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Thread: portable phantom power

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    portable phantom power

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    Having a bit of a clearout and came across the attached drawing.

    The idea is for an 'emergency' phantom power supply or one that can be used for kit that has none, e.g. many small PA speakers have XLR mic ins but no spook power. Many Acoustic guitar amps are similarly lacking. It also has the advantage of being TOTALLY quiet and ripple free.

    "BUT" You say "eighteen volts?" Well yes, two PP3s, many modern cap' mics will run quite happily on just 12V. Of course you can multiply the battery count ad. n. You could use 12V 'camera' batteries. The unit only draws power when a mic that takes phantom power is plugged in. The switch "sw ?" is an option, not really needed.

    The Bits!
    R1, R2 are the 6k8 feed resistors. Buy 10 and match them on a DMM. The closer the match the better is the Common Mode Rejection Ratio. Same goes for C1C2, 47mfd 22V or higher and could be 100mfd or bigger. If you can match pairs. (N.B. the matching is lily gilding a bit, 1% Metal Films are good enough but since almost no budget pre will hand match critical components, might as well get the best CMRR you can for next to nothing?)

    R3 R4 are just 'tie down' Rs of about 22k+ and 1% MF is fine.

    The case ------ is shown 'DC' bonded to pin one. You might find it better to 'float' it but if you do, put a 100nF cap from P1 to case.

    Dave.
    Attached Files Attached Files

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    Nice one, Dave!
    Looks like you bonded case to pin three by mistake?

    For the idiots reading...I get that r1+2 are your DC current limiting, and I guess c1+2 are there to protect gear beyond the output.
    What is the purpose, and effect, of r3+4?
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  3. #3
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    Quote Originally Posted by Steenamaroo View Post
    Nice one, Dave!
    Looks like you bonded case to pin three by mistake?

    For the idiots reading...I get that r1+2 are your DC current limiting, and I guess c1+2 are there to protect gear beyond the output.
    What is the purpose, and effect, of r3+4?
    Ah! You are first to spot the delib...No! I 'ked up. In fact, as I found it the circuit did not have the case indicated and so I added that as an afterthought..and connected it to the wrong pin!

    Yes, R1&2 are the 6k8 feeds you always have in a phantom source. Yes the caps keep DC off the pre amp because if it does not have phantom power it will not like it up it! R3 and 4 drain the charge that would otherwise build up on the output side of the caps.

    Dave.

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    Ah, ok. Thanks for that.
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