Pull down resistor vs. bias resistor, the showdown.

[acehobojoe]

So, I figured out that the 1m at the beginning of this good ol' pepper mill was having beef with the 1m past the capacitor. On the schematic, it isn't even there, but on the perf layout, it has an extra 1m at the beginning.

Somehow, I must assume that the bias is escaping.. this is probably why the PDR on most circuits like this are 10m or greater. I've also seen it called a "dehum" resistor, which would explain the annoying hum that the circuit now posseses.

http://www.runoffgroove.com/peppermill.html

[wilrecar77]

If I'm not mistaken, the 1M resistor to ground before the cap is to eliminate pops when switching the effect on and off. It lets the voltage on the input side of the cap be equal to ground, which in turn eliminates popping that would be caused by a sudden DC shift. I'm not sure what the biasing issue you describe is - did you measure a difference in the transistor bias with and without that resistor present? The PDR should not affect the DC bias of the transistor at all.

Edit: Just remembered what PDR would stand for 

[PBE6]

The pull down resistor is isolated from the DC bias by the 0.022uF input capacitor, so it should not have an effect on bias unless the cap is leaking.

Pull down resistors are often chosen to be as large as possible (provided they still reduce switching pops) in order to keep the input impedance high. This circuit appears to have an input impedance of 1/(1/1M + 1/1M) = 500k without the pull down resistor, and 1/(1/500k + 1/1M) = 333k with it. Increasing its value to 10M brings the input impedance back up to 1/(1/500k + 1/10M) = 476k which will retain more high end.

The hum is likely being caused by something else. How are you powering the circuit, battery or power supply?

[acehobojoe]

power supply. the hum isn't too big of a deal. I'm sure it's something easy to fix.

Possibly one cap is leaking?

[GibsonGM]

power supply. the hum isn't too big of a deal. I'm sure it's something easy to fix.

Possibly one cap is leaking?

Power it with a battery and see if the hum goes away.  

Other sources for hum are being too close to your computer monitor, or a florescent light, etc, with the circuit out of the box unshielded or on a breadboard.  Even a bad ground on your guitar cable will cause this.    IF the .022u cap were leaking, it COULD cause a hum, but the more likely culprit is probably the simpler thing - the power supply.  

Even if a cap were bad, the hum is still coming from the power supply, remember. To test a cap for DC leakage, just measure for DC on its 'outside' leg, opposite the side that is getting the DC....

[GGBB]

I'm not sure you need a pull down with this circuit.  It's main function is to allow the input cap to discharge when bypassed so that when engaged again there is no DC shift pop. In this circuit, the 1M after the input cap allows it to discharge as it is non-polar. Or so I have been led to believe.

[bluebunny]

I built mine without any pull-down resistor.  I no longer have it (one of my tame guitarist buddies snapped it up), but I don't recall any popping.  I do recall it being dead good!

[acehobojoe]

sounds good to me. it is unshielded now, so that's likely the culprit.