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RSRecords

Well-known member
Joined
Jun 8, 2009
Messages
320
Hey, tying to teach myself eagle. Just simple stuff for extender cards, guitar pedal, etc...

I drew up this little pcb for adding phantom power to a mic pre. The bottom layer is all ground plane.  I did a copper pour on the top but I don't know if that sort of thing is necessary.

If anybody feels like looking it over and giving some feedback it'd be much appreciated.
 

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You making them yourself? If so, perhaps increase the polygon pour isolation a wee bit, just to be safe; the cap pads look awfully close to mask. 

Minor adjustment: Perhaps give PAD2 a bit more room away from the neutrik? 
 
oh man, never get a notification.

I redrew it a little bit with better clearances and couple things moved about. I didn't allow for the thickness of the front panel so cut down on some of the area under the xlr.

Quick question: Is there a standard hole to hole measurement for a 1/4 watt resistor?  I see that they are ~6.3mm long.

Thanks,
 

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"for a 1/4 watt resistor"
consider to recalculate the rating needed of R1 and R2 by applying ohm's law, just in case a shorted mic cable is connected with phantom power activated, IE  P=U2/R when you fill in 48 for U and 6800 for R ...
 
ruffrecords said:
The standard through hole quatrer watt resistor footprint is known as RC05.

Cheers

Ian

Awesome thanks!

Harpo said:
"for a 1/4 watt resistor"
consider to recalculate the rating needed of R1 and R2 by applying ohm's law, just in case a shorted mic cable is connected with phantom power activated, IE  P=U2/R when you fill in 48 for U and 6800 for R ...

Well damn...that's a good point--.339 watts.  I will upgrade to 1/2watts to be safe.

I received the rev A1 boards from dirty pcb.com.

I'm getting 48v on the output to the mic pre. I'm not sure why though. Any thoughts?

here's the schematic I based the pcb off of. I've omitted the zeners for now.

 

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pvision said:
You're measuring at the input not the output?

Nick Froome

Ha, if only.
Measuring on the mic pre side behind the 22uf cap. ~48vdc between each leg and ground.
 
I used the bottom ground plane to tie pin 1 to the power gnd. Is that bad practice? I’m kind of baffled by this. Its such a simple circuit without the diodes but there’s +48 on both sides of the blocking caps. :-\
 
RSRecords said:
I used the bottom ground plane to tie pin 1 to the power gnd. Is that bad practice? I’m kind of baffled by this. Its such a simple circuit without the diodes but there’s +48 on both sides of the blocking caps. :-\

For general grounding goo practice check this out:

https://groupdiy.com/index.php?topic=68869.0

Without the diodes the is no dc path for the caps to charge up through so the voltage across the cap will remain zero so both sides will be at 48V. Connect resistors from the output side of each cap to ground and they will charge up. Make them quite large, say 47K, so the do not load the input and you will see the caps charge up and the output side will drop down to 0V.

Cheers

Ian
 
ruffrecords said:
For general grounding goo practice check this out:

https://groupdiy.com/index.php?topic=68869.0

Without the diodes the is no dc path for the caps to charge up through so the voltage across the cap will remain zero so both sides will be at 48V. Connect resistors from the output side of each cap to ground and they will charge up. Make them quite large, say 47K, so the do not load the input and you will see the caps charge up and the output side will drop down to 0V.

Cheers

Ian

Oh duh! Here I am looking at the pcb for an issue. Thanks for you help. That's why this forum rocks.

 
"That's why this forum rocks. "  It's like a handful of extremely competent and generous people that do 95% of the heavy lifting.  I feel somewhat compelled to start a patron page they can dip into at will.  I wouldn't have a clue how to enforce fairness or funds withdraw criteria...
 

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