Why Does Two Circuits In Same Box Cause Popping Problems?

[Paul Marossy]

So I have built or worked on three different pedals that have basically two circuits in them, and all of them have issues with making popping noises when operating the switches.

The first one was my Blackstone MOSFET OD clone. Either switch would pop on a power supply, but not when you used a battery only. Next I just build a Deadringer (Fulltone Fulldrive II clone) and every single switch pops, toggle switches included. It's technically not two circuits, but it does have a boost feature. And the third one I can't say what it is, but it also has a boost circuit and it pops as well. It's only two circuits in series that work fine individually, but when you put them in the same box you get popping problems.

I've been trying to think of why the heck this happens, I'm baffled. Is it a power supply thing? Does the higher gain of two cascaded gain circuits make it so that the microphonically pick up the switch noise? What the heck is it?!?!?!?!?! I would really like to figure this out because it's driving me crazy.

[HOTTUBES]

I'm going to assume you have tried Pull Down Resistors to help kill the POP !

or

Try this ( as seen below )


http://www.diystompboxes.com/pedals/images/3PDTbwiring.jpg

[Gus]

For the power supply the answer is most likely related to the step response of the power supply and the current draw.
Do this test load the power supply more if it is rated for it 10ma 20ma and listen if the pop decreases.  Ohms law voltage / current = resistor load  then calculate the power rating of the resistor(s)

Search for step response also search for transient load testing of power supplies.  Also read up on power supply design.
DIY power supply?

Basic troubleshooting the thing that causes the change(the power supply) is the section to look into.

For the Fulltone clone you could have a microphonic part(s) that "senses" the mechanical shock from the switch.  If you float the board from the case switches does it go away?  

Using a scope to see what is happening on the +9VDC or nodes of the circuit as you switch the different switches.

[R.G.]

The short answer is that two circuits in the same box does not *necessarily* cause popping. The company I work for started with two effects in a box, and a fair amount of our production is still two-in-ones. There is some side effect causing this. Good guess on power supplies, I think.

Gus' comments are appropriate.

I distinguish between ticks, pops, and thumps. Ticks are momentary spikes. Pops have some more low frequency in them, and thumps have only the lows. Generally ticks are AC-coupled edges, pops are instant DC level jumps, and thumps are slowed-down level shifts. When they get too slow, the frequency content is too low to hear.

Ground or power supply shifts when something is switched, often an LED, as that is a big power user, can cause ticks. Usually not pops. DC levels on outputs usually cause pops. If you're not careful about bias supplies, shifts in current use on power or ground can cause sudden movements in a bias supply, and a tick or pop because the circuit is (correctly, in its view) amplifying the movement of the bias supply.

Detective work is needed here.

[Paul Marossy]

Of course all of my circuits have pull down resistors. On the fulldrive clone it will still pop even with no LEDs in the circuit.

I'll have to try Gus' suggestion on the power supply.

I also will try switching the 9V on the LEDs instead of the usual way. That might help on one of my builds. But the fulldrive clone is the most perplexing one of all. I'll have to try pulling everything out of the enclosure and see what happens with the"pop". I'm not sure what I should call it. It's kind of a thump I guess you could say. It just seems like there is DC floating somewhere on this one because some of the pots will be scratchy depending on how the switches are set. But I used a verified layout so I don't know what the deal is. I reflowed pretty much all of the solder joints too. Just perplexing!

[midwayfair]

Next I just build a Deadringer (Fulltone Fulldrive II clone) and every single switch pops, toggle switches included.

Actually, this sounds like something's contacting the case or possibly a solder bridge to ground ... somewhere that doesn't prevent it from working properly but does leak stuff from the case back into the audio circuit. I remember you had some issue with the bias supply on that one -- maybe there's something else going on that requires a magnifying glass? otherwise, you can also try a 100R in series with the input and output; that might at least help with the stomp switch noises in a way that the pull-down resistors don't.

By the way, the only incurable switch pop I've ever had was also a 3-in-1. I ended up wiring everything for buffered bypass. :/

[R.G.]

Of course all of my circuits have pull down resistors. On the fulldrive clone it will still pop even with no LEDs in the circuit.

I was pretty sure you had, but I had to ask to be sure I didn't miss a basic. I've inadvertently left off pulldowns before myself.

