blocking preamp phantom power (& cap discharge) - Page 1
Forum section: electronics / beginners

author: electrolust
author_rank: Unknown
timestamp: July 03, 2020, 07:55:34 pm
content: I'm trying to block phantom power from a mic preamp. Phantom power is supplied as 12 or 48V (most commonly) from the preamp so as to power a condenser mic. A dynamic mic will ignore it. I do have a dynamic mic but the rest of my circuit (not shown here) cannot handle the phantom power.I've found 3 versions of this on the interwebs, they all havefundamentaldifferences of opinion. Not just different component values. 2 of them are definitely outright wrong. I am not sure which 2.In "my" version, attached, R6&R7 are described as load resistors, and R8&R9 as discharge resistors. This, I believe is wrong or rather incomplete. Sure, R6&R7 do in fact present a load, but we don't need that. We are just blocking the DC. Sure, R8&R9 do discharge the cap, but not on their own -- they require R6&R7 to complete the discharge circuit.Q1: So, in fact I could remove all the resistors, yes? I just wouldn't then have a way to discharge the caps.By removing the resistors I also no longer have any concern about the RC filters formed and the impedances presented to either side of the filter.Here's the tricky part. I do want to discharge the caps. At some point the battery will need to be changed and I don't want 48V/10mA of charge there waiting to surprise someone.My device is self-powered, with an LED indicator, so I would discharge the caps via that LED and an SPDT power toggle. (on -> switch out the discharge resistor; off -> poweroff but switch in a discharge resistor "R_d)". Bonus: LED fades to dark indicating cap was discharged. In the off state it won't actually go off until you also disconnect from phantom power.Q2: Now, if I discharge the caps that way, do I have to worry about polarization while still connected to phantom power?When R_d is switched in, the negative legs of the caps are open circuit (I'll be sure to use break-before-make switch).The 48V circuit is not completeas there is no path to ground, however what is the effect, if any, of the 48V being now present on the negative side of the cap, across R_d? That negative side was previously referenced to the same ground potential.My guess is that even though that negative cap leg is at the same "charge" potential as the 48V ground, there's no circuit because it doesn't present a return path. And even though we are now presenting the 48V potential to the negative side of the cap it's via R_d acting as a load, not as a wire. The + side of R_d sees 48V, and the - side 0V.Q2.5: The cap in fact will not discharge as long as it's connected to 48V. Is that correct? But will discharge after removing the phantom power.

author: dmills
author_rank: Super Contributor
timestamp: July 03, 2020, 10:42:04 pm
content: This mostly deals with the issues at the supply end of the cable, buthttp://www.thatcorp.com/datashts/AES5335_48V_Phantom_Menace.pdfmakes some good points.The major thing you need to protect against is a charged cap with a sudden cable short because that charge has to go somewhere....Regards, Dan.

author: electrolust
author_rank: Unknown
timestamp: July 04, 2020, 01:24:26 am
content: Thanks Dan. That paper is very good reading. I haven't digested it fully yet but it appears to be related to a similar culprit, the patch bay. In this scenario, a TRS patch bay is used to connect mics (and intermediate devices) to the mixing desk. The TRS connector often shorts one of the signal conductors to ground on its way in or out of the jack. (this shorts the other conductor as well, because the 2 signals are just the 2 legs of a voice coil.) ribbon mics are the most talked about fatality from this, and most discussion around blocking phantom power are around that.my device is a one-off for home use. it will only "ever" be connected between mic and preamp via XLR cables in good condition. i do not expect to ever see the scenario described above. i'd have to review the paper a bit more to see if that could ever happen.that said, the cap + side of the circuit leads to a male XLR, which will definitely get in contact with fingers. so i suppose that indeed i better make sure to discharge the caps somehow.it's easy enough to test my discharge solution on the bench, but i'd rather not blow up a cap, not even for science.

author: themadhippy
author_rank: Super Contributor
timestamp: July 04, 2020, 02:03:02 am
content: Another way is to use a 1:1 transformer,not a cheap option but guaranteed to work

author: electrolust
author_rank: Unknown
timestamp: July 17, 2020, 11:58:27 pm
content: I've decided the 2nd option, discharge on power-off, is not safe. someone might disconnect the cable and not flip the power switch. then that juicy charge is waiting for them at the connector.