Members Anderton Posted December 9, 2018 Members Share Posted December 9, 2018 Here's why I ask, although maybe this already exists. So many audio interfaces and mixers these days make phantom power an all-or-nothing proposition where either all XLRs have phantom power, or no XLRs have phantom power. Granted, phantom power won't necessarily kill any gear that doesn't require phantom power, but it can still be kind of a nail-biter when you wonder if you can connect an expensive multieffects with XLR outs to a mixer input that has phantom power applied. My lab isn't up and running yet so I can't check, but wouldn't it be possible to create an XLR female to XLR male adapter with capacitive coupling that would keep the phantom power out? Seems like a couple capacitors and a couple drain resistors would be all you need. Paging Mike Rivers...or any other solderheads out there. Link to comment Share on other sites More sharing options...
CMS Author MikeRivers Posted December 10, 2018 CMS Author Share Posted December 10, 2018 A 50 µF 50 v electrolytic capacitor in series with each of the signal leads will do the job. Drain resistors are already built into the phantom power supplied to the mic input so no additional parts are required. Simple. 20 Hz will be less 1 dB down [a correction from my original before-coffee post] for an average mic input, which will probably be fine for most signals you'd put through a signal processor, but you can find 100 or 250µF that will fit in a barrel if you want even less low frequency loss. But it's worth a little over-thinking. A better way to connect a typical pedal to a mic input would be to use a DI box - there's probably already one around the studio. Then the DIY article wouldn't need to explain how to wire an adapter from the pedal to the XLR. A DI will also present a higher load impedance to the pedal designed to work into a guitar amplifier input and may not be totally happy working into a 2kΩ load. Maybe build the in-line adapter in a barrel with an XLR-M on one end and a 1/4" phone jack on the other. Or build a box with several channels for your multi-output effect processor. If you're making unbalanced inputs on your box, it would be a good idea to tie Pin 3 on the XLR end to ground through a capacitor (of the same value) rather than ground it directly to avoid putting a DC load on one leg of the phantom power supply. However, if you have a signal processor device with line level XLR outputs, chances are very good that there are already blocking capacitors, or maybe even a transformer, behind the outputs in order to protect it from people who don't think about these things. And, conversely, a line level input that's an attenuated mic level input nearly always has blocking capacitors, so if the input is a combo XLR-1/4" connector, just plug it in and play. Link to comment Share on other sites More sharing options...
Phil O'Keefe Posted December 10, 2018 Share Posted December 10, 2018 As an alternative to building something that would cancel out the phantom power (or making your own external phantom power supply, which is another option), you could just pick up an external phantom power supply and use that for whatever condensers and other phantom powered mic(s) you needed to run and leave the phantom power on the board turned off so that the rest of the channels that don't need phantom aren't having it fed to them. Rolls makes a six channel rackmount phantom power supply that retails for about $125, and single and dual channel phantom power supplies are considerably less expensive than that. Depending on how soon you need it, and how much your time is worth to you, it might be easier to just buy one instead of building one. Link to comment Share on other sites More sharing options...
CMS Author MikeRivers Posted December 10, 2018 CMS Author Share Posted December 10, 2018 As an alternative to building something that would cancel out the phantom power (or making your own external phantom power supply, which is another option), you could just pick up an external phantom power supply and use that for whatever condensers and other phantom powered mic(s) you needed to run and leave the phantom power on the board turned off so that the rest of the channels that don't need phantom aren't having it fed to them. That's a solution, of course. Keep in mind, though, that the phantom power source can affect the common mode rejection of the mic signal. For $125, you don't get six, or even one, high quality center-tapped transformers to feed phantom power through. So they do what most console designers do for phantom powering transformerless mic preamps, which is to use two resistors to feed DC to the two signal leads, and capacitors to block the DC from the input stage of the preamp. With 1% tolerance resistors off the shelf, worst case mismatch could cost close to 20 dB of common mode (hum/noise/EMI) rejection. Responsible console manufacturers do better than that, either by using smaller-tolerance resistors or in-house matching of pairs. Excellent common mode rejection may not matter much in a system that you only want to spend $125 upgrading but if you already have a decent console and decent mic, you shouldn't mess it up. Even at power line frequency, the reactance of two 50µF10% tolerance capacitors won't differ by more than about 2.5Ω. The difference between two 1% tolerance 6.8kΩ resistors for a standard phantom power feed could be as much as 68Ω. Capacitors are cheap, about 65 cents, and if you use a barrel (about $10 for a Switchcraft S3FM) you don't have to make any holes, which, for most would-be builders, is the most difficult part of a home DIY project. Link to comment Share on other sites More sharing options...
Members Hush Posted January 17, 2019 Members Share Posted January 17, 2019 https://www.fullcompass.com/prod/257...QaAlxrEALw_wcB [ATTACH=JSON]{"data-align":"none","data-size":"full","title":"191688.jpg","data-attachmentid":32453156}[/ATTACH] Link to comment Share on other sites More sharing options...
Moderators daddymack Posted February 2, 2019 Moderators Share Posted February 2, 2019 https://www.tritonaudio.com/phantom-blocker.html Link to comment Share on other sites More sharing options...
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