Rode VXLR+ hiss

[DerFetzer]

Hey,

I recently bought a Rode VXLR+ which is an adapter that converts 12V-48V phantom power to plugin-power.
The first one I got introduced a massive amount of hiss to the microphone signal so I decided to send it back for replacement. But the distributor let me keep it and send me a new one.
Unfortunately the one I got as a replacement shows the exact same hiss.

I found a tear down and the schematic here https://www.zachpoff.com/resources/testing-xlr-adapters-with-plug-in-power/.
And in this post (https://groups.io/g/MicBuilders/topic/rode_vxlr_noise/76813908?p=) someone with the same problem replaced C1 with a 100uF low ESR one (KEMET T520B107M010ATE070) which fixed the noise.

I could not get the exact same one but I bought a PANASONIC 6TPC100M which also has 100uF and an even lower ESR according to the datasheet.
Unfortunately this had almost no effect on the noise level.

Since I am by no means an expert in analog circuit design I would like to ask some questions:

1. Did I overlook something when selecting the Panasonic capacitor that makes it not a good fit for the job?
2. Could the noise come from some other part of the circuit other than the zener diode?
3. Are there zener diodes that produce less noise?
4. Do you have other ideas for getting rid of that noise?

Thanks in advance!

[moffy]

To test the inate noise of the voltage converter place a shorted connector into the 3.5mm jack and listen to the hiss. Is it better, worse or the same?

[DerFetzer]

When I short tip and sleeve the noise gets worse compared to when nothing is connected.

[moffy]

According to the diagram you referenced, noise increasing with shorted inputs, kind of implies that the noise is coming from the source of the phantom supply. Both XLR_2 and XLR_3 are inputs and shorting the 3.5mm input puts C2 as a filter cap across XLR_2, so if the noise on XLR_2 and XLR_3 is roughly the same with the 3.5mm open circuit, and subtracting from one another, when the 3.5mm is shorted that balance is upset because the noise on XLR_2 is reduced by C2 and no longer subtracting from XLR_3 to the same degree.
I am making a lot of assumptions here, assuming the XLR inputs are true differential, the wiring is correct etc. but you might want to look at the piece of equipment providing the phantom power. The circuit is inherently unbalanced and that can cause problems on its own.

P.S. If you connect the tip and sleeve of the 3.5mm socket with a 2.2k resistor that should restore the balance and the noise should reduce. See R4/C3.

[DerFetzer]

Thank you very much for the detailed explanation!

I did some measurements with my Scarlett 2i2 Gen.1 and a Akai EIE Pro and the unmodified adapter directly plugged into the XLR port:

2i2
Open: -60.63 dB
Short: -52.57 dB
2k2: -56.87 dB

EIE
Open: -67.24 dB
Short: -65.8 dB
2k2: -32.65 dB (huge amount of 50Hz + harmonics)

The 2i2 does behave like you assumed. The EIE seems to have better phantom power but strange 50Hz noise with the resistor.

Recordings with my Rode Lavalier II have very high hiss on both interfaces.

When I connect the microphone to my Tascam DR-07 MKII there is hardly any noise at all.
With all my other microphones connected directly to the interfaces there are no problems either.

I have a feeling that the components do not like each other.

[moffy]

Very nice and systematic investigation! The noise levels are very poor.

[wody21]

Any news on this one? I have bought an Arturia Minifuse 1 and a Rode VXLR+ and I experienced the same phenomenon. Have you managed to fix the hiss caused by this annoying adapter? Thank you for your help in advance.