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...

[bluegrass_brad]

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?).

Active DI's still have transformers. Thats how they balance a signal. Some also really color the sound. Ive used several different types in FOH work.  A Countryman doesnt sound like a Radial Labs, which doesnt sound like a BSS, etc.

[BayTaynt3d]

Ome more thing, does anyone know if the R4 puts phantom out on TRS? It's quite possible it doesn't, which in this case might solve the problem without even needing an inline?

[BayTaynt3d]

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?).

Active DI's still have transformers. Thats how they balance a signal. Some also really color the sound. Ive used several different types in FOH work.  A Countryman doesnt sound like a Radial Labs, which doesnt sound like a BSS, etc.

Just curious, I've been thinking about buying a DI for taping b/c I tape jazz trios a fair bit where I frequently use an extra spot mic or two with my main pair going into an R4. Got any suggestions for an active/passive DI to be used mostly for double-bass? And possibly guitars? That isn't too "colored"?

[bluegrass_brad]

your gonna run the mics into a DI then into your R4?

[BayTaynt3d]

your gonna run the mics into a DI then into your R4?

No man...

Maybe I should spell it out more? Say I'm recording an acoustic jazz trio, say bass drums, sax (or maybe piano or trumpet). Often I'll run a stereo pair into the R4, but knowing that the bass and/or piano often end up being weak in the mix, I'll spot mic one or both of them as well. That's using all four channels on my recorder. At other times, in a trio format in particular, I might just spot mic everyone and skip the stereo pair altogether (sometimes I'll use one overhead and one kick for drums, them mic the bassist's amp, and stick a mic in the piano or in front of the sax depending on instrumentation). Anyway, micing a bass amp often is muddy as hell for my tastes, so I'd like to have a DI with me to get a direct from the double-bass. Remember these are mostly acoustic gigs, so there is no PA and no engineer and the bassist typically won't bring a DI, just an amp or possibly no amp at all. If I had my own DI, and I know the bassists who would be cool with it, I can get them to stick my DI b/w their bass and their amp so I can take a direct feed into one channel of my R4 (and NOT use a mic at all). Especially in cases where I'm running a stereo pair, being able to reinforce the bass with a dry/clean extra punch might sound great cause I'll have all the ambient/room I need in the AUD pair.

[bluegrass_brad]

Gotcha.  Honestly I think they all color it to some degree, even when they claim that they dont.  I have a couple of Countryman Type 85's that I use.  I bought those mostly because lots of venues have them and I had used them often and knew their sound. You can get them for around $150 or so.  Radial Labs makes some really nice DI boxes as well. BSS too. Are these guys playing upright acoustic bass or electric bass?

[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.

Well, the good news is that i remembered where I saw them.

The bad news is that he's no longer marketing them:
http://www.naiant.com/studiostore/cleverdevices.html

No sign anywhere but I know with absolute certainty that they were there at one time. According to the front page of the store (I used an old link to get in) he's on vacation until Jan 8 or so but after that you might want to drop him a line.

[BayTaynt3d]

Gotcha.  Honestly I think they all color it to some degree, even when they claim that they dont.  I have a couple of Countryman Type 85's that I use.  I bought those mostly because lots of venues have them and I had used them often and knew their sound. You can get them for around $150 or so.  Radial Labs makes some really nice DI boxes as well. BSS too. Are these guys playing upright acoustic bass or electric bass?

Stand up double/contra-bass...

[bluegrass_brad]

with a stand up bass I would try to have some type of mic blended in with the DI. Like if you were running a stereo pair onstage and blending the DI in with it, or had another instrument miced close enough to the bass to get some bleed.  I have worked with a bunch of acoustic instruments doing sound and no matter how nice the pickup and how nice the pre they are running it into, I just dont like the sound of a DI all by itself. When you mic the bass are you micing the amp, or the bass itself? 

[BayTaynt3d]

with a stand up bass I would try to have some type of mic blended in with the DI. Like if you were running a stereo pair onstage and blending the DI in with it, or had another instrument miced close enough to the bass to get some bleed.  I have worked with a bunch of acoustic instruments doing sound and no matter how nice the pickup and how nice the pre they are running it into, I just dont like the sound of a DI all by itself. When you mic the bass are you micing the amp, or the bass itself? 

