SM23 strange noise issue
Author: Lutz Schmidt
Date: 10/30/2011

I'm runing into an issue with my SM23; perhaps someone might come up with a suggestion before I send it in for repair.

Situation: SM23 (mfd 1961) with 2x NT52a (heating stabilizer updated) into RME Micstasy / RMEFireface / Laptop

I wanted to check it for recording sessions next week. At the beginning everything was fine. No issue (other than the inherent self noise).
Then after 1 hour or so it develloped (on both channels !!!) a kind of oszillation noise, souding a bit like cell phone interference into audio. It varies sporadically. RTA shows massive peaks 12kHz and 24kkHz.
No cell phone around; not sure about WLAN in the building. Distance to Laptop does not seem to alter it.
Once I move my hand around the mike (without touching it) it alters.
Adjusting the polar pattern voltage makes it lounder for a short moment. Once the capsule bias is stable, it lowers to what it was before.

I checked the grounding and the connectors but could not trace the issue. I think it is RF interference. Any suggestions are welcome.

Lutz Schmidt
Lutz Schmidt