Limiter Rod Elliot

[emosms]

Hi, this circuit adds hum to the signal chain:



I don't quite understand how VTR1 controls the limiter sensitivity, but if it is not shorted (reducing NFB to 1k) it adds audible hum.
Quite a lot at end position.

Even at min setting, if I take out the limiter pcb and bypass the signal, there is less hum.

I am examining my pcb and connection for ground loops, but why it adds hum at relatively small gain (up to gain 10 I think)

 

[emosms]

The actual schematic and pcb layout. Two channel limiter (fr/ horn), the Full range channel adds hum:





At min TR1 setting the limiter is not supposed to work at all..