Phantom Power Damage

[benthen]

Hello! I am finally creating my first post after reading the blue room for more than a year.

 

I should know the answer to this question but I just don't know how to word it so your help would be fully appreciated.

 

Does Phantom Power damage passive DI boxes?

 

Now I know that phantom Power cant do much damage to dynamic Mic's as the voltage across pins 2 and 3 it reads 0V but how does this relate to a passive DI box?

 

Say for example an acoustic guitar is plugged into the DI box will this instrument receive any voltage?

 

I personally believe that no damage will take place for the same reason as dynamic microphones and that it is transformer isolated however I need it worded in a style that suggests I know what I'm talking about. (alougth I really should considering what I am doing)

 

Ive searched around for a previous topics and there is plenty that state there being to dangers of phantom power with dynamic microphones I cant find anything completely related to passive DI boxes.

 

Thanks in advance

 

Ben

[dbuckley]

A passive DI box is nothing more than a transformer, and as such is quite happy to sit across phantom power, and power cant get past the transformer, so there wont be any voltage on the instrument jack.

[mikienorth]

However, when using a DI box to go between a big midi rack full of equipment for a keyboard player and a desk running phantom across all channels, be very very wary of keyboard player who doesn't like DI boxes unplugging them and replacing them with Jack/XLR leads to connect to the desk.

 

And not telling you.

 

Ah well, his fault, the DI boxes were there for more than one reason!

[benthen]

could you explain why the power cannot get from one side to the other please.

 

take a step down transformer, 230v in one end 12v out the other. Is that not power going through the transformer, so how does a passive DI transformer differ?

[bruce]

Completely different environment.

 

The "step down" transformer, 230<>12 is for AC. If you apply a DC signal, the output will be zero.

 

In a DI, or any similar transformer isolation, there will be a 1:1 transformer which means that the output will effectively be an isolated replica of the AC component of the input. But any DC offset (which is what phantom power is) will not be replicated on the output.

[benthen]

so in essence the transformer will block any DC signals?

 

If this is the case is it safe to say that devices with a balanced output will quite happily accept phantom power be it a DI box or a banjo with a balanced (self powered) pickup? (note: second example was a random pick out the a hat)

 

Just to make my reasoning for this topic:

 

Having set up some E614s to Mic up a drama on stage the sound op removed all of my set up and instead used some £20 maplin dynamic Mic's. His reasoning was that any phantom power would be dangerous as 48 volts on a DI box wasn't healthy and was dangerous. I tried to explain that it would be blocked, sadly he was not going to listen and this lead to the actors performing what sounded like a mime.

 

After not being to fully explain why it would be blocked, I took it upon myself to learn exactly why. Annoyingly all of my text books are 300 miles away so I had to lean on you guys.

 

Thanks you've been a great help

 

Ben

 

-edit: to make it eaiser to read

[dbuckley]

If this is the case is it safe to say that devices with a balanced output will quite happily accept phantom power be it a DI box or a banjo with a balanced (self powered) pickup?

Sadly, no.

 

Anything with a transformer or a direct untapped coil should be Ok, but some things with balanced outs have electronics powering the output, and then it's down to the designer. Wireless mic receivers should be Ok, as it is entirely reasonable that they'll get plugged into consoles and get phantom shoved their way. Those that were once upon a time unable to take this treatment have subsequently been fixed. Last time this got discussed I seem to have some memory of a Midas desk output not liking phantom applied it's way...

[Chris_R]

so in essence the transformer will block any DC signals?

 

If this is the case is it safe to say that devices with a balanced output will quite happily accept phantom power be it a DI box or a banjo with a balanced (self powered) pickup? (note: second example was a random pick out the a hat)

 

Indeed, for a transformer to work there needs to be a constant change in the magnetic flux, caused by a constant change in the current (AC).

 

In my experience lot of amps with a balanced output will be fine with phantom such as Line 6 and Ashton amps but its always worth checking.

[Modge]

so in essence the transformer will block any DC signals?

Yes.

 

I feel worth being conscise occionally.

[Bean]

think of the signal pins as being at 0v while the earth pin is at -48v (for this example only)

 

no power gets through the transformer as the difference between the signal pins is null.

 

the reality is that both signal pins are at 48v relative to earth and 0v relative to each other. So Passive DI boxes are fine.

 

 

Please correct me if I'm wrong.