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Old 31st January 2007, 01:01 PM   #1
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Default Input pin distortion?

I have a mosfet amp which is performing quite nicely -- THD is 0.003% at 1 kHz, 10 watts -- in testing it out I wondered whether the distortion might not just be an artifact from my test setup -- so I observed the distortion on the input capacitor -- when there is no power on the input stage the distortion is just a little higher than what you should expect of the SG505, i.e. 0.002%. (the SG505 when connected directly into the AA501 measures 0.0008% THD at 0 dBm). When I power up the input stage it shoots up to 0.007%, sometimes 0.010% -- i wonder what causes this little bit of unhappiness.
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Old 31st January 2007, 01:13 PM   #2
moamps is offline moamps  Croatia
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Ground loop?
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Old 31st January 2007, 01:15 PM   #3
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Maybe input offset current creating a small bias across the cap? If it's an electro then the bias might make a lot of difference.
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Old 31st January 2007, 01:18 PM   #4
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Noise on the rails?

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Old 31st January 2007, 01:58 PM   #5
Tim__x is offline Tim__x  Canada
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Nonlinear input impedance.
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Old 31st January 2007, 02:09 PM   #6
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Do you use a series R at the input? What if you short that?

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Old 31st January 2007, 03:53 PM   #7
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I think that I have found the problem -- I switched to a Pomona cable to the input connector and connected directly to the input capacitor -- now the THD before and after doesn't change -- and it went down to 0.0022% -- this is using an 80kHz bandwidth.
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Old 31st January 2007, 04:09 PM   #8
AndrewT is offline AndrewT  Scotland
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Hi J,
tell us more about the before and after cables
regards Andrew T.
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