I’ve been testing a few inexpensive XLR to 3.5mm adapters that contain a circuit to convert 12-48v phantom power to low-voltage “plug-in power”. (See Richard Mudhar’s Microphone Powering for great background info and vocabulary.) These adapters are designed for unbalanced electret capsules like lav mics, providing 5v bias via a 2.2k resistor.
The Rode VXLR+, Deity D-XLR, Movo FXLR-Pro, Boya 35C-XLR all seem to contain the exact same circuit, but with different board layouts (and possible component substitutions). With the exception of the Deity, they all sound fine, with no added background noise or obvious frequency anomalies.
I traced the circuit to see what’s going on in there. Download the KiCad schematic file here: XLR-Mini-Phantom-Adapter.sch
- $25 USD
- Seems like the best choice for most people. It sounds fine, fits well, and is supported by a reputable company.
- No complaints.
- $25 USD
- My sample produced lots of hiss and low output from every mic I tried. The board had a 10k bias resistor value (R1) so it wasn’t delivering enough current to the mic.
- I returned it after taking these pix. Hopefully it was a manufacturing mistake because I can’t imagine that they’re all like that.
Movo FXLR-PRO + Boya 35C-XLR
- $15 USD (The Movo FXLR-PRO & Boya 35C-XLR are identical products with different labeling
- This thing is pretty useless straight from the box. The underlying circuit sounds fine but there are several problems that need fixing:
- The 3.5mm TRS input is wired wrong! (Signal is on the ring instead of the tip, so it won’t work with most 3.5mm mics.) After taking these pictures I de-soldered the jack and flipped it, but it required running a flying ground wire around to the other side of the board and endangering the tiny cheap PCB traces. Not for the inexperienced.
- The housing is a bad fit for most XLR jacks! The pins are recessed too far from the rim of the connector (see comparison image against Rode VXLR+). So it might fall out of your recorder!
- Considering all these caveats I bought two and packed their boards into a little shielded box so I could plug stereo electret mics into 2 XLR inputs without a mess of adapters. It works great! (I used Switchcraft AAA low profile XLR connectors.)
If you’re building your own self-contained mic with an electret capsule, you can use the clever “SimpleP48” circuit that has been bouncing around the micbuilders email list for a few years. It has only 2 parts! Tom Benedict has an in-depth description here: https://tombenedict.wordpress.com/2016/03/05/diy-microphone-em172-capsule-and-xlr-plug/
NOTE: It’s really only suitable for finished mics, not an adapter because:
- The resistor value needs to be selected to match the FET inside the mic capsule.
- The resistor selection is only valid for a specific phantom voltage. (If you build it for 48v then the capsule will be under-powered if you plug it into 12v phantom.)
- The capsule is floating at the phantom voltage, so the capsule body needs to be electrically insulated and then shielded to prevent RFI. It won’t work with traditional 3.5mm electret wiring like the adapter circuits above.