ruffrecords
Well-known member
I have been doing some detailed tests on the Classic Solo. With the gain turned up to full, there is almost exactly 50dB of gain from the DI input to the balanced output. With the DI input shorted, the output noise is about -56dBu which makes the EIN around -106dBu. If I could add a noiseless 1:10 transformer to that I could have a mic input that would have an EIN of -126dBu which would make me very happy. When I do this though, the output noise rises to -51.5dBu which means the EIN is only just -121.5dBu. Not a million miles away from what I normally measure but not as good as it could be. (It is about 1dB worse if I take the top off the enclosure. ) When I listen to the noise I notice it has a small mains hum/buzz in it. This is completely absent when the DI input is shorted so it would appear to be entering either at the mic input or the high impedance secondary of the transformer.
With the top off, I probed about with my finger and discovered the orange drop input capacitor is very hum sensitive and the hum/buzz increases significantly as my finger approaches it. There is no hum from my finger if my other hand is touching the chassis. I also notice the hum disappears if I disconnect the AC input to the HT supply leaving the mains transformer running and supplying the heaters. The noise also drops about 3dB.
I suspected it might be E field from rectifier spikes so I added a 100 ohm in series with the bridge and the reservoir capacitor. No change.
So no I am left wondering if this is in fact just some general E field pick up by the Orange drop cap.
Any thoughts?
Cheers
Ian
With the top off, I probed about with my finger and discovered the orange drop input capacitor is very hum sensitive and the hum/buzz increases significantly as my finger approaches it. There is no hum from my finger if my other hand is touching the chassis. I also notice the hum disappears if I disconnect the AC input to the HT supply leaving the mains transformer running and supplying the heaters. The noise also drops about 3dB.
I suspected it might be E field from rectifier spikes so I added a 100 ohm in series with the bridge and the reservoir capacitor. No change.
So no I am left wondering if this is in fact just some general E field pick up by the Orange drop cap.
Any thoughts?
Cheers
Ian