Caps or resistors?

[Andy S.]

Folks,

Hi. New Here. Was looking through the posts and was trying to find a definitive answer to a problem. Working on a Dunlop Cry Baby 535Q pedal.
Everything seems to work fine but when first using the footswitch, there is what sounds, through the amp, as an explosion.

After the first "pop", it can go for a while without the noise. If it sits, yeh, BOOM, again. Also, on the Q rotary switch, it can happen there or on the boost switch.

So, one post mentions putting a high value resistor across the output and input. And I thought I read another, either here or on another forum, that mentions "recapping", replacing older caps.

I have no schematics to work with and the two physically large (maybe 3/8") caps are 220 micro-farad. There are a few physically smaller ones on the the board that I can see. I have NOT taken the board out as of yet, so I don't know what surprises are waiting there.

Anyhow, suggestions on which to do to silence the Pops/explosions on this pedal.

Thanks,

Andy

[bob p]

someone already mentioned the 1 meg resistor beetween signal and ground. how did it work?

[Andy S.]

bob,

Been so busy with the day job, I haven't had a lot of time to take the thing apart farther and try it. Hopefully things will slow down in the next day or two. There's light at the end of the the tunnel!! Hopefully it ain't a semi-truck coming from the other direction!!

I'll post the results as soon as I try it.

Andy

[Andy S.]

Well, the 1M ohm took care of the pop from the off/on switch for the Wah function as well as the boost switch.

However, the Q rotary switch on the side still is popping a bit. Usually louder the further along the contacts you go.

Any suggestions for that?

The switch is on a separate,small pcb with some micro wafer caps on it, with a heavy three lead ribbon connector. I tried putting another 1M ohm resistor across those leads, no help. In fact, the popping came back on the power boost switch. Removed the resistor there and the popping was only left on the rotary switch.


It's a bit frustrating since I can't find a schematic for the 535 Q anywhere and a couple of other posts on other sites were somewhat confusing as to where they put the resistors. On one, the guy put a "47uF cap from the middle terminal to the non-grounded side of the pot" But I didn't understand whether he was still referring to the rotary switch or the actual Wah control pot.

And goofus me!! I didn't mark the site where I read it!!! (having teenage kids and getting old is NOT good for the memory cells!)

So, any ideas are greatly appreciated.

[R.G.]

However, the Q rotary switch on the side still is popping a bit. Usually louder the further along the contacts you go.

Any suggestions for that?

I wrote a long polemic on "why does your stompbox pop?". It's at GEO.

All pops are generated by the sudden connection of a voltage that is different from the existing signal voltage. Unless you're playing, the signal voltage is nominally 0V (ground), so any sudden up or down causes a pop.

There are two common ways this happens. If the Q control switches to one of a number of capcitors, then you're getting capacitor leakage pop. All capacitors leak. If you charge one up to 10V, then come back a while later and check it, it will be at a lower voltage, some of the 10V having leaked down. For really good teflon or polystyrene caps, the leakage from 10V to 9V may take a week. For mylar/polyester, it may be a minute. For electrolytic it may be a second. But if you have a rotary switch connecting one of a number of caps into the circuit, all but the single connected cap will be leaking down, and you'll typically get a pop when you switch.

Enter pulldown resistors. The 1M resistor you used to cure the bypass popping is the classical solution. The input and output caps on the effect are disconnected when the effect is bypassed. Their outer end just floats around in space, open circuited. The cap is there to block the effect's internal DC level and so the cap has a few volts on it. When it's left open, it leaks down. When you REconnect it, the voltage is now different than it was, so the cap sucks in a batch of DC to get back to the correct DC voltage. You hear that as a pop.

I think the Q resistors are similar. If this is like the popular mod, there are a batch of caps with one end in common. The other ends are selected one at a time by a rotary switch. The non-selected caps are left open. When you flip to a new one, it pops because it's leaked down. The solution is simple though. You put a very high value resistor from the free end of the cap to the pole of the rotary switch FOR EVERY SINGLE CAP. This reduces to a 1M or higher resistor from the pole of the rotary switch to every single throw. The 1M's keep the free ends of the caps that are not selected pulled to the DC voltage that they will be at when selected, so no pops happen.

The second major way pops happen is discontinuous selection of DC voltages on resistors. This can happen with switches, but it's more common on pots. DC across a pot puts a different DC level at every place on the pot element. The pot wiper can't keep perfect contact as it moves, so it comes off the element and back on as it moves. This can be a "bounce" in worn pots. When it comes back in contact to a different DC potential, that's heard as a pop or click. Generally these are smaller, and is heard as a scratching sound. This is so common that scratching when a pot is moves is almost completely diagnostic of DC on a pot where it shouldn't be.

[Andy S.]

This reduces to a 1M or higher resistor from the pole of the rotary switch to every single throw. The 1M's keep the free ends of the caps that are not selected pulled to the DC voltage that they will be at when selected, so no pops happen.

Excellent explanation! I will try the resistors on the switch as you described and let you know how it goes.

Much appreciated!

Andy

[Andy S.]

R.G.,

Resistors worked perfectly. No pops!! No change in tone control. Works perfectly!


The article you wrote was well done and easy to understand.

I guess what I don't get is why don't the manufacturers put these resistors in the circuits at the start? Or, with the newer models, are they doing that?

Just thinking out loud, no need to respond to that unless you have some insight on it.

Thanks so much again!!

Andy

[R.G.]

Great! I'm glad it helped.

The effects maker I work for does put anti-pop resistors on all of its effects. It's such a simple thing and so effective that I think it should be universal.