connect a bridged negative output to ground or even to the other channels negative output.

hax 27th February 2013 04:42 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.

theAnonymous1 27th February 2013 04:57 PM

The TDA8566 is still a bridged output amplifier, so the same rules apply.

There should not be a direct connection/path from the RCA ground to the speaker negative outputs. If there is, then I'm stumped.

hax 27th February 2013 05:09 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.

theAnonymous1 27th February 2013 05:18 PM

A single supply bridged amplifier will have Vcc/2 on both the positive and negative outputs. That means that if the amp was powered by a 12VDC supply, the speaker outputs (both + and -) will be at 6VDC when referenced to ground.

In a nutshell, the amp sent 6VDC into the tablets audio output. :/

hax 27th February 2013 05:23 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 -.-

theAnonymous1 27th February 2013 05:45 PM

We all make mistakes, you just have to learn from them. I've killed hundreds of dollars worth of parts and equiptment from stupid mistakes.

hax 27th February 2013 06:06 PM

About going to mono,my plan was to feed the L/R into a summing op amp then split it into the L/R input, would that work ok?

Saturnus 27th February 2013 06:11 PM

EDIT: Oh. It didn't. Ok, common sense restored, there are after all no such thing as miracles.

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