portable phantom power

ecc83

Well-known member
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.
 

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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?
 
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.
 
Personally I would make the voltage higher as the gain of the mic would be better. Maybe not 48Vdc but closer up. Pin 1 being ground would be to the box case no? 9V batteries would have a limited length of run as they are too small- try using the 3.7V lithium cells that could be combined to make any multiple of that- 8 would give you around 30Vdc.
The idea with Lithium is that they can be recharged and they have a much longer discharge time than 9 V batts.
 
Personally I would make the voltage higher as the gain of the mic would be better. Maybe not 48Vdc but closer up. Pin 1 being ground would be to the box case no? 9V batteries would have a limited length of run as they are too small- try using the 3.7V lithium cells that could be combined to make any multiple of that- 8 would give you around 30Vdc.
The idea with Lithium is that they can be recharged and they have a much longer discharge time than 9 V batts.
Yes, rinse and repeat, many variations are possible. If I were building one such for myself I would use a DC-DC converter to get 48V (as per just about every AI) and use a 12V Lithium or similar.

Where and how to ground the tin is moot. Such portable kit can touch other metalwork and a loop established. Probably wise to fit a ground lift switch shunted by 1nF 'RF stopper' and a static bleed R? Just wanted to K.I.S.Sir.

Dave.
 
Pin 1 = the case? You'd think so but would probably get hung drawn and quartered on some forums. Practically, it makes sense but the electrical ground is often a different voltage from the audio 'technical' earth in the studio. Often called the Pin 1 problem. One device with it's case connected to ground can inflict noise on other items in the chain not designed with this in mind. It's like the linking tag on XLRs - should the metal plug be connected through this tag to Pin 1? Most people would say absolutely not. In practice, sometimes it makes things apparently better - but it's a great way to get ground loop current into your system if you ever go somewhere where ground potential is different from the local ground. Long extension cables, and big buildings can often have the mains earth at two points a few volts apart - and that is current having to go somewhere.
 
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