Discharging Caps and Wiring Transformer for my LM3886 Amp...

[mydementia]

Hi Guys.
I've been highly distracted with all the new stompboxes over on Aron's forum, but am finally getting back to this project.  I soldered up my chipamp.com boards last night - what a joy!  I bought the mono kit from Brian a couple months ago... very professional looking. 

So...
1) I bought this transformer from PartsExpress following this forum (and Brian at chipamp.com) recommendations:
http://www.partsexpress.com/pe/showdetl.cfm?&Partnumber=122-615
I'm still not satisfied with my enclosure - but really want to put this beast together.  I want to build it up on a wood block just to get it all working... so my question here is about grounding... Am I creating my 'star' grounding from the 'ground' pole of my AC Jack? 

2) The 10000uF caps on the power board scare me... Do I need to treat them as high-voltage power caps (using a 5W, 10ohm resistor to the positive lead with the other end clipped to ground) - or are they more innocent than I think?  I just don't want to get dead...   Other 'drainage' techniques appreciated...

Thanks for looking.
Mike

[joecool85]

Star grounding works excellently.  As far as the 10k uF caps, they could hurt you, but you don't have to worry about draining them.  The boards are set up with a resistor connecting the two pins of the caps so that when there is no electricity flowing they drain themselves.  They drain in about 1 minute or so.  If you are worried, just check the voltage at the caps before doing anything.  Better to be safe than sorry.

[R.G.]

Signal ground and AC safety ground are different things. AC safety ground (on the AC input jack) should be connected to the metal chassis you have this thing in. Signal ground should be connected to the input jacks and to other signal ground places in the circuit and to the "ground" point in your power supply. There should be 1.00000 connections between the signal ground and safety ground. Construct your chassis, hook AC safety ground to it using proper safety grounding techniques, then run a wire from your signal ground star point to the AC safety ground.

The 10kuF caps are charged to about 30V each, so they're not really a hazardous voltage. They are a hazardous *current* though. No rings or watches on your hands when you reach in. Bridge a 10kuF cap charged to 30V with a gold ring and you just might get a third degree burn all around your finger, not to mention welding the ring to the cap.

If it were mine, I'd put about a half-watt of DC bleeder resistor across each one, then you could rely on the resistor to always run the caps down when the AC power is off. Then you can just turn off the power, wait a minute, and the caps are drained down, even if the circuit didn't drain them down - which it probably did.

Stay away from that AC power line voltage, though. That will kill you dead.

[joecool85]

The LM3886 kits from Brian come with resistors and spots on the PCBs for bleeding the caps.  Assuming he put the kit together properly, it's a pretty safe kit.

You did raise a good point about current as opposed to voltage on those caps though.

[teemuk]

I was about to answer this but my computer crashed. In short, separate power and signal common returns and have them connect only at the common point. To save me from writing same things over and over again see:
http://www.ssguitar.com/index.php?topic=288.msg2219#msg2219

Another thing about the caps: Sudden, fast discharges (i.e. shorting with a screwdriver to chassis, slip of probe etc.) are also bad because this might damage the semiconductors as well. Bleeder resistors are a good idea - some might argue they waste power, though: To add some more functionality to them I use a bleeder that is combination of power indicator LED and it's current limiter resistor.

[mydementia]

Thanks guys - I DID put in the resistors across the big caps... now I know why - Thanks!!
teemuk - looks like a good read... more than I should do at work.
I'm going to crawl back into my hole and read tonight... I'll be back...
Mike