Level Question

[aziltz]

referring to this TS article... http://www.geofex.com/Article_Folders/TStech/tsxfram.htm


Why is the level control (after the tone circuit) connected to 4.5V and not ground?  (4.5V is also called "AC Ground" in this article)

I've seen level controls located at the end of the overall circuit that are connect to ground instead, and that makes sense to me.  Why is it different for controls that are mid-circuit?

[alanlan]

Probably meant to be connected to gnd assuming it's feeding the output jack.  You wouldn't want 4.5V DC on your output.  In mid circuit AC gnd is biased at 4.5V to keep the op-amps running at mid supply for maximum headroom so it makes sense to do it there.

[aziltz]

Probably meant to be connected to gnd assuming it's feeding the output jack.  You wouldn't want 4.5V DC on your output.  In mid circuit AC gnd is biased at 4.5V to keep the op-amps running at mid supply for maximum headroom so it makes sense to do it there.

its not feeding the output, that's why i'm asking.  and i'm seeing it in many designs.  i'm just trying to understand the differences and why.

[grapefruit]

In this case the volume pot is going to 4.5V (ac GND) because they want to bias the FET switches at 4.5V. If you were using a "true" bypass switch the pot would go to ground.

The CCW of the pot is also often returned to 4.5V (ac GND) if you have a volume control on the output of an op amp feeding into another op amp. This way you don't have to have a coupling capacitor on either side of the volume control, as the output of the op amp should be at 4.5V, and you want the input of the next op amp to be biased at 4.5V. You do have to be carefull about offset voltages though, depending on the circuit.

Stew.

[aziltz]

The CCW of the pot is also often returned to 4.5V (ac GND) if you have a volume control on the output of an op amp feeding into another op amp. This way you don't have to have a coupling capacitor on either side of the volume control, as the output of the op amp should be at 4.5V, and you want the input of the next op amp to be biased at 4.5V. You do have to be carefull about offset voltages though, depending on the circuit.

Stew.

awesome, i think that's the case i was asking about.  thanks!