Vactrols in a preamp

[black mariah]

Looking over the Soldano X88R schematic I notice a number of vactrols (VTL5C1, to be exact) in the circuit going straight to ground. What's the story on that? What use are they in a preamp? Aid to dynamics? 

[Arno van der Heijden]

Switching (by shunting the signal to ground). They are less noisy than relays, but also more expensive.

[black mariah]

I thought about that, but where they are in the circuit it doesn't make sense.

How do they work as switches? I'm FET-inizing the lead channel and might have to keep that setup if it works well... 

[Arno van der Heijden]

Just like I said: they shunt the signal to ground.

When there's little or no resistance, the switch is 'closed' and the signal is directly connected to ground. When there's a big resistance (several megs), this is almost like open circuit and hence the switch is 'open'. By making the right combinations of open and closed switches, one can switch certain parts of the circuit in and out of the signal path.

There's no need to use vactrols for your Fet-version of the amp. A normal mechanical switch will work here and is much cheaper.

[black mariah]

I didn't explain myself well enough. I know they shunt to ground. What I don't know is what activates them, and for what purpose. Would these be the channel selection switches? If so, how do they function with the footswitch? They way they're set up I could see the footswitch being used to drive the LED half of the vactrol, but other than that I'm lost.

[Arno van der Heijden]

Yes they're there for channel selection. The footswitch drives some switching logic, which turns the right combination of LEDs (inside the vactrol) on or off.

[black mariah]

Alright, cool. 

I have the lead channel schematic redrawn in Eagle. The pre and post buffers are not included at this point. I think those will be shared by the two channels I'm planning.

Am I correct in assuming that the B and E leads would just go to +9v?