Input pin distortion?

[jackinnj]

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.

 

[moamps]

Ground loop?

 

[richie00boy]

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.

 

[pinkmouse]

Noise on the rails?

We could go on for ever.

 

[Tim__x]

Nonlinear input impedance.

 

[jan.didden]

Do you use a series R at the input? What if you short that?

Jan Didden

 

[jackinnj]

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.

 

[AndrewT]

Hi J,
tell us more about the before and after cables