Switching op-amp input in Rebote w/o popping?

[sfr]

( A continuation of my previous thread here: http://www.diystompboxes.com/smfforum/index.php?topic=49876.0      Rebote 2.5 schemo is linked from here:  http://tonepad.com/project.asp?id=51  )

Okay, I'm still having wierd issues with attempting to create a momentary switching setup for the Rebote 2.5.   Basically, after the first half of the TL072 (IC2b in the schematic) I want to break the input to the begining of the delay line (the first integrated opamp in the PT2399), so no new sound is added to the delay, but any repeats occuring finish their fade out.   The point where I need to break this is somewhere after pin 7 of the TL072.  It appears the ideal place to break it is either before of after the 1µF capacitor feeding into the first of the three 12K resistors at the inverting input of the first integrated opamp in the PT2399.  As far as I can tell, if I break any later than that in the circuit, I mess with the repeats that are already occuring.

Below is a snippet of that section of the schematic, including some of the switching I've tried.  It's a little convoluted, but I hope it makes sense.



(If the mess with the two switches doesn't make sense, imagine it as basically either taking that 1µF cap and either grounding it or connecting it to pin 7 - the first switch basically just routes to the momentary switch, so that I can still use the pedal "stock" if I wish.)

The first opamp, on the left, is the TL072, the second opamp, on the right, is part of the PT2399.    Also, contrary to what I've drawn, I've used a non-polarized cap for the 1µF cap there.  (I've tried breaking that connection both before and after the 1µF cap, before seems to work better, and I didn't think I should ground the positive side of the cap)

Here's the thing - *without* that ground connection on the NC side of the momentary switch, simply making or breaking that connection without redirecting the input of that second opamp to ground - when the "feed" connection is broken, that first 12k resistor is hanging, and the gain of that opamp rises, and the repeats both get louder, and the delay line can oscillate much sooner than it could in "stock" mode.  You basically have to figure out your knob settings all over again in comparision to using the pedal in it's normal mode.  Depending on the settings you end up with, you can wierd volume changes on the delayed signal as you open and close the momentary switch. 

So I ground that 1µF cap/12k resistor combo, and the gain structure doesn't change.  However, the cap induces some nasty popping.  (Obviously enough.)  And of course, being a delay, "nasty popping" is actually nasty popping ... popping... popping...  popping...

I've tried 1M and 2M2 pulldown resistors to earth on either side of that capacitor, I've tried switching from both ends of that cap, and no luck avoiding that pop.

So does anyone have any ideas?  Would some sort of FET or other electronic based switch work more appropriatly here, to prevent the popping?  Could I leave that connection "hanging" and then use a switch with more poles (electronic or otherwise) to change the resistance in the feedback resistor or one of the second two 12K resistors to set the gain of that opamp back to what is "normal"?    Is there way to get rid of this capacitor?

[sfr]

Actually - is this just something that could be solved with a "make before break" switch?

[R.G.]

Popping happens when you connect different DC levels.

What's the DC level on pin 7?

When you ground the 1uF cap, it goes to zero volts. When you then connect it to pin 7, it suddenly changes to whatever pin 7 is. Unless you're using +/- supplies to get pin 7 to ground, it will pop. In fact, it will probably pop anyway, as most opamps have a vew millivolts of offset.

The way to do this is to connect another 1uF cap to pin 7, and then connect that cap through a 1M to ground. Likewise, add a 1M to ground from the existing 1uF cap. Now you can connect the existing 1uF cap to either ground or the new cap from pin 7 and not change the DC voltage any.

That should help a bunch.

[sfr]

Wow.  Breadboarded that up, ran the wires off the rebote, and whaddya know, it works.  (As if I expected anything else when taking advice from R.G.)  Awesome. 

Once again, thanks for help. R.G.  Guess I learned something today.  I kind of understood why the cap made popping, but I think I get it much more clearly now.

[R.G.]

You got it. Popping may mystify you in the future, but it will be "where??" not "why?".

I once heard: "Experience is what you have left when you've forgotten her name."

[rogeryu_ph]

R.G.,
Hi. I just recently read this topic, I need also your help. I just finished my Rebote 2.5 its nice also from Tonepad. My problem is when I switch on the pedal it POP. I built two project before TS clone and Big Muff from Tonepad also but didn't experience this popping problem. I use the same DPDT switch and same millenium bypass for all of my project. I have I meg pull resistor on pcb between input and ground. What seems to be the problem? Thanks in advance....

[monkeyxx]

referring to R.G.s advice, is it necessary to use an electrolytic in this situation?  or if I use a metal film 1uF cap will I still also need the 1M to ground?  trying to denoise some switches, thanks