Found also this doc on phantom blocking in my archives,
here it goes
here it goes
Phantom power blocker
- Thread starter trashcanman
- Start date
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Brian Roth
Well-known member
I know we're on a veer here, but those Rat Shack "isolators" are indeed quite handy. Looks like they are currently out of stock:
https://www.radioshack.com/products/radioshack-ground-loop-isolator?_pos=2&_sid=3f10c89e8&_ss=r
I was first introduced to them many years ago when I was working on a TV shoot (as the client's rep) with one of the huge TV "trucks" (they normally traveled to do NFL/NBA/etc games).
We had to feed some monitors in the venue's lobby and the audio and video was full of hum. The chief engineer of the truck added a balun to clean up the video, and had a stash of those Rat Shack audio isolators with the RCA plugs cut off and XLRs installed.
I bought a couple and ran them through various bench tests and they Don't Suck. Not exactly Jensens, but plenty good enough for many tasks.
/veer off.
Bri
user 37518
Well-known member
Bri, around 10 years ago, before the era of Zoom meetings, a friend of mine who used to be a very important executive at Universal Music Mexico asked me to help him because their conference room was having serious noise issues. It was a considerably large conference room, they had a center table for something like 20-25 people, they even had a small PA in it to listen to the new records or videos from the artists. They had screens all over the place and there was a particular screen with a very sophisticated conference system. Whenever you turned that system on, you could instantly see the conference room at Universal Music Los Angeles, Brazil, etc... real time, without even logging in and without even asking permission to the other end, just turn it on and bam, you were watching what was happening at the other side of the world. I believe it must have been a dedicated connection only for that, extremely expensive I guess and probably useless these days with Zoom meetings.
Anyway, you can figure out what the problem was, as soon as someone connected a laptop or anything to a video screen and to audio simultaneously, a big ground loop would blast all over the place. As you can imagine, they were tired that every time they got in touch with some big Kahuna in LA to show them the recordings or videos of the new artists, the song would be accompanied by a huge amount of hum and buzz, and this was present everywhere, it was even transmitted to the other side of the screen at LA or to whomever they happen to be talking to in that particular moment.
Long story short, I quickly determined that it was a bad grounding and cabling installation, and I couldn't do anything that didn't involve some serious redesign. The big issue was that they had tons of points where to connect stuff, the mixer they used also had multiple inputs and everything was stereo. There wasn't really much discussion after I told them that their options were that they could either pay something like 1.6K+shipping+customs taxes for 16 Jensen transformers or, they could pay something like $100 for 16 unbranded Chinese transformers. You can figure out what they chose. So I bought tons of these cheap transformers and placed them in every single audio box they had and the problem was solved, not only that, but they told me that they received a lot of compliments from everyone: managers, A&Rs, producers, etc... saying that the audio sounded extremely clean.
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Jensen Iso-Max and Radial Twin Isolator might be two other options. The Jensen looks easier to deal with for your purposes, as it has RCA inputs: SMMDB | Jensen Transformers I owned an RCA -->RCA version of this at one point. Seemed OK...
I clicked on that link and wouldn't advise anyone to use that product. It's not idiot proof. It clearly states that you plug it to the input, make sure 48v is on, THEN plug in the mic. If you turn off phantom and toggle it back on, you will still fry your equipment with the initial voltage. If you don't have 48 on, and turn it on (the reason why this was brought up) no bueno. I'd go with 1:1 iso transformers if Di's aren't working. Di's probably don't work great for laptops because they are looking for passive High Imp sources, not headphone level sources.
There are many stereo DI type devices designed specifically to accept headphone level, low impedance sources like laptops, cell phones, etc.
Probably the circuit that's inside of it is exactly the same schematic that is linked in the first post
user 37518
Well-known member
I can see that the "ribbon mics get burned if you turn on phantom power" myth is alive and well...
Of course there is a lot of misunderstanding as to the how and why's, but still the Phantom menace is real.
I think if Mr Gallagher of Sweetwater made a real scientific exposé, most of his potential customers would be lost.
The big challenge in audio is passing the message in common language without lying.
