Boom, amp goes up in smoke!

[Jonkan]

So I had my truepath up and running yesterday with unbalanced inputs.

I then hook up my input transformers and result is that the sound is very low and faint without bass.

I double check the wiring according to the transformer data sheet. I then think, "what if its a grounding problem?"

So i connect pin1 on xlr to AGND on truepath input and power up.

BIG SPARKS FLYING EVERYWHERE!!!!

What the hell just happened, and why???

Doesnt truepath want to have a GND reference? Or do you think it was something else?

The chip is fried, one of the legs broke off. And also the ferrite that connects AGND and Digital GND is totally melted.

Please help me, and more important, help me realise what I did wrong so that I dont repeat my mistake after I can get a new chip+ferrite.

/J

[krilli]

:'(

I'm sorry to hear. I wasn't happy when I melted an Amp11 - I can only imagine your frustration.

Just to ask some dumb questions in the hope of shedding some light:

What DC voltage is at XLR pin1? Referenced to the other pins, for instance.

Or could it be that the -2.5V input bias voltage in the Tripath had a path to AGND?

One more: What are the XLR pins galvanically connected to?)

[Jonkan]

Dc voltage on pin1 should be zero, since they are connected to chassis+safety ground.

The input capacitors where still in place, so input bias voltage should have been blocked.

I had it connected this way:

Xlr pin1 and cable shield to chassis/safety ground.

Xlr pin2 to transformer primary +

Xlr pin3 to transformer primary -

Transformer secondary + to truepath input.
Transformer secondary - to truepath AGND.

It was when i connected the xlr pin1/safety ground to AGND that the sparks started flying.

I just hope that the pcb isnt damaged. Theres a small indentation in the pcb just under where the ferrite used to be before it melted. I can see some tracks inside the pcb where the indentation is. I hope nothing has melted inside the board.


Oh, and it seems the pin on the ta3020 chip that has been most damaged (it fell off!) is pin 39 VNN.
/J

[krilli]

OK I think the ferrite bead is the only place where a connection is allowed between chassis/power ground and AGND. This is something off the top of my head though.

[Jonkan]

Ok. I wish i had known that before...

Im not sure how to connect the tranformer then. Any ideas?

/J

[krilli]

BIG, IMPORTANT DISCLAIMER:
I haven't worked with input transformers! So I don't know how they work!

But ... I'm not sure about this:
"Transformer secondary - to truepath AGND."

Doesn't the secondary-neg leg on the transformer carry an inverse audio signal? I would think that a center tap or something like that was to be connected to AGND.

[Jonkan]

Youre right I think.
The signal that comes out on the secondary side is normal unbalanced signal+gnd. The input side is +/- and Gnd.

I think i hooked up the transformer the wrong way round somehow. Its really not an input transformer, but an output transformer. Im not sure that it matters, but I forgot to connect it backwards to use it as an input.

The main problem being that the primaries were connected in paralell, and the secondaries in series. I think its supposed to be the other way round.

I wish i had thought of that before i blew up the amp... Feel like shit now. I just hope its fixable. I dont want to have to buy a whole new truepath kit.

Well, atleast the psu:s all still work.

/J

[audiophool]

quote:Originally posted by Jonkan


...
So i connect pin1 on xlr to AGND on truepath input and power up.

BIG SPARKS FLYING EVERYWHERE!!!!

What the hell just happened, and why???

/J

Read the Truepath documentation. AGND and GND are supposed to be floating and connected together only by the ferrite. The ferrite is a low current component.

You connected AGND to chassis earth. There was a potential difference between GND and chassis earth. Voila, your ferrite popped.

Why was there a potential difference between GND and chassis earth?

Can you provide us with more info on how you wired up your power supply? A wiring diagram, some photos, perhaps?

[krilli]

Because I was guessing at stuff I don't know, I just want to say +1 to Audiophool's instructions; Even with my limited knowledge, it is immediately obvious to me that he's on to something.

And it's usually fixable. I'd guess that it takes a hotwire to fix the GND<>AGND trace, and a new chip.

[Jonkan]

The psu is wired exactly like this picture: http://www.maedos.com/Images/AI/AI_image024.jpg

The only connection to safety ground/chassis is done at the IEC for the psu.

Should i connect main power transformer Centertap to chassis aswell?

The xlr pin1 goes directly to chassis according the rane pin1 standard.

How likely is it that i need to build a whole new truepath module? If the pcb has been damaged perhaps? Its very hard to tell on these 4 layer boards.