How do you stop pops in rotary switch when selecting capacitors?

[KH602]

I am wanting to add a mod to my wah pedal using a rotary switch to switch between different capacitor values.

When switching between capacitors it pops as you rotate the rotary switch. Does anyone know how to stop this without

affecting the frequency or the capacitor?

i am basing it off this schematic



it looks similar to the pic below, same rotary switch, same layout

[JDoyle]

Put a 1 megaohm resistor to ground on each cap, from the leg of the cap attached to the switch.

[anchovie]

Turn the switch when the effect is bypassed as it's unlikely you'll want to change it in the middle of playing!

[tiges_ tendres]

If I recall, I had one of those snarling dogs wahs with the rotary cap switcher.  If you turned the knob a few times, backwards and forwards through each of the presets, it stopped popping.

[KH602]

cheers for the replys i will give it a shot and test it

[Mark Hammer]

If I recall, I had one of those snarling dogs wahs with the rotary cap switcher.  If you turned the knob a few times, backwards and forwards through each of the presets, it stopped popping.

That's right.  It happens because the popping is stored charge being let out all at once.  If you rotate the switch a bit, then each of the caps has a chance to drain off, which means they won't pop when engaged.

Jay's suggestion of strategic use of high-value resistance is intended to steadily drain off the cap, even when not in use.  The value doesn't have to be 1M, but that strikes a compromise between efficiency of drainoff and impact on the signal itself.

Another way to do things is to have the caps wired in series, and simply use the rotary switch to shunt off as many as you need to to achieve the value.  That way, no cap is ever left "hanging" when not in use.  The only difficulty with that is that: a) caps don't drain off quickly through other caps, and b) sometimes the math works against you in nailing the values you want for musical reasons.

[zyxwyvu]

Put a 1 megaohm resistor to ground on each cap, from the leg of the cap attached to the switch.

The large resistors should go across the switch, not to ground. In this circuit, and in many other cases, the sides of the switch are not at ground potential, so a resistor to ground may cause more popping.

[bean]

Or set up a blend pot and get no popping and everything in-between...this is how I did my Wah.

[DougH]

You can also stick big multi-megaohm resistors between each switch lug, as in this schematic (page 2):

http://www.schematicheaven.com/newamps/matchless_dc30_old.pdf

This connects all caps to ground through the volume control network.

[PRR]

> You can also stick big multi-megaohm resistors between each switch lug

That also saves one resistor and maybe a common-lug, and does not need 7 leads stuffed on one lug.

For the case in the top post, the resistors want to be much-much more than the 33K and ~~50K impedances, much-much less than the ~~200Meg cap leakage. A Meg or so is very often a good answer.

[Milothicus]

what if you put the switch before the caps?

wouldn't they build up less of a charge if they're not connected to the circuit when not being used?

[mdaudet]

You can also stick big multi-megaohm resistors between each switch lug, as in this schematic (page 2):

http://www.schematicheaven.com/newamps/matchless_dc30_old.pdf

This connects all caps to ground through the volume control network.

Thats your ticket, as DougH said. If you have 6 capacitors, then you need five resistors and place them between the lugs. Try 3M9 or 5M1 resistors.

Regards,

[KH602]

cheers for all the replies. how have dunlop done this then on the crybaby 535q, because that has the cap selector on there. just never had the chance to properly look at one and how it works with out popping etc.

[PRR]

> what if you put the switch before the caps?

Works exactly the same.



> wouldn't they build up less of a charge if they're not connected to the circuit when not being used?

It is the circuit which charges them up. One side sits near 3V DC, the other side near 1V DC. The cap holds a 2V charge.

Doesn't matter which "side the switch is".

If it was just the one cap, there is a 2V "pop" at turn-on and turn-off, but these are masked by the whole circuit coming up from zero to 9V and back.

Here we have one cap charged to 2V, and five more un-charged. When switched-in, these caps must charge-up to 2V. This kicks right into the input, and is large compared to guitar signal, POP!

Meanwhile a cap which has been charged to 2V and then switched out self-discharges to zero. On a good cap this should be "slow". I bet if you switch through all the caps, and then switch back, the pops are much less the second time. However if you play a while, they discharge back to zero, and will give a 2V pop when put back in.

The 1Meg-5Meg resistors across the switch keep the "idle ends" near the same DC voltage they will have in use. Pop is much less. Assuming 100Meg capacitor leakage and 1Meg bleed resistors, the pop is about 1/100 of before, a big reduction.

There's other ways. You can redesign the circuit so both ends of the caps are the same DC voltage. But for a simple system like this, that's a major effort and surely more costly.

> have the caps wired in series

In this case, that reduces the pop by roughly the number of caps. All-in, the 2V divides across all caps, at the other limit only one cap has the 2V. You can add bleeders on all but the last cap, then it takes the whole 2V at all times and the others are at zero VDC whether bled or shorted. Saves one resistor. However I -think- the series-string is liable to need more total nFd than the parallel, so the cost may be the same. Even if I'm wrong, the cap-math for series is too tedious for one-off DIY construction.

[KH602]

thanks for that. how would you go about making both sides the same voltage to get rid of the pop?

the wah pedal with the rotary switch looks like this



rotary switch is in the top right corner. can't really see any caps....just trying to incorporate the rotary switch into another design

[PRR]

> how would you go about making both sides the same voltage to get rid of the pop?

I think this was covered?

zyxwyvu "large resistors should go across the switch"

DougH "stick big multi-megaohm resistors between each switch lug"

[Unlikekurt]

PR, if using this method as a tone control would a shorting switch perform better than a non shorting?
Meaning, does that moment of "break" from a non shorting have the potential to create a pop?
Also, obviously if one were making selections during times of non playing, then this wouldn't be an issue but lets say you were playing and wanted to select a new cap, again would there be a perceived difference between using a shorting and non shorting switch?  ie: would the momentary "break" from the non shorting switch cause a perceivable tonal shift / noise during the switching?

thanks

[merlinb]

PR, if using this method as a tone control would a shorting switch perform better than a non shorting?

Add a resistor across the switch so both sides are always at the same DC potential. This should cure the pop whichever type of switch you use.

[Mark Hammer]

The classic Gibson ES-355 Varitone.  You will note that each cap is in series with a 10M resistor.  The switch selects between each of the cap/resistor junctions.  When the switch is NOT selecting that junction, the cap is draining.  When the switch DOES select that particular junction, it's like the resistor is not there.

[Unlikekurt]

Thanks Merlin&Mark

Mark, the varitone schematic is one that i'm aware of and what sort of stemmed the question. 
Is there a value in doing it that way as opposed to the method posted by PRR earlier in the thread (large Megaohm resistors between the switch throw lugs)?

Merlin, I see how that would work.  But that being said, simply put, would the use of a shorting switch eliminate the need for that?