Pulldown resistors but still popping on in/out switching, solution inside

[Dirk_Hendrik]

Today I was working on a Arion chorus with the request of making it TB. As I find the use of those blue 3PDT's for this job absolute horrible (they destroy the pedal in looks and the possibility to reverse the mod) I tend to use a small DPDT relay for these tasks. The modification was fairly somple and done in some 30 minutes. Problem was that when testing the results the pops were way too loud. Mechanical switching may cause some popping but not like this. So back to the bench to determine where these pops came from. When scoping on the effect input I noticed peaks of about 700 mV which, depending on switching in or out would have a upward or downward slope and would take up to a second to come to ground level. Serious DC. The solution, eventually turned out to be the decrease in value of the input cap which was chosen at the incredibly large 1uF (High impedance input stage following after this cap) which caused the cap to charge up through the bias resistor which makes the iput circuit at half the supply voltage. Decrease of this cap to 47 nF did not give audible changes but minimized the pops. When switching in/out at a lower rate than 1hz the pops weren't noticable as the cap was fully discharged.

in a schemmo:

http://www.dirk-hendrik.com/example.pdf

This may be a point to look at when unable to eliminate pops in DIY circuits

[Connoisseur of Distortion]

interesting idea. maybe a resistor to groundon the other side of the cap could help, too, by offering a continuous path to ground... and not letting that cap charge...

[343 Salty Beans]

but wouldn't a decrease in the input cap mean a decrease in bass response as well? It's kind of a bummer to change the intregral sound of a pedal to stop a technical problem.

Perhaps CoD is onto something, though...

[Connoisseur of Distortion]

he DID say that it didn't affect sound... but my idea would probably put a hurt on the input impedence,.

[Dirk_Hendrik]

interesting idea. maybe a resistor to groundon the other side of the cap could help, too, by offering a continuous path to ground... and not letting that cap charge...

Interesting, However,

At the "primary side of the cap there's the 1M pulldown already. At the other side is the pullup that baises the input stage at half vcc. Havin a pulldown at that side will lower the bias voltage but not the charging. Therefore there's still charging up/down.

[amz-fx]

Ideally, the caoacitor will have the DC bias on one side and nothing on the other (0v) because a capacitor should not pass any DC current...  often, changing the input capacitor to a different one of the same value will stop the pop, or even getting the same value in a different brand will work because they might have less DC leakage.

The pop is exaggerated when the circuit has a high gain, as in fuzz pedals or boosters, and usually less of a problem in low gain devices like flanger/chorus/phaser units.

regards, Jack

[R.G.]

It's quite likely that the 1uF was electrolytic and leaky. The 0.047 was most likely non-electrolytic and possibly new, so it had almost zero leakage.

Pull down resistors will not eliminate pops from leakages that are too large for the resistor to pull below about 20mV.