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Old 13th April 2007, 08:47 AM   #1
impsick is offline impsick  United States
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Default preamp supply w/ 48v power

hi, can someone please check this layout for me? its for a mic pre with 48v phantom power. My transformer is a toroidal 16.5v - 0 - 16.5v ct.
so im a little confused with the ct in regards to ground and stuff.
thanks.
Click the image to open in full size.


also say i have another tranformer that is dual sec. 8.5v
can i make a supply like the one i made?
what should be done differently?
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Old 13th April 2007, 09:36 AM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
the 317
the 317 needs a cap on the adj pin to bring it's performance up to the already poor regulation of the 78xx/79xx regulators.
The 1uF (or is it 100nF?) on the output may be too far away from the reg pins.
The input pins have no small stability cap. Do you take the chance on stability with the remote caps? or put in a pair of pads just in case?
When the 48V+ supply starts up and the output is sitting at 0v what is the input voltage from the smoothed and rectified supply? Does it exceed the maximum for the regulator?
How do you get +48v from a 16.5Vac or 33Vac CT supply when the 0v is already committed to a dual polarity supply?

the 7815/7915
the input and output caps are too far away from the pins.
the input smoothing 0volt connection should not be commoned with the supposedly quiet 0v going to the pre-amp.
It needs a separate connection going to the audio ground @ the amplifier.

Finally, do you want to consider RC snubbers at the output?
Interference suppression at the AC input?
Is 317/7915/7815 performance good enough for a microphone supply?
How did you post that 176kb pic?
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Old 14th April 2007, 01:44 PM   #3
paulb is offline paulb  Canada
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You should post a schematic. The +48 supply doesn't look like it will work.
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Old 15th April 2007, 05:55 PM   #4
impsick is offline impsick  United States
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hey thanks for the replies. i've been away.
the schema is from esp i'll get it. i've been working on another one though too. ill post that one also
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Old 15th April 2007, 06:01 PM   #5
impsick is offline impsick  United States
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Click the image to open in full size.
ok actually im not sure if this was from esp. but is this schematic wrong? did i not fallow to suit with it? besides the caps on the ac lines.

Click the image to open in full size.
here is another one that im trying to make. is this one correct?


Andrew what do you mean how did i post the pic?
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Old 15th April 2007, 08:52 PM   #6
AndrewT is offline AndrewT  Scotland
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Hi,
the 176kb pic is above the 100kb limit.
What did you do to get the pic to show?
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Old 15th April 2007, 10:28 PM   #7
impsick is offline impsick  United States
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i dont know i believe its a .bmp file. i just uploaded it to photobucket. i didnt know there was a limit.
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Old 16th April 2007, 01:48 AM   #8
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Schemo #1: I would put C6, C7 and C11 on the output side (or way downstream on the pre-amp board ...)

A single common ground point on the PS board ... with a single ground connection to the powered boards & devices ... Lookin' good!

Image below = note that the terminal block (center / top) is where all the grounds come together in the multiple PSUs, ampboards and filter caps. Each amp board also has a single ground connection (bottom / foreground) for +/- power in, signal in & speaker output. (Chassis has its own power ground lug [lower right foreground almost hidden] for line cord power in and completely isolated from all other modules.)
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Old 16th April 2007, 03:20 AM   #9
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thanks eddie! hey i got that pre amp down sized to 3"x4". i'm putting it together now. i'll post a pic on the other thread.
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Old 22nd April 2007, 10:37 PM   #10
impsick is offline impsick  United States
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how is this? im not sure if i got the phantom power section right. i think its like a voltage doubler or something. i used the schematic from the Green Pre power supply but used 78,79xx's instead of 317,337.
Click the image to open in full size.
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