Yeah, I hear you bigtime. That usually isn't a problem b/c I usually am using a stereo pair as my main source, then just adding a tiny bit of extra bass to get the balance a little better -- so in that instance, I'm thinking the clean DI might be perfect. Right now, I will mic their amp if they are using one, and if they aren't, then the bass itself, but since I'm more of a taper (and not their engineer), I try not to be too demanding of the here's my mic, let me put it right in front of you so you can play into it, and please don't knock it kind of thing, cause many times they aren't really thinking that their there to make a recording. So, in those cases (no amp), I might a little farther back than I'm sure the bassist would do in a studio or other live sound scenarios. Also, if they are amped, I tend to get bass bleed into all of the other mics anyway -- cause although I may be spot micing, it's not like ppl are playing directly into an SM57 or anything, I'm usually a bit back from each instrument so as to get a little more ambient and be a little more out of their way. That bleed is good and bad I've found. Sometimes it screws me on the phasing/imaging front, other times it makes it sound freakin' awesome cause it gives some spread to some of the instruments. It's been real hit or miss for me. Anyway, I hear you about the dry DI sound with the bass, but usually I'm just reinforcing a stereo pair, and if not, I usually do have bleed from the bass into the other mics, so I think I'll be alright, but point taken. Also, I've been reading up on micing the bass amp, and I think I might try something different next time I have the chance -- I've been putting my mic offset from the center of the cone, but next time I might try pointing it DFC straight into the cone cause I've read I'll get a little more definition and highs (and less mud) that way, but not sure about the SPLs b/c I'm doing this with a SD condenser (maybe I should buy one SM57 for the job, but my phantom is linked in pairs on the R4, will phantom fry a 57?).

[bluegrass_brad]

That explains some of it.  Another thing I would suggest if close micing is backing the mic off of the bass cabinet a little bit to reduce proximity effect, which could be contributing to the muddiness. using a DI as reinforcment is right on though, and should work well for what you are wanting since it will have some acoustic reinforcment through other mics.  Phantom power wont hurt a dynamic mic like a 57. Ribbons it can, but dynamic's like a 57 wont be hurt and it wont change the way they sound.

[BayTaynt3d]

Thanks for the help, this close-micing stuff in new for me, but I've been experimenting and reading a lot about it. I appreciate your help for sure. Good to know about the 57 and phantom, I might just get me one of those. I've actually thought about close-mcing the bass amp with an omni b/c of the proximity effect thinking that if I was close enough, the bleed might not be that bad, and since I'm only recording, feedback isn't a concern of mine, but that's probably not a good idea. Also, I wonder if I could damage a 481 or C4 by putting it right in front of the cone? Both those mics can handle decent SPLs, but not sure about in this application? It seemed fine with the C4 slightly off-center on the cone, but I did have to run line-in on the R4 to handle it the times I've done that. (Whoa, talk about a thread-jack... LOL!)

[bluegrass_brad]

A 57 would be a good all around mic investment, You can use them in lots of places.   57's work fine and are cheap and tough but they are nothing special. I use them often cause every venue has tons of them. If I was investing in a dynamic for myself for recording I think I would go with a Beyer M88 or a 201 (there is some discussion in another thread here on the Beyerdynamic dynamics), or a Sennheiser 421. But those are alot more pricey than a 57.

[Chuck]

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.

Well, the good news is that i remembered where I saw them.

The bad news is that he's no longer marketing them:
http://www.naiant.com/studiostore/cleverdevices.html

No sign anywhere but I know with absolute certainty that they were there at one time. According to the front page of the store (I used an old link to get in) he's on vacation until Jan 8 or so but after that you might want to drop him a line.

Yeah, they were. E-mail him. He may have some left.

[BayTaynt3d]

A 57 would be a good all around mic investment, You can use them in lots of places.   57's work fine and are cheap and tough but they are nothing special. I use them often cause every venue has tons of them. If I was investing in a dynamic for myself for recording I think I would go with a Beyer M88 or a 201 (there is some discussion in another thread here on the Beyerdynamic dynamics), or a Sennheiser 421. But those are alot more pricey than a 57.

What's interesting is that I already own a dynamic mic that I use for ENG field interviewing type of stuff. It's an Electro Voice RE50/B, and it's BOMBER and used in electronic news gathering by major networks all of the time. But, it's an omni and I'm not sure how "musical" it is. Even though it's an omni, it's NOT an omni like we're used to around these parts, you really have to shove it in someone's face to pick up their voice well, but it's got an AMAZING lack of handling noise b/w the omni and the fact that the capsule is internally shock-mounted. Anyway, I wonder if I could get away with using it for micing a bass cabinet just as effectively as buying an SM57?

OOPS. Just checked the specs, it bottoms out at 80 Hz, so that kind of sucks for bass amping, LOL! And I can't find the Max SPL in the specs anywhere. Oh, well... It's still an INCREDIBLE interview mic... 

[BayTaynt3d]

Brian,

Did you ever find a solution for this?

I just had this problem the other day. I plugged directly into someone's bass amp (they all seem to have built in DIs these days, direct outs), but I wanted to use a fourth mic and got paranoid that if I turned on phantom, it'd fry the guy's bass amp? I kind of doubt it considering the whole point of that direct out of the bass is to negate the need for a DI isn't it? So, I'd think it was designed to plug into a snake, etc., that might very well have phantom running down it (although most SBDs let you turn that off I guess, but some snake boxes are self powered aren't they?).

Anyway, if I could find an inline phantom-blocking adapter, seems like that'd have been the safest thing to use, but I can't seem to find anything like that on the web and this thread never posted a resolution except for homemade (which isn't something I'm capable of, lol)?

Unless someone could tell me that the direct outs on all bass amps will block phantom and not fry? I do that trick all the time, and yes I know it's a dry signal, but I get plenty of wet bass off my main stereo pair when I do this trick (for acoustic jazz recording).

