Can Phantom Power be bad?

VellaJ

So we've just acquired a mixing desk, with the view to eventually build up a complete PA system for the band. I have a question about Phantom Power though...

I've just bought myself a DI box for dirt cheap at a clearance sale. Now this thing can use phantom power from the mixing desk, to save on batteries. Great, but I also have a Shure SM-57, for mic'ing up amps etc. I'm guessing the following question applies to any dynamic mic though; Is using a dynamic mic while using phantom power detrimental to any of the equipment in any way? The info on the desk says it has "Phantom power for studio condenser mics."

Basically, my question is: Can I use the phantom power from the desk to run the DI box, while using dynamic mics into the other inputs?

Gearhead

Yes. The phantom power works by putting a DC voltage on top of the mic's AC signal. They get separated on the desk side with a capacitor in both the hot and the cold, which are in the signal path regardless of whether the DC voltage is there or not. On the mic side, the design usually takes into account that if a DC voltage is present, it does not harm the mic.
If you actually would have read the SM-57 manual, you would have been aware of this ?

VellaJ

Gearhead wrote: Yes. The phantom power works by putting a DC voltage on top of the mic's AC signal. They get separated on the desk side with a capacitor in both the hot and the cold, which are in the signal path regardless of whether the DC voltage is there or not. On the mic side, the design usually takes into account that if a DC voltage is present, it does not harm the mic.
If you actually would have read the SM-57 manual, you would have been aware of this ?

Thanks GH. Yeah when I got the mic I was a typical man. "Man not need manuals. Man know how to work this. Manuals make Man unhappy." Now I can't find the manual... :-[

Gearhead

I am usually the same way but after buying a TCE G-Force, a MB Triaxis and a Behringer FCB1010 this Man is almost nowhere without his MANuals. How did they even think one could work this gear without?
So I went one day and read the other paperwork as well. More interesting than the phone book, but otherwise...

Riaan-Combrink

OK, GH - so having the phantom power switch on can't harm the mics and instruments connected to the PA (or the PA itself) in any way? This is contrary to what I was taught (by word of mouth I must say - I have minimum technical savvy :-[).

VellaJ's thread may have solved a conundrum I recently had. I played at a venue that had a lovely PA set up. I plugged the cables from the respective multifx units I use for my two guitars into the two ins of the DI box marked accordingly, but couldn't for the life of me get any guitar sound over the PA. When I plugged straight into the two channels on the mixer, I had guitar sound.

Now I had never used either phantom power or a DI box, and don't understand either. What I did notice when I arrived was that the phantom power switch on the mixer was on. Based on what I had been taught (basically to expect everything connected to the mixer and all speakers, etc to spontaneously explode because phantom power was on), I switched the phantom power off. Is this perhaps why the DI didn't work? Is it possible the phantom power was running the DI, and I'd essentially tuned off the power supply to the DI?

VellaJ

Riaan C wrote: OK, GH - so having the phantom power switch on can't harm the mics and instruments connected to the PA (or the PA itself) in any way? This is contrary to what I was taught (by word of mouth I must say - I have minimum technical savvy :-[).

I'll have a go with this one ? What DI box were you using? The box I have is active, so requires power, either from a battery or from the phantom power from the desk. No power, no sound.

Having done a bit of Googling, the rule seems to be:
Dynamic mics aren't harmed by PP
Condenser mics require PP to work
Old ribbon mics will spontaneously explode with PP, but modern ones are protected against it...

Riaan-Combrink

I'm not sure whether DI was battery powered or not. But I'm pretty sure that was the problem. Thanks!

AlanRatcliffe

VellaJ wrote: Old ribbon mics will spontaneously explode with PP, but modern ones are protected against it...

Yup. Some old ribbons can be harmed by it, but generally anything else is safe.

Johnny-B

I had an instance once when I plugged my LR Baggs Para DI box in to a Yamaha portable powered mixer and it killed the battery on the DI box although it didn't damage the DI box itself. I think the problem was that the Baggs DI requires 48V phantom power and the Yamaha mixer pushed out less than 30V. I suspect that the phantom power switches a relay in the DI box to disconnect the battery and connect the externally supplied phantom power and the voltage was too low to switch the relay so I ended up connecting the phantom power to the 9V non-rechargable battery.
Having said that I connect the Baggs DI through the church house system with phantom power from the Yamaha digital mixer and I've never had a problem with it. I suggest that you just check the phantom power voltage specs on the desk and the DI box to check that they are compatible.

TomCat

Essentially Phantom power is 48Vdc across both pins 2 and 3 of the XLR connector with respect to earth (Pin-1). This means that if you measure across pin 2 and 3 with a multi-meter there will be no DC voltage present as both pins are +48V. The 48V is between either pin 2 or 3 to pin-1.

Microphone capsules are connected between pin-2 and pin-3 so the capsule essentially does not see the 48V, hence no hard comes to the mic capsule for either condenser or dynamic microphones.

DI boxes work essentially the same way with most of them having an output transformer with the secondaries connected across pin-2 and pin-3.

The danger of damaging the mixer's pre-amp comes in when an unbalanced mic cable is plugged into the mic input of the mixer and pin-3 gets shorted to pin-1 through the unbalanced wiring of the cable.... ? ?....better quality mixers use 6k8 DC buffer resistors on the phantom power feed to each of the input connectors of the mixer to prevent dead shorts to the power supply...This protects the power supply to a degree but the transients created can damage the pre-amp though.....