Switched Input Capacitor Popping

[phillip]

I've made a switched input capacitor setup for the upcoming Axis Face Revision 4, and I'm wondering how severely the pop will be with the switched setup, and how to possibly resolve the problem.  Here's a schematic of what I've drawn up so far:



In the UP position you have 0.47uF, in the MIDDLE position you have 0.23uF, and in the DOWN position you have 1.05uF.

Will the voltage divider "PreGain" control on the input of the circuit prevent any pops from getting to the input?

TIA!
Phillip

[Nasse]

Just quessing but I would try large resistors to ground from "between the caps"

[phillip]

Here's an improved, simplified input capacitor switching...ala Keeley Java Boost adapted to Axis Face:



UP is 1.04uF, MIDDLE is 0.22uF, and DOWN is 0.44uF.

Will the 1M resistor to ground have any negative side effects on the PreGain control?

Phillip

[R.G.]

OK. Let's think about this.

Why does a pulldown resistor keep input caps from popping? It's because all capacitors leak a little charge, even if it's only through the air around the terminals. The capacitors leak toward a state of 0V on the cap.

Input caps have a voltage on the circuit side, and 0V (ground) on the other side when connected to the circuit. When they are opened up, the caps start their leaking towards 0V across them.

When an open-ended cap is suddenly connected into a circuit where there is a voltage difference, the sudden change in voltage is coupled through the circuit as a pop. This usually happens in a true bypass circuit which opens up a cap as part of the operation.

A pulldown resistor prevents this by pulling the flying end of the cap to the same voltage it has when it's connected into the circuit. That keeps it from relaxing to 0V across it, and then when the switches close, there is no voltage difference to make a pop.

Can you see where that same thing is happening here?

The second 0.22 and the 0.82uF cap are both open-ended when the switch is flipped away from them. The 1M pulldown and the 100K control to ground have no effect whatsoever on their leakage, and when you flip the switch to one of them, you get a pop as the cap charges up.

The fix is simple - put a pulldown resistor on **every** cap that is going to be open circuited. For this circuit, both the second 0.22uF and the 0.82uF need a high value resistor to ground. The always-connected 0.22uF doesn't strictly need even the one pulldown, as it's connected to ground through the wiper of the pot. However, it's really good practice because moving contacts on pots sometimes open up then close, and you could hear this after the pot wears.

The size of the pulldown resistor just needs to be much larger (defined as more than ten times) than the other resistances. I would put a 2.2M resistor where the 1M is now, and a 2.2M to ground from each of the other two caps. That should prevent any pops when switching caps.

[phillip]

Thanks RG!  I had a feeling that would be the case, so I'll have to go back in and add a couple more pulldown resistors to the loose ends of the two capacitors.  

Will the pulldown resistors cause any problems with the PreGain?

Phillip