Buffer Popping When Switched?

[Mandobird]

I put together a simple buffer circuit, with the addition of a switch activated pot to lower the volume.


I took the advice given in this thread and put the volume pot between 2 buffers.  The switch basically selects between 1 buffer without the pot, or 1st buffer, then pot, then 2nd buffer.
http://www.diystompboxes.com/smfforum/index.php?topic=102496.0

However when pressing the switch it gives off a loud pop through the amplifier.  I've tried putting 1m resistors to ground at the input & output capacitors but the problem persists (although I'm guess this would probably only work with true bypass switching).
I also tried wiring it up to 2 different switches (a toggle and a footswitch).

The circuit is boxed up, and I've tried it on a breadboard.  I've also tried using a 9v power supply and a 9v battery (there's no battery inside, so it's not stereo jack switching that's causing the pop).

I've overlooked something I imagine.  Anyone got any ideas?  Is there a fundamental flaw with the schematic?  Thanks.

[amz-fx]

The pop might be coming from C2.

Here is an alternate version that I think does what you want:



regards, Jack

[R.G.]

Any time you have a capacitor with a DC voltage across it and open one end, the cap keeps the voltage, but starts running down a little from internal leakage. This is why we use pull-down resistors on hard-switch bypassing - the cap "relaxes" while it has one end open, and when you reconnect it, that "relaxation" voltage appears as a pop when the cap recharges.

That's exactly what you were doing with C2 - and C4, by the way. Rearranging the circuit so that caps always have a wire or resistor pulling each end to the DC voltage they should be at will eliminate pops caused by opening the caps' leads.

[PRR]

> when pressing the switch it gives off a loud pop through the amplifier.

Look at the DC voltages on the switch:



The opamps sit at +4.5V. The cap (C2 C4) on the other side of the switch idles at an un-known voltage, but probably near Zero.

Throw the switch and there is a 4.5V transient.

Since guitar is more like 0.2V, the POP!! is about 20 times bigger than the guitar.

There are many ways to fix this.

Jack's suggestion looks fine. Different from yours (aside from dealing with the DC), it makes the first opamp drive the pot and second opamp even when not needed. If second opamp were a HIGH gain stage, we might choose to kill its signal so its distorted output did not leak to the desired output. But with a unity gain stage, it does not matter. (Jack also saves one switch-pole.)

[Mandobird]

Thanks for the replies, really helpful stuff.  Jack's suggestion worked fine, just boxed it up and the deafening pop is no more.