Topic: cable to block phantom?

[Brian Skalinder]

I'm wondering if a cable or adapter or box or some such exists that will block phantom power?  (preferably small as possible, with XLR connectors)

I'd like to run the occasional 3-mic mix + a 4th SBD channel.  However, the R4 only provides paired controls for turning phantom on/off.  In order to use the 3rd mic, I need the 3rd channel's phantom ON, which would turn phantom ON for the 4th channel I'd use for the SBD feed.  But I don't want to send phantom to the SBD, I assume.  Looking for something I can insert in-line with the channel that's receiving the SBD signal to block phantom from hitting the SBD.

Any ideas?

[Shawn]

brian I was thinking of the same thing and didn't even consider that. I guess if something like that doesn't exist then I could always just get a seperate phantom power box for the third mic.

[Church-Audio]

You should be able to use a dc blocking cap with a value of 10uf or 47uf at 100volts in series with pins 2/3 to block it out. You should try and find some nice Mylar caps, the only problem is they will be huge. You might have to make an in line box, using a nice steel Hammond project box. They make some that are very small just big enough to mount a panel mounted XLR on. If you don't want to do this your self I can build it for the cost of parts.


Chris Church


Here is an interesting idea from Audio upgrades.
This has some interesting info on caps and what types to use and why.
This was taken from a website link http://www.allfouraudio.com/Capacitors-in-the-signal-chain-817726x1307.htm

"Here's $.04.
Capacitors make a huge difference. Anyone versed in direct coupled gear
would know this. All electrolytic caps dampen transients. Their
dialectric absorbtion converts small signals into heat. This is a
filter effect.
You don't need to put huge Polyprop or mylar film caps in there as
large film caps are also slow due to their size. Bypassing the
electrolytic caps with small value high quality films overcomes the
losses presented by the electrolytics.
Good choices include WIMA from Germany or the exotics like MIT's,
InfiniCaps, etc.
For an experiment, try this: Replace the 47 uf phantom blocking caps
with either Rubycon Black Gates, Z series, Panasonic FM or Nichicon HE
caps. Install WIMA MKP-02 .01 uf, 250 volt polypropylene film caps
across them.
Report back.

Jim Williams
Audio Upgrades"

[BayTaynt3d]

I'm thinking a DI will do it. Heh...

I don't know that much about this kind of thing, but many (if not all) DI boxes block phantom coming from the snake so as not to fry instruments. Thing is, I think many DI's can take a line-in signal right? So, couldn't you go SBD > 1/4" > DI > XLR > R4 w/ phantom? It's just like taking an instrument in, but the source is the soundboard line-outs? Does anyone know if that would work? B/c if so, that'd be a hella-easy solution...

[cmoorevt]

Could you use an unbalanced cable?

[Church-Audio]

I'm thinking a DI will do it. Heh...

I don't know that much about this kind of thing, but many (if not all) DI boxes block phantom coming from the snake so as not to fry instruments. Thing is, I think many DI's can take a line-in signal right? So, couldn't you go SBD > 1/4" > DI > XLR > R4 w/ phantom? It's just like taking an instrument in, but the source is the soundboard line-outs? Does anyone know if that would work? B/c if so, that'd be a hella-easy solution...

A dI that  uses a transformer will do it only one problem the dI'S input has a transformer in it that will degrade the sound quality, unless it is a very good dI. Capacitors on the other hand when well selected will not.

Chris Church

[Church-Audio]

Could you use an unbalanced cable?

No its never a good idea to try and unbalance a balanced line with phantom you would end up shorting out the phantom supply and at the very least blowing the fuse for the +48 on the preamp.

[SparkE!]

The input impedance of the R4 is 4kohms.  In order to get down to 20 Hz (3 dB point), you'd need about a 2 uF cap.  If you blocked both the + and - sides of the signal, you'd need twice that value.  The R4 itself claims a 20 Hz lower corner frequency and if you did not want to degrade that significantly, you'd want to at least double the value of the caps again.  So, Chris is right about the 10 uF value for the caps and you'd need 2 of them per channel.

[Chuck]

If you want to, you could bypass the (4) 10 uF caps with some .1 uF film caps too.

[Church-Audio]

If you want to, you could bypass the (4) 10 uF caps with some .1 uF film caps too.

.1 is too low and would act as a high pass filter in this circuit. I picked 10uf because it would allow everything to come thru the lower the value the more roll off you get.

[SparkE!]

If you want to, you could bypass the (4) 10 uF caps with some .1 uF film caps too.

.1 is too low and would act as a high pass filter in this circuit. I picked 10uf because it would allow everything to come thru the lower the value the more roll off you get.

Chris, Chuck is right about this.  It is a good idea to put .1 uF caps in parallel with the 10uF caps in order to reduce the effect of dielectric absorption.  While we're on the topic of dielectric absorption, it's also a good idea to use non-polarized versions for the 10 uF caps.  If you can't find non-polarized versions, you can make your own by putting two 20 uF polarized caps in series with their negative terminals connected together and their positive terminals as the leads of the resulting non-polarized 10 uF cap.

[Church-Audio]

If you want to, you could bypass the (4) 10 uF caps with some .1 uF film caps too.

.1 is too low and would act as a high pass filter in this circuit. I picked 10uf because it would allow everything to come thru the lower the value the more roll off you get.

Chris, Chuck is right about this.  It is a good idea to put .1 uF caps in parallel with the 10uF caps in order to reduce the effect of dielectric absorption.  While we're on the topic of dielectric absorption, it's also a good idea to use non-polarized versions for the 10 uF caps.  If you can't find non-polarized versions, you can make your own by putting two 20 uF polarized caps in series with their negative terminals connected together and their positive terminals as the leads of the resulting non-polarized 10 uF cap.

Yes I misread what chuck was saying. He meant bypass with these caps not remove and use these .1 instead hehe. the .1's by them selves would be a high pass filter.

[BayTaynt3d]

I'm thinking a DI will do it. Heh...

I don't know that much about this kind of thing, but many (if not all) DI boxes block phantom coming from the snake so as not to fry instruments. Thing is, I think many DI's can take a line-in signal right? So, couldn't you go SBD > 1/4" > DI > XLR > R4 w/ phantom? It's just like taking an instrument in, but the source is the soundboard line-outs? Does anyone know if that would work? B/c if so, that'd be a hella-easy solution...

A dI that  uses a transformer will do it only one problem the dI'S input has a transformer in it that will degrade the sound quality, unless it is a very good dI. Capacitors on the other hand when well selected will not.

Chris Church

What about using an active DI (no transformer at all right?)? That seems perfect for this, and if you look, you can find many DI's with pads that can take line-in easily. These boxes will be powered by the phantom, but not pass it back to the source, plus they will turn it into a balanced connection, which means you could do longer cables runs from the SBD to your deck. Some of them can even sum two inputs, so you could sum a stereo SBD to mono, block phantom, and do a long cable run from the SBD (maybe even back through the returns of the house snake if you want to setup closer to the stage?).

[rowjimmy]

I've seen an inline device for this... the stated intent was to protect a ribbon mic from 48v...

Gonna have to dig around to remember/find where.

[BayTaynt3d]

I've seen an inline device for this... the stated intent was to protect a ribbon mic from 48v...

Gonna have to dig around to remember/find where.

That's funny, cause someone offered me the chance to use their pricey royer ribbons for a gig about a month ago, and when they told me phantom would fry them, I decided not to risk gear that wasn't mine and were so expensive. Inlines to block phantom sound like a good application on the ribbon front...