Electrolytic Cap DC bleed (causes pop)

[phi1]

In two recent builds, I have had issues with a loud pop when the pedal is activated or deactivated. One build was a reverb, and one build was a 4-channel buffer for a studio application, so technically I've had the issue 5 times.

In each case, I observed DC voltage (0.3v~0.5v or so) on the PCB output, despite the presence of a properly wired pull-down resistor after the capacitor. In each case, I replaced the electrolytic cap with a film cap, and the voltage (and pop) was gone. One project used 1uF and the other 10uF, so it wasn't just a single value of capacitor.

Has anyone else experienced this? Are electrolytics generally more prone to DC leak? Or, does this sound like just bad parts from Tayda? If so, do you recommend another supplier (I'd probably start with SmallBear since I order stuff from them a lot already)?

 

[falzhobel]

I try to use Tantalum Capacitor in those small uF polarized application. I only had problem with pop once (See my Sea Bed post) and it was gone with an output resistor to the ground.

 

[Chuck D. Bones]

Aluminum electrolytics are leaky, some worse than others. Tantalum & film (especially film) are much lower leakage. Whe you use polarized caps, it's crucial to observe polarity. An aluminum cap wired backwards is very leaky. Also, observe the voltage rating. The higher the voltage rating, the less leakage at low voltage.
I see some pedal designs with HUGE output caps and I have to wonder why. Most pedals and amps have a very high input impedance, so what is the point of a 10uF output cap? If the next device in the chain has a 100K input impedance, then 100nF is more than enough, even for bass guitar. Fuzz Faces have a very low input impedance, so if you want to drive the shit outta one of those at low freq, then you might want a cap in the 10uF range. Otherwise, a 100nF film will work great.

 

[phi1]

Thanks Chuck. I will definitely be on the look out for Electrolytic output caps.

 

[Chuck D. Bones]

I have to admit that when I built my Frost Drive, I installed a 22uF aluminum output cap in accordance with the BOM. So far, so good. But if I get any popping or DC on the output, out it comes.