1 Topic by Mark_S (edited by Mark_S 2024-07-22 15:08:57)

  • Registered: 2021-03-05
  • Posts: 39

Topic: UFX II mic input for MC phono - input impedance

Hi RME team,

I am considering getting rid of my (quite noisy) phono preamp and connecting my MC cartridge with the UFX II mic input and doing RIAA equalization via PC based convolution.

I am using fully balanced wiring between MC cartridge and phono preamp already, so that would not be an issue.

Input impedance of UFX II mic input however is 3.4 kOhm, and would need to be ideally 100 Ohm. Is there an easy way to adjust input impedance as seen by the cartridge (e.g. by adding resistors inside the XLR connectors on UFX side), and what would be the wiring scheme? Any pros/cons or things to consider?

BR Mark
ADI-2 Pro FS R BE / UFX II
Neumann KH120, KH310, KH810 / Audeze LCD-X
  • waedi
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Re: UFX II mic input for MC phono - input impedance

I would just ignore that and crank up the preamp until the signal is fine.
Manipulating a cable will not lower the input Ohm.
You would have to open the device and do some frickeling after that the unit is destroyed and need repair.

The mic input is sensitive enough for professional music production, so it will be fine to listen LPs.

M1-Sequoia, Madiface Pro, Digiface USB, Babyface silver and blue

3 Reply by MC

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Re: UFX II mic input for MC phono - input impedance

It should be no problem to add a single and simple resistor of 100 Ohms into the cable or connector...

Regards
Matthias Carstens
RME

MC's Website

4 Reply by Mark_S (edited by Mark_S 2024-07-23 10:24:02)

  • Registered: 2021-03-05
  • Posts: 39

Re: UFX II mic input for MC phono - input impedance

Hi MC,

My electronic knowledge is very basic, so - how to wire the resistor(s) exactly? Between + and GND as well as - and GND (pin 1 to 2 and pin 1 to 3)?

Can I assume this in parallel with the 3.4kOhm input resistance of the UFX II?

Which grade of resistors would you recommend? 0.5% would be sufficient for good CMRR?

BR Mark
ADI-2 Pro FS R BE / UFX II
Neumann KH120, KH310, KH810 / Audeze LCD-X

5 Reply by MC

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Re: UFX II mic input for MC phono - input impedance

Between + and -, or pin 2 and 3. % does not change CMRR, but you should use metal film version (1% typically). The 3.4 k can be ignored when using 100 Ohm.

Regards
Matthias Carstens
RME

MC's Website

6 Reply by Mark_S (edited by Mark_S 2024-07-24 06:50:41)

  • Registered: 2021-03-05
  • Posts: 39

Re: UFX II mic input for MC phono - input impedance

Thank you!

Tried connection without this additional resistor, and sound quality seems fine. But good to know how to proceed when changing the cartridge loading :-)

Btw. the MC cartridge loading seems to be recommended primarily to dampen very high frequency resonance (in several 100 kHz range) of MC generator coils with cable capacitance. This is supposed to negatively affect phono preamps with less than good RFI, and reduces distortion in amp stages which can affect the audible range - so the theory.

https://forums.stevehoffman.tv/threads/ … 997/page-3

(Ralph Karsten seems to know what he is talking about)

Not sure whether this kind od loading (dampening RF) is required with the UFX II mic amp at all, or whether it can handle such high frequency content without distorting into the audible range. Maybe you can comment on this?

BR, Mark

BR Mark
ADI-2 Pro FS R BE / UFX II
Neumann KH120, KH310, KH810 / Audeze LCD-X