almost....

[purpletheory]

Hey guys,

I almost have my amp done, though i've run into something wierd. I wired everything up, but when I flipped on the switch the lamp came on for a second and burnt out. I thought maybe the lamp was defective, so I replaced it, now it powers on fine, it's just that I noticed that when the power cable is connected and the switch is turned off, the lamp lights up ever so slightly. I tested it and found out that 11v are mysteriously powering it, and I can't for the life of me figure out where it's coming from. Don't know if the part about the lamp burning out is of any significance, just thought i would mention it just in case. Any ideas?

[joecool85]

By lamp what do you mean?  Are you doing the LM3886 and it is the blue LED on the snubberized PSU board?  Or something else?

[teemuk]

It would be helpful to know what kind of circuit you are building - without that information it's pretty difficult to help you at all. Also your description about the problem's symptoms leave me a bit clueless... I didn't quite understand is the amp operating normally or is there a fault condition. I gathered the lamp has a slight dim while the circuit's power is switched off? That could be caused by reservoir capacitance draining through the lamp circuit. Does the voltage drop eventually?

[purpletheory]

Oops sorry, here's the PS: http://sound.westhost.com/project04.htm
and I built two of these: http://www.generalguitargadgets.com/pdf/ggg_lm3886_amp.pdf
The lamp is 120v connected directly to the switch along with the mains power.

I kinda think that it's not stored charge from the caps because the lamp only lights up when the power cord is connected and disappears as soon as it is disconnected.

thanks,
matt

[teemuk]

I gather the lamp is connected between the Line and Neutral and you use a douple pole-single throw switch shown in the schematic? (This is at least what you're supposed to do). If the switch is off then, following the PS schematic, it should indeed switch off both L and N and no current at all should flow there. (This happens at least when you remove the coord completely). In this case the lamp should not function - even slightly. Is the lamp one of those that are integrated to the switch itself? If yes, then it's hard to say anything about them.

Based on this information I'd guess the switch is faulty: Normally it should have nearly an infinite resistance between it's contacts when open (a resistance of air to be exact); I guess you could check this with an ohm-meter to make sure. You have to disconnect the switch from the circuit to make a proper measurement though. There might have been some arcing that carbonized the switching mechanism causing it to constantly leak a little current - thus powering the lamp as well. There will still be a resistance high enough to drop the voltage to 11V but this one is no near infinite. If so, then change the switch: It's not a reliable On/Off switch anymore.