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LIMP and Focusrite Scarlet 2i2
LIMP and Focusrite Scarlet 2i2
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Old 7th April 2020, 03:33 PM   #1
rajestanna is offline rajestanna
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Join Date: Apr 2020
Default LIMP and Focusrite Scarlet 2i2

Hello.

I am trying to use LIMP with Focusrite Scarlet 2i2 ist Gen. The DUT is driven through an Amplifier

Please help me with the following queries:

(1) The Scarlet has mic , Line and Instrument level inputs.

Line In: input impedance is 10K and noise in presence of signal is -104dBu at min. Gain

Instrument In: Input impedance is 1Meg and noise in presence of signal is -87dBu at min. gain

Would it be better to use Instrument in or Line In and why?

(2) Limp manual suggests using reference resistor of 10 to 27 ohms. Professional systems use low value current Sense resistors. What would be the problem if I use a 0.47ohm or 0.22 ohm for the reference resistor. ( I am using an Amplifier to drive the DUT)

Thanks and regards
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Old 7th April 2020, 07:01 PM   #2
celef is offline celef
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Join Date: Nov 2012
if i were you i would test different restor values and compare the results, i would also try both inputs and see which one gives best results. i would also try different test levels, but be sure to protect the input from excessive levels

i would also try measure impedance using the soundcard headphone output
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Old 8th April 2020, 09:15 PM   #3
Mark Tillotson is offline Mark Tillotson
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Join Date: May 2018
Location: Cambridge UK
Instrument is for electric guitars which can't drive 10k, use line input, much cleaner and should have a flat response,
which the instrument input might not.


The sense resistor should be close to the impedance being measured, due to the limited
connection setup (not a differential measurement of the shunt). Use ~10 ohms for a speaker, ~50 for a headphone.
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