Our man covered this years ago, I'm sure this demo is posted elsewhere on GroupDIY. Doesn't address old center tap to ground transformers.
https://shinybox.com/ribbon-microphones-and-phantom-power/
https://shinybox.com/ribbon-microphones-and-phantom-power/
user 37518
Well-known member
Agreed, but when phantom power problems arise is not just ribbon mics which are affected, for example, patching a mic in a trs patchbay with phantom turned on can wreak havoc for dynamics as well as ribbons, even the input stage of the preamp can burn. So my argument is that, whilst ribbons are more sensitive to this, it is not a problem related to the nature of a ribbon mic alone.
The most common failure due to phantom malpractice is base-emitter junctions going to zener, making them noisy, and often taking with them other components. Second is I/O stages. Ribbon mics are a distant victim because the damage is seldom fatal: it becomes so by accumulation. I've never seen a dynamic mic seriously damaged by phantom misapplication.
The focus on ribbon mics is out of proportion with the real nuisance factor IMO.
user 37518
Well-known member
Totally agree
At the studio, both the 8028 room and the A-range room have Phantom at the xlr panel hardwired. On the A-Range, it comes from the channel, so it's present at the channel input on the bay. We use 44s, 77s, Coles every day. This has been going on for 40 years in 2 and I haven't had an issue. Just Make the crosspatch in the A-range room before patching the mic, I guess.
XLR's offer almost zero possibility for inadvertent shorting, contrary to TRS or Bantam jacks.
user 37518
Well-known member
When I used to teach at a very famous international audio school (wink, wink), I used to ask this question to my students: "What happens if you connect a ribbon mic to a mic pre and you suddenly turn phantom power on?", all of them, without exception or hesitation would answer "It will burn", it was a plain giveaway for them. When I told them that it wasn't true in almost all cases, they would to look at me like I was some sort of idiot, they were amazed that I couldn't even know something as basic as that. But I don't blame them, every single one of their other teachers used to teach them that, and they used to teach it as if it were gospel truth.
After I explained them with circuit diagrams why this wouldn't happen, some of them would get it but the vast majority would still not believe it, after all, all of their teachers told them that and I was the only one telling them otherwise
It doesn't surprise me at all that Sweetwater is able to sell $45 phantom power blockers with the pretext of "protecting those delicate ribbon mics", as the guy in the video says.
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That's the problem with that kind of questions that call for the answer "it depends". It starts early with "don't talk to strangers".
Many people, educators and teachers included, prefer simple binary answers rather than one that involves applying one's own judgment and accepting the responsibility that goes with it.
Confirmation bias is a strong pollutant.
swpaskett
Well-known member
So, as I understand things, and I can always have it wrong, many transformers are not designed to operate with DC on their coils. This is because the DC shifts the operating point somewhere on the BH curve that is unfavorable to expected operation. IOW, you can "bias" a transformer into a region where it works better or worse (essentially affecting its "gain" but I am really hesitant to express it in those terms because there are plenty of people will challenge me on that and they would be right -- gain is based on the turns ratio), just like you can bias a transistor into a region where it does not work well. Sometimes this feature is used in something called a magamp or magnetic amplifier. A former employer used magamps to regulate power supplies.
As for magnetic fields cancelling, I don't know enough to speak out on that. I do know that at DC a coil of wire looks like pure resistance, so with a grounded center tap you have DC flowing through the coils.
Tube mics are a different story altogether. I presume they have transformers designed to have some amount of DC across their coils. IDNK, never seen a tube mic let alone worked on one.
Back in my early days I used to see trannys with DC current ratings. I used to wonder what that was about, then I asked the magamp guy at work and he gave me more or less the explanation I gave above. He knew a lot, but I don't think he ever got his magamp power supply to work. A competing design, using more conventional regulation techniques, seems to have flown on that bird. I cut my losses and found a job where I understood what I was doing. Magnetics and RF are still voodoo in my book.
That's correct. Magnetic fields will cancel out, as long as they are well balanced and both sides receive equal currents. These conditions are not always met.
Gain is not the subject here. There are two major things that happen when submitting a xfmr to DC. One is the shift in magnetization curve, which results in creation of harmonic distortion (despite the name, harmonic distortion is often unpleasant) and decrease of inductance, which results in loss of bass.
