Anyone know owt about pahntom power?

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rojash | 11:26 Fri 25th Mar 2011 | Electronics
6 Answers
In our band, we don't use any back line amps. All the guitars go straight to the desk via Amp/Cab modellers.
I was thinking about making some stage boxes, and wondering if there was a way I could use the phantom power from the desk to power the FX units.

Mine is the most complicated setup. I have:
Two guitars, connected: Compressor->Amp/Cab Modeller->unbalanced to desk.
One Bass, connected: Amp/Cab modeller->unbalanced to desk
One bouzouki, connected: DI Box->Balanced to desk
Dynamic Mic, connected: balanced to desk.

The DI box for the bouzouki uses phantom power from the desk over the balanced line.

I'm wondering if there's any way that I can take the (unused) phantom power from the Mic line, and drop it to 9 volts to power the FX and modellers without (a) compromising the Mic signal and (b) blowing up the desk.


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Question Author
Should have mentioned, the total requirement for all FX units is 900mA at 9V
Been a long time since I've played with mixers, but I thought phantom power normally only supplied very low current to run mics and a quick check seems to indicate the standard for it only mentions upto 10mA.... are you sure the desk can supply 900mA ?
Question Author
"are you sure the desk can supply 900mA ? "
No idea, mate, that's why I was asking :-)

Seems such a shame to have all these cables around with 48 volts on them and have to run mains leads out to power 9 volt adaptors for the FX. The other plan was to use a single big unregulated PSU at around 12 volts. Send that to each of the stage boxes and regulate it to 9 volts at that end.

I'm trying to get to a position where the roadie is just responsible for the connections at the desk end, and rolling out a single snake to each musician. Each musician would then be responsible for connecting his own Mic, FX, and instruments at the stage box end.
Powering multiple devices with the one supply could run into ground level mismatch problems on different equipment. One terminal of the power supply input is usually connected to the signal ground and the choice could be arbitrary for the manufacturer.

Running from a plug pack transformer isolates the the power ground for each piece of the equipment from the others. Without it there is the possibility of shorting the power supply through the signal ground. Even if all your gear is say outside positive you never know when someone will come along with a different unit and you will have forgotten the restriction.

Note that this is an entirely different from the loop problems fixed with ground lift often found on this kind of gear. Moreover you may also introduce ground loops if each supply is not isolated.

The safest bet is to power each individual unit using a separate isolated power supply. These are available with 48 volt inputs typically operating from 36 Volts and sometime up to as much as 72 volts.

The input current is much lower with 48 volts than 12 volts and will allow the power to be taken long distances on small gauge wires. It also means supplying less than a third of the current needed for a 12 volt distribution. If you tried to use a linear regulator to get 9V from a 48 volt supply you would be faced with a substantial amount of heat in each regulator.

Lets say each FX unit is five Watts so is going to pull less than 100mA from the phantom supply. This might not run from the phantom power for a single line on your mixer. You should be able to find this spec from the manufacturer.

Alternatively, inject a more powerful phantom supply externally.

Failing that
Failing that , a separate very small figure eight twin will do the job. The wire size required is limited by durability rather than conductivity.

The output voltage of the converters in the link I provided are not available at 9 volts but there are other types from different manufacturers with adjustable output voltages.

On the other hand following the 12 volt converter with a linear regulator would ensure any noise in the switch mode unit was thoroughly cleaned. Ideally check which side of the FX unit supply is at signal ground and regulate the other using either the negative (7909) or positive (7809) linear regulator as required. This keeps it all with the same ground.

The FX unit probably has regulators inside. I expect most nine volt equipment will be quite happy with a maximum of twelve volts. The regulated output from the converter won't suffer the low load over-voltages seen in most transformer power supplies. Many nine volt supplies will expose the equipment to twelve volts at low load anyway Impossible to be sure if it is safe without knowing the design of the equipment.

Ideally get hold of the isolated switchmodes with the nine volt or adjustable output.
Question Author
Thanks for the advice.

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