Switch popping.... help me stop it please!!

[BDuguay]

I've built a simple switching box - 1 input to 6 DPDT switches for led status outputs. The input jack goes directly to a buffer circuit.
There is a noticeable pop when selecting from the 6 outputs. Any and all suggestions to correct this will be greatly appreciated.
Brian[/quote]

[smoguzbenjamin]

Is there a pulldown resistor after the buffer?

And do you mean one signal being split in 6 paths, or
6 signals, of which you then select one to "let through"?

[Mark Hammer]

Hi Brian, and happy new year.

First step to reducing popping is to identify if and where there might be any input or output caps with one end "hanging".  If the cap gets a chance to accumulate and store charge, but has no opportunity to discharge when you're not using it, then the pop you hear on switching is all that charge coming out the first chance it gets - a sort of "hard discharge".  Ideally, you want to tie that "free end" to ground via a suitable drain resistor so that when you are not busy charging it up with input signal it has drained off and is effectively "reset".  The value of that resistor would depend on how big the cap is (more current charged, = bigger need to drain while "off-duty" = lower value resistor), and the various input/output impedance needs of the devices in question.  E.g., a 47k resistor would provide terrific current draining capabilities, but would likely degrade the signal quality because it is too low a value.

The anti-popping resistors you'll see folks like RG and others add to designs are generally not critical values.  For instance, you'll see input resistors of 2M7 and up (4M7 is common).  Feel free to use smaller values if you have them, since I imagine many parts bins are not too stuffed with values between 1M and 10M.  I'm fairly confident the values of add-on parts usually found in schems are simply the largest resistor value you can get with adequate current draining capabilties.  E.g., a 4M7 resistor is pretty darn close to open circuit, so it will very likely not interact with the output resistance of any preceding stage in any audible way and that makes it an ideal choice.  A 1M resistor will also drain caps nicely, but *may* not be a large enough value to avoid interacting with *any and all* preceding stages.  You can usually use 1-2M2 and sometimes even 470k-1M with reasonable confidence, though.

Another source of popping is poor decoupling.  Remember how when you were younger your mom's old sewing machine, or the compressor pump on the fridge or freezer would screw up the radio or TV reception in a completely different room (usually when it was the 4th quarter of the Grey Cup or 7th game of the Stanley Cup)?  They were all sharing the same power lines and none of them had adequate power-supply filtering.  So when one of the mechanical appliances turned the motor on, each revolution of the motor resulted in a momentary current drop in the TV or radio as current was sucked away to elsewhere.  The solution is to have enough current stored up in the power supply caps of each device, such that when one of them suddenly draws on a common supply, the others have enough juice stored up in their "camel humps" (supply filtering caps) to ride out the momentary drop and surge.  
This is why whenever you see resistor-dividers providing a floating ground, you will almost always see a filter/smoothing cap from their junction to ground.  Chopping 9v in half to get 4.5v from the same battery is no different than a radio and sewing machine plugged into the same wall wiring, with one needing 120v and the other needing 40v.  That cap to ground filters any brief surges and drops in available current so that ticks, pops, and other power-related transients from other parts of the same overall circuit are "decoupled" from the circuit fragment in question.

Hope that's clear.

P.S.: I was out your way during the holidays, but it was such a breeze-in, have dinner 3 different places, and breeze-out visit, that there would have been precious little time to get in touch.

[BDuguay]

smoguzbenjamin,
It is a "6 signals, of which you then select one to "let through" " type.
Thanks for the reply!
Where does this pull down resistor go exactly?

[BDuguay]

Oh hey there Mark!
Thank you too for the reply.
I've been unavailable, for a myriad of reasons, to participate in the forum these past 4 or so months. It had a lot to do with moving and staying with the inlaws and changing jobs ( I got my old one back.... YIPPEE!!!) and blah, blah, blah.
I may need a little more splainin 'bout these pull down resistors.
B.

[smoguzbenjamin]

So if I understand right, you've got 6 signals, of which you send one to a buffer and then to the output. So what you do is put a resistor to ground after the last component before the output. Don't make it big though, as it'll raise (or lower) the output impedance. Have you got a schematic of the buffer you're using?

The pulldown resistor is there to let electricity from the capacitors to drain off when the circuit is disconnected. A sudden change in voltage on your signal path sounds like a big 'pop'. So when the caps have somewhere to drain off to, they don't suddenly start pumping out electricity because they bleeded off their charge allready. If those resistors aren't there, the caps remain charged and pop when they discharge. I remember someone here saying it's like taking a whiz after holding it up for ages, and it hurts like hell at first, and that the cap issue is sortof the same story That enough 'splaining?

[BDuguay]

Yup.
The input of the switcher is connected directly to the input of the buffer. The output of the buffer then goes to the 6 output switches. The output switches are wired such that only one output at a time will operate. Pressing another output switch re-routes the signal from the output of the buffer to the switch that was pressed.
I'll try and post the site where I got the buffer from.
B.

[smoguzbenjamin]

I'm still confused at what you're trying to accomplish :? How many signals have you got going in, and how many signals are there going out?

Could you draw a schematic for the whole thing? a picture speaks 1000 words

[BDuguay]

I can't really draw one right now.
I'm picking it up on Friday from the guy I built it for, obviously to fix the problem. I've kinda forgotten how I did it to be honest with you, however, it did work when I built it. I'll try and get photos to post. Essentially he wanted to be able to change instruments to feed different outputs ie. mixer, direct boxes, amplifiers etc., one at a time and have the convenience of "silent" switching to accomplish this.
B.

[smoguzbenjamin]

Ok, i sortof get it, I think. Let's get one thing straight "Input" is where a signal enters the unit, and "output" is where it leaves the unit Because I think I got confused with that. :? I'd like to see the way you wired your switches!