Seeking Advice re Popping A/B Box

[gaussmarkov]

Greetings!  I am new to this forum and to making stompboxes.  I have read through some posts to try to answer my question but I am not confident of my conclusions, so I am posting.   :?

I just constructed the simplest (some will say naive   :oops: ) A/B box. I would like to use it in both directions, but my immediate goal is to switch conveniently between two guitars going into one rig. When I switch channels (guitars) I get the proverbial popping sound. I was hoping that this would not happen because there is no power in this stomp box.

Will a "pulldown resistor" diminish the pop? If so, what value would you suggest?

The signal and ground wires are all insulated from the aluminum box (I checked the continuity). Where should the resistor(s) go? The single channel side, across the signal and ground lugs of the jack?  Or does the direction I am using the box and the simplicity of my design dictate a choice of direction for a simple solution like a pulldown resistor?

Here's a description of the circuit: I am using a DPDT switch. This switch flips from the A jack to the B jack (both signal and ground).  This may be the best newbie project of all time!  :wink:

[Satch12879]

Usually people place 1M resistors from the hot to ground on ALL of the jacks.  Use really good, metal film resistors to cut down on noise.

[gaussmarkov]

Usually people place 1M resistors from the hot to ground on ALL of the jacks.  Use really good, metal film resistors to cut down on noise.

Thanks!

[gaussmarkov]

I am stumped.  My passive A/B box is popping loudly when I switch jacks.  I picked up some metal film resistors at my local electronics store.  They were out of 1M, so I bought the closest available:  1.2M.  I put these on all three jacks, hot to ground.  The resistors did not change the popping.

I must be doing something wrong, but I just cannot see it.  Here's my very simple shematic:


As I said before, I checked continuity.  The circuit is insulated from the alumninum enclosure.  All the connections are fine.  When there are 1.2M resistors on all of the jacks the resistance measures .6M across the hot and ground for the connected jacks and 1.2M for the isolated jack.  The parts are all standard from Small Bear:  Switchcraft mono jacks and an Alpha DPDT pushbutton switch.

I am connecting two guitars to the A and B jacks.  The other jack goes to an amp.  I have tried two amps with the same results.  The guitar signal is great but there is loud popping when I switch jacks.  I have used another pedal (Barber Burn Unit EQ) to confirm that pops don't occur for everything that I plug in.

I also tried removing the resistors from the A and B jacks.  No diff.  Any ideas?

[puretube]

put pulldown resistor only on "amp" jack;
disconnect the ground wires from the switch, and interconnect them instead;
solder a wire from the "amp-jack" ground-lug, and screw it to the chassis.
(I assumed your jacks are isolated plastic ones? - switchcraft to me is just a company name, sorry...)

[R.G.]

I don't know that this is the case, but one possibility is that the AB box is working perfectly - switching a DC level from one of the connected units. No amount of pulldown resistors or magic foofoo dust will help if that's the case.

Get out your meter and measure the DC levels on each jack in all switch conditions.

[LyleCaldwell]

Since you are switching the ground, I think it's likely that there is a short time when switching that the ground is disconnected, hence the pops.

Check out (and copy) the switch wiring here:

http://fulltone.com/PDfFiles/AB_switcher.pdf

Ignore the LED stuff if you aren't using one (the three lugs on the right in that photo), but note that Mike Fuller has the grounds continuous from jack to jack.  There's no reason not to.

Also notice that he has it wired so whichever side is not used (A or B) is grounded out, so there will be no noise on the unused side (important if you ever use it for connecting two amps).

And I agree that you should ground the chassis.

[gaussmarkov]

Once again, thanks!  I was beginning to think that I might be fated to get a bad switch on my first try.  That's a possibility mentioned by R.G. on his site, and implied elsewhere.  I guess I should try another switch, too?

Lyle, at one point I did try hooking up all the grounds together.  That's a feature of Fuller's switcher.  It did not help then.  But I will try that again.  And I will try Fuller's version.  I am not sure that all A/B boxes run equally well in both directions and it seemed like he was shooting for two output channels.  I'm not objecting to your suggestion, just explaining my own confusion.

[vanhansen]

When looking at Fuller's design, pay close attention to the lugs on the left.  There are two wires connecting a couple of lugs in an X formation.  Can be tough to see because the switch and wires are both black.

[gaussmarkov]

put pulldown resistor only on "amp" jack;
disconnect the ground wires from the switch, and interconnect them instead;
solder a wire from the "amp-jack" ground-lug, and screw it to the chassis.
(I assumed your jacks are isolated plastic ones? - switchcraft to me is just a company name, sorry...)

Thanks for this suggestion also.  And I guess I should've been more detailed.  :oops:  I am using the Switchcraft mono #11 jack, insulating it from the chassis using shoulder washers (listed as Keystone #3069 on Small Bear).  The switch I am using is the Alpha 107-SF12020-L DPDT.

Again, I really appreciate the help.  This seems like it ought to be a no brainer newbie project.  Maybe there's no such thing.  :?  

[gaussmarkov]

I don't know that this is the case, but one possibility is that the AB box is working perfectly - switching a DC level from one of the connected units. No amount of pulldown resistors or magic foofoo dust will help if that's the case.

Get out your meter and measure the DC levels on each jack in all switch conditions.

Thanks, R.G.  I should've done this earlier.  I did read your note about this beforehand and the thought did pass through my mind but ... well, no excuses.  I just checked the DC.  It's zero at every jack switched both ways. (And an old 9v battery measures 8.2v   )

[LyleCaldwell]

Once again, thanks!  I was beginning to think that I might be fated to get a bad switch on my first try.  That's a possibility mentioned by R.G. on his site, and implied elsewhere.  I guess I should try another switch, too?

Lyle, at one point I did try hooking up all the grounds together.  That's a feature of Fuller's switcher.  It did not help then.  But I will try that again.  And I will try Fuller's version.  I am not sure that all A/B boxes run equally well in both directions and it seemed like he was shooting for two output channels.  I'm not objecting to your suggestion, just explaining my own confusion.

Won't matter if it's one input and two outputs or vice versa.  If you wire it like Fuller's and still have popping then you most likely have a bum switch.  Remove the shoulder washer from whichever jack is not being switched (A and B are switched, so remove the shoulder washer from "C").  That way the box will be shielded.

[gaussmarkov]

Using the suggestions posted above, I got my A/B box to switch channels quietly.   I left a pulldown resistor on the single channel side, grounded that jack to the enclosure, and used Fuller's method of grounding the open channel.  I made all of the changes at once, so I have know idea whether (for example) the pulldown resistor is needed.

Thanks everyone!  It's great to get my first stompbox working.  I really appreciate your patience and helpfulness.

[puretube]

the pulldown in your case (single output to amp) is there to take care, that if your amp got an input-cap, it won`t be "floating", but referenced to ground...

[ESPguitar]

So what about my loop box? 1 loop and switchcraft 11 jacks.. Can i totally prevent popping there?

Thanks,

RB

[barret77]

Usually people place 1M resistors from the hot to ground on ALL of the jacks.  Use really good, metal film resistors to cut down on noise.

what's a good US source for cheap, really good low noise metal film resistors?