• Registered: 2021-10-12
  • Posts: 2

Topic: Measuring noise level using FF 400

Hello,

I'm trying to use a FF 400 sound card to measure THD+N with DSSF3 software of my DIY phono preamp. Here is the link http://www.ymec.com/products/dssf3e/

When I connect FF output to the amplifier input the noise level goes up, and shows approximately -80 dBu (around -65 in peak) in 5-100 Hz range. If I connect it to my Macbook Pro phones output, the noise level drops to -100 dBu. My amplifier is a phono preamp, and it has +30 dB gain at 1kHz. Does any have a clue why there is such a difference? I have a gut feeling that it has something to do with source impedance difference. I've tried both balanced outputs and phones. Result is the same.

Thank you!

A.

  • cyrano
  • DAW Mastermind
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  • From: Belgium
  • Registered: 2015-04-28
  • Posts: 701

Re: Measuring noise level using FF 400

Hum? Greatly amplified because your amplifier uses RIAA correction?

MB Pro - 2 X FireFace 400, FF800 & DigiFace USB
ADAT gear: Korg, Behri, Fostex, Alesis...

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3 Reply by MC

  • MC
  • MC
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  • From: Germany/Thailand
  • Registered: 2007-04-06
  • Posts: 25,582

Re: Measuring noise level using FF 400

You need a passive attenuator at the FF400 output with about -40 dB to get the Phono amp input driven with the expected 5 mV. Anything else won't work correctly - too high noise and distortion.

Regards
Matthias Carstens
RME

MC's Website

  • Registered: 2021-10-12
  • Posts: 2

Re: Measuring noise level using FF 400

MC wrote:

You need a passive attenuator at the FF400 output with about -40 dB to get the Phono amp input driven with the expected 5 mV. Anything else won't work correctly - too high noise and distortion.

Hey, thank you for the answer.

Could you describe the motivation please? What design decisions of FF400 are requiring it? Why it is not required when input is driven by iPhone for example?