Soundcraft (Spirit) Multitracker MicPreamp Problem

[peterh]

Hi,

i have two Spirit Soundtracker, both showing the same issue:

Setting the gain above approx. 40db the mic-preamps become very noisy. But the noise sounds more like an oscillation. As both units show the same effect, i assume that this is a design problem. I like these uniits very much and the issue was not a problem so far, because i uses CondensorMics, but now i also want to use ribbons, and there, i need the gain....
Does anyone one of you knows about this issue, and knows about a solution ?
As Soundcraft only sells the complete ServiceManual, does anyone have the circuitdiagram of the MicPreAmp ?

Any hint is very welcome.
Thanx, Peter

 

[bernatvm]

The cause can be dirty or worn gain potentiometer or leaky capacitor 1000uF.

Don't know what console are you exactly talking about, but this reffers to Spirit live mixing console which I assume would be similar...

 

[JohnRoberts]

All preamps reveal noise when turned up to high gain... Mic preamps are designed for low input Z termination so turning up with inputs floating can be noisier.

JR