SSL Shield Question

I'm having doubts about the shielding on the pots of the SSL 9K schematic (highlighted in red)  and I would be really thankful if you could help me answer the following questions:



[list type=decimal]
[*]Why are the shields connected to the output of the opamps instead of just going to ground?

[*]Are the shields supposed to be floating on the opposite end? Or should the opposite ends go to ground?

[*]Or should there be a single shield that connects both pads together?
[/list]

All DIY designs based on this preamp just ignore these shields and the resistors attached to them. Why would SSL put that additional shielding circuitry if it is (apparently) not needed?

Thanks!!!

"Driven Shield".

Capacitance to ground should be avoided, since it can be a filter. Shields DON"T HAVE TO BE ground. They can be something with a low impedance relative to ground.

tomas1808 said:

I'm having doubts about the shielding on the pots of the SSL 9K schematic (highlighted in red)  and I would be really thankful if you could help me answer the following questions:

[list type=decimal]
[*]Why are the shields connected to the output of the opamps instead of just going to ground?[/list]

Click to expand...

As Keith already answered they just need to be connected to a low impedance. In this case connecting the shield to ground introduces capacitance that could destabilize the circuit.

• Are the shields supposed to be floating on the opposite end? Or should the opposite ends go to ground?

Click to expand...

float... if you ground the free end you effectively short the op amps output to ground.  In a high RF environment (like broadcast) you might use a small C (or RC) to ground, but C to ground is what we are trying to eliminate with this technique.

• Or should there be a single shield that connects both pads together?

Click to expand...

different signals require different shields, you can't short the two op amp outputs together.

All DIY designs based on this preamp just ignore these shields and the resistors attached to them. Why would SSL put that additional shielding circuitry if it is (apparently) not needed?

Thanks!!!

Click to expand...

Shielding for the gain pot depends on how far away from the rest of the circuit it is, and the environment. I did one console back in the '70s when the gain trim pot was almost 2' away from the circuitry,,, I used a driven shield for that.. Slightly different version but same concept.

JR