Any help would be appreciated, thanks!

[Brian Skalinder]

Seems like transformer isolated splitter box is the way to go.  The cheap ones have low quality transformers, the expensive ones obviously high quality.  Most of the advice recommended staying away from the cheapos, and I didn't pursue it further.  However, never actually listened to a lower end transformer isolated splitter box, so I don't know the audible impact of the lower quality transformers.

Perhaps one of our TS tinkerers / retailers could make a good quality transformer isolated splitter box at a decent price point, dunno.  I'm thinking Church or Busman, though I'm sure there are others.  Chris and I actually talked about it a bit, but I neglected to follow-up.

[dean]

Wow, this is absolutely fascinating, seriously.  I don't really follow it, but it makes me sure of one thing, that I, along with each of you, am a complete, utter and hopeless NERD!!!

[rdfager]

I've had this problem too.  Add me to the list of people who would buy one if someone comes up with a reasonably priced inline device to remove phantom power.

[SparkE!]

What's wrong with just a series capacitor in both the hot and cold leads?

[BayTaynt3d]

Does anyone know the answer to my other question? Anyone have any experience with the direct out capabilities of newish bass amps? I got to think they block phantom just like a DI would don't you think? I guess I'm just not willing to risk frying someone's amp to find out, LOL.

[BayTaynt3d]

I looked through a few random user manuals for bass amps/heads last night, and the funny thing was that although they pretty much all had direct outs, they pretty much all were XLR out, and they pretty much all described it as a "built in DI," none explicitly said they'd block phantom. But all DIs block phantom right?

I guess the only really safe thing to do is to bring my own DI for these cases, but it's not only easier to just plug into the amp, but it also doesn't require putting my gear in the musicians chain -- not everyone is going to like that cause my cheap DI could color their sound, not to mention fail or something, causing problems for the performance. Whereas when I plug in directly, there's no chance of any of that.

I wonder if I could do something like this:

XLR-to-1/4" adapter > DI > XLR > R4

But the DI would be expecting unbalanced on the bass side, and in that scenario I'd be feeding it a balanced signal wouldn't I? Or could I do that if the XLR-to-1/4" was wired appropriately. Sorry, I don't know enough about the technical side of this to know any better...

[SparkE!]

A normal XLR to 1/4 adapter will not block phantom power.  It just sends the XLR hot to the 1/4 tip and XLR ground to the 1/4 ring.  The XLR cold is not connected.  So, if you have 48 V on the XLR hot and 0 V on ground, you end up with 48 V between the 1/4's tip and ring.

[bluegrass_brad]

A normal XLR to 1/4 adapter will not block phantom power.  It just sends the XLR hot to the 1/4 tip and XLR ground to the 1/4 ring.  The XLR cold is not connected.  So, if you have 48 V on the XLR hot and 0 V on ground, you end up with 48 V between the 1/4's tip and ring.

But he is talking about using the XLR > 1/4" TS to run into a DI box.  the DI box would block the power.

[BayTaynt3d]

A normal XLR to 1/4 adapter will not block phantom power.  It just sends the XLR hot to the 1/4 tip and XLR ground to the 1/4 ring.  The XLR cold is not connected.  So, if you have 48 V on the XLR hot and 0 V on ground, you end up with 48 V between the 1/4's tip and ring.

But he is talking about using the XLR > 1/4" TS to run into a DI box.  the DI box would block the power.

Yes, exactly. And as you said before, I guess taking that balanced XLR out of the bass amp and adapting it to 1/4" TRS (that's the cable I already have) and then sticking that 1/4" TRS into the input of the DI will still work? I ask cause I'm kind of clueless about this, I do know that typically you'd be putting an unbalanced 1/4" into the input of the DI. In a way it's like I'd be going DI > DI > R4 (yuk). But from what you said earlier, sounds like I probably don't have to worry about frying the direct out of the bass amp anyway?

[DSatz]

I don't know the configuration of the bass amp's output, but if it's using an output transformer, it shouldn't mind the phantom powering. If not, you might consider some of the toys on http://www.jensentransformers.com/iso_aud.html. Jensen transformers are among the best available anywhere. Pamper your signals a little ...

--best regards

[Church-Audio]

Seems like transformer isolated splitter box is the way to go.  The cheap ones have low quality transformers, the expensive ones obviously high quality.  Most of the advice recommended staying away from the cheapos, and I didn't pursue it further.  However, never actually listened to a lower end transformer isolated splitter box, so I don't know the audible impact of the lower quality transformers.

Perhaps one of our TS tinkerers / retailers could make a good quality transformer isolated splitter box at a decent price point, dunno.  I'm thinking Church or Busman, though I'm sure there are others.  Chris and I actually talked about it a bit, but I neglected to follow-up.

I will build you a transfomer isolated input if you need it let me know.


BTW there was mention of a "bass amp" what is this all about?

Chris

[George2]

Leave the +48 off on channels 3 and 4 and use one of these.

http://www.professionalsound.com/Catalog/48ph.htm

http://www.denecke.com/Products/Audio%20Accessories/audioaccessories.htm#PS1A