Fun With Measurements

BRSHiFi

Previously known as kingden
2008-08-28 4:27 am
Evanston, IL
I just purchased a Quant Asylum audio analyzer unit and have been playing with it for the past few days.

Attached is data I took from the left Channel of a phono preamp I made some time ago. I wanted to verify RIAA compliance and the signal to noise ratio of the unit. The zip file contains an excel spreadsheet with the results.

Noise is a huge problem with this unit as is evident from the data, but RIAA compliance looks good, though distorted a tad due to the noise.

My question is how do you properly measure the signal to noise ratio with the input shorted? Since it is an RIAA preamp, the floor is obviously higher in the low frequency ranges.

To tell which photo is which, move your mouse over the thumbnail and look at the name in the tag that pops up.
 

Attachments

  • Left Channel, Analyzer Connected, Unit On.jpg
    Left Channel, Analyzer Connected, Unit On.jpg
    242.3 KB · Views: 112
  • Left Channel, Input Shorted, Unit Off.jpg
    Left Channel, Input Shorted, Unit Off.jpg
    252.2 KB · Views: 108
  • Left Channel, Input Shorted, Unit On.jpg
    Left Channel, Input Shorted, Unit On.jpg
    244.6 KB · Views: 103
  • Left Channel, Input Open, Unit On.jpg
    Left Channel, Input Open, Unit On.jpg
    244.1 KB · Views: 103
  • Setup.jpg
    Setup.jpg
    104.2 KB · Views: 108
  • phono_left.zip
    37.2 KB · Views: 12
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BRSHiFi

Previously known as kingden
2008-08-28 4:27 am
Evanston, IL
I put a 1 KHz tone into the unit so the output reached -3dBV. The unit with the analyzer connected measures a signal to noise ratio of 25dB.

When I short the input (disconnect the analyzer output), the noise floor drops considerably. The 60Hz component looks to stand out (see left channel input shorted picture).

So, with the input shorted, do I measure the -3dB output against the noise floor at just 1 KHz or the whole noise floor? What is typically done mathematically here since the floor varies across the audio band so much?
 
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