I also will try switching the 9V on the LEDs instead of the usual way. That might help on one of my builds. But the fulldrive clone is the most perplexing one of all. I'll have to try pulling everything out of the enclosure and see what happens with the"pop". I'm not sure what I should call it. It's kind of a thump I guess you could say. It just seems like there is DC floating somewhere on this one because some of the pots will be scratchy depending on how the switches are set. But I used a verified layout so I don't know what the deal is. I reflowed pretty much all of the solder joints too. Just perplexing!

I've traced similar situations by using my meter to measure the DC on all the pots and output cap pins, etc. The meter tells me what is there, not what I think it should be.

[Paul Marossy]

So I tried switching the 9V to the LEDs instead of ground, that doesn't help at all. In fact, I think it made it a little worse.

Does it make any difference that one of the two circuits on the same PCB has a Vref supply and the other one doesn't? I keep coming back to that, but I am not sure why...

[R.G.]

So I tried switching the 9V to the LEDs instead of ground, that doesn't help at all. In fact, I think it made it a little worse.

Does it make any difference that one of the two circuits on the same PCB has a Vref supply and the other one doesn't? I keep coming back to that, but I am not sure why...

Time to get some answers.
- Is it the power supply? You can get some info on this by temporarily putting a BFC across the power supply, one on each circuit. 1000uF should show some change if this is the problem.
- Is it the bias supply? You can tell this in a similar way by putting a BFC across the bias supply.
- Is it the LEDs? You can tell this by removing the LEDs temporarily. If it's the sudden change in current when the LEDs come on, this should stop it.
- Is there DC loose where it should not be? You can tell this by getting out the meter, and measuring the DC voltage on each pot terminal (as these scratch) and each switch terminal in each switch position. If you get a DC difference across a pot, it will scratch. If you get a different DC across the switches when they switch, this can cause a pop.

[Paul Marossy]

I'm really talking about two different projects here, but I am focusing on one at the moment. I will try those suggestions on both of them and see what I can find. Thanks for the help R.G., I will report back with my findings in the next day or two.

[Paul Marossy]

OK, so now I feel like an idiot. I finally plugged the Fulldrive 2 clone into a tube amp last night and tried it - no popping from any of the switches. But if I plug it into my little LM386 test amplifier, pop city, ALL the switches pop - and that's what I had been testing it with previously . So at least I feel like my sanity has been somewhat restored on that one.

On my other problem pedal, this is a really weird one. When it was first sent to me, the boost section popped when switched on/off. I added a couple of caps and resistors and no more popping. However, when I sent it back to the owner, they tell me it still pops. Any ideas on what the heck is up with that?!?!?! Can this be something to do with building grounding, or lifted grounds in equipment? All my stuff is properly grounded, and they say they run everything thru a power conditioner. There has to be an X factor somewhere here...

[wavley]

OK, so now I feel like an idiot. I finally plugged the Fulldrive 2 clone into a tube amp last night and tried it - no popping from any of the switches. But if I plug it into my little LM386 test amplifier, pop city, ALL the switches pop - and that's what I had been testing it with previously . So at least I feel like my sanity has been somewhat restored on that one.

On my other problem pedal, this is a really weird one. When it was first sent to me, the boost section popped when switched on/off. I added a couple of caps and resistors and no more popping. However, when I sent it back to the owner, they tell me it still pops. Any ideas on what the heck is up with that?!?!?! Can this be something to do with building grounding, or lifted grounds in equipment? All my stuff is properly grounded, and they say they run everything thru a power conditioner. There has to be an X factor somewhere here...

Maybe the stuff they have it plugged into doesn't have a pulldown on it's input.  I had a problem with a pedal that I built that had pulldowns, but it made my Space Echo pop.  I only figured it out when I switched to a buffered pedal before my space echo and it didn't pop.  So instead of just adding a resistor to my space echo I replaced it's stupid, low impedance, and noisy input with a Mini-booster with a pulldown.  Problem solved.  I guess the pulldown resistor I had on the output just wasn't doing anything for the input cap on my space echo after 10 feet of cable.

[Paul Marossy]

Never thought about cable length being a factor. I use the shortest possible cables. Maybe they use 20 footers? Could be a contributing factor I suppose. When I did my testing it was straight into the amp from the pedal, so guitar --> pedal --> amp.

[wavley]

Never thought about cable length being a factor. I use the shortest possible cables. Maybe they use 20 footers? Could be a contributing factor I suppose. When I did my testing it was straight into the amp from the pedal, so guitar --> pedal --> amp.

Well, I don't know if it actually is a factor, it's just my guess.  RG is probably the one to answer that.  I can't imagine that my 23pf a foot george l cable is adding so much capacitance that it negates the pull down on my pedal.  But then again we often need local bypass caps on things that are far from a power supply and it's such a small change in signal that makes such a big noise so it makes sense to me.  To clarify, I skipped a step in my process, I added a resistor in my space echo for just long enough to confirm that it worked, then I decided to finally fix it's miserable 16K input impedance.

[samhay]

OK, so now I feel like an idiot. I finally plugged the Fulldrive 2 clone into a tube amp last night and tried it - no popping from any of the switches. But if I plug it into my little LM386 test amplifier, pop city, ALL the switches pop - and that's what I had been testing it with previously . So at least I feel like my sanity has been somewhat restored on that one.

On my other problem pedal, this is a really weird one. When it was first sent to me, the boost section popped when switched on/off. I added a couple of caps and resistors and no more popping. However, when I sent it back to the owner, they tell me it still pops. Any ideas on what the heck is up with that?!?!?! Can this be something to do with building grounding, or lifted grounds in equipment? All my stuff is properly grounded, and they say they run everything thru a power conditioner. There has to be an X factor somewhere here...

Not an answer, but perhaps a hint: If your 386 test rig doesn't have an input buffer then it has, IIRC, an input impedance that is signficantly lower than your typical amp or effect. What was the 'owner' plugging the other pedal into?

[Paul Marossy]

Not an answer, but perhaps a hint: If your 386 test rig doesn't have an input buffer then it has, IIRC, an input impedance that is signficantly lower than your typical amp or effect. What was the 'owner' plugging the other pedal into?

That applied to the one pedal I had where every switch (toggles included) would pop. This is a different pedal I'm talking about now. It pops with his amps and not mine. Not sure what amps he has, but I tested it with five different amps that I have and it popped with none of them.

[earthtonesaudio]

Quick sanity check:

Grab a lead with alligator clips at both ends. Plug a cable into the input of the poppy pedal.  Short tip to ground using the alligator clip lead.  Test for pops.  Repeat with poppy pedal's output.  If either setup silences the pops you have a place to look.

[R.G.]

Meters are our friends, folks. They tell us stuff about mysterious electrical doings that we otherwise can't see, hear, taste or smell.

Most electronics has an input cap and an output cap. This is to block the internal DC level from getting out on either the input or output. If a circuit is missing either an input or an output cap, it puts a DC level out on the signal lines to/from it, as these are (mostly) just wires - shielded wires, but wires. If an input cap or output cap is electrolytic, but turned the wrong way round, it leaks prodigiously. If an input pulldown is missing, the tiny leakage from even non-electro caps can leak the capacitors so they have a non-ground voltage when switched by "true bypass" switching.

If an output is leaking DC it puts DC on everything following it that's connected by wires and the beloved "true bypass" switching. That can make everything after it pop when switched. If an INPUT is leaking DC it can put DC on the caps of everything before it. If an amplifier - which is just another circuit, after all - leaks DC out its input, it can make a pedal that is otherwise blameless pop.

To track down leaks, first measure the input and output pins of each circuit separately with your meter. If DC is under a few millivolts to a 10M input resistance meter, you may assume it's not the culprit. But you have to measure everything.

Capacitance value per se doesn't matter to DC leakage. The added capacitance of a cable won't change popping much. LEAKY caps will lead DC. It's the leaked DC charge that does it. Bigger caps may store more of it.

If it pops, DC's getting in somewhere.

Meters are our friends.

[Paul Marossy]

If it pops, DC's getting in somewhere.

This is one golden rule to remember for sure.

A little background on the LM386 amp tester thingie. It used to be a RadioShack mini-amp that I rehoused in a decommissioned enclosure. I guess my mistake was in thinking that it was OK as is. I'll have to look at it now, I wonder if even has an input cap.

It was like this:


Now it is like this:

[Paul Marossy]

I finally got around to looking at this little audio amplifier to see if it had an input cap. It didn't. So I added one. Live and learn.