[279] Pulldown Resistors

Date: October 17th, 2008 | Comments : [8] | Categories: DIY.

Pulldown resistors

There are a number of stompboxes that do not have “pulldown resistors” on the inputs and outputs. As long as the capacitors on those connections have very low leakage, there will be no problems with popping sounds when the pedals are switched on. As shown above, the dc voltages at the circuit capacitors that interface to the outside world should be 0v because no dc is able to pass.

But in the real world, many capacitors will leak a tiny voltage which will build up on the input (or output) as a small voltage potential (5mv in the example). When the pedal is switched on, the voltage is discharged into the signal path and a “pop” is heard.

Pulldown resistors

The dc voltage that accumulates must be eliminated to stop the popping sound. It is frequently asked on the forums if the pulldown resistors can be placed on the jacks. The answer is “NO” and the next drawing will make it obvious why.

Bad pulldown application

If the resistors are added to the jacks, they will do nothing to drain off the voltage when the pedal is bypassed and the leakage will still accumulate as a small voltage potential. The proper way to add pulldown resistors is mount them at the input of the circuit boards, as shown below.

Proper pulldown resistors

The small charge is now leaked off through the pulldown resistors (shown in blue) and the input (or output) will have no voltage to cause pops as it is switched on.

Should your pedal still pop after the addition of the pulldown resistors, the sound may be coming from sudden voltage supply variations caused when the LED is switched on. There is another AMZ article that deals with this type of noise/popping.

 

8 Responses to “Pulldown Resistors”

[595] friender Says: 5:58 pm, January 9th, 2009

If you use a 3pdt switch and ground the effect input when bypassed, there’s no need for a pulldown resistor, right?


[605] yk Says: 11:54 am, January 30th, 2009

Wat if I use a 3pdt switch and ground both the effect input and output when bypassed?


[629] johnD Says: 10:25 am, March 10th, 2009

What is the ideal value for the pull down resistor? 1M, 2.7M ?


[630] admin Says: 11:57 am, March 10th, 2009

It is the largest value that will eliminate the pop. I usually start with 1M or 2,2M and then reduce the resistance value, if required, to bring the pop down to a tolerable level.

-Jack


[691] FEDERICO Says: 7:04 pm, April 29th, 2009

hi master Jack tnx for your advise!!!! i´ll do my homework and let you know….


[2889] PeeBee Says: 11:39 am, May 31st, 2012

When adding a pulldown resistor to a positive ground guitar effect eg treble blaster, should the resister be connected to ground i.e. positive or to the negative of the battery?


[3852] Mike Says: 10:02 pm, April 25th, 2013

I use a piezo pickup on my guitar in addition to the regular magnetic pickups. When I switch from one to the other, I get a loud pop. Would adding a pull down resistor help stop this? I’ve asked this question on all sorts of forums, with no real answer. Thanks for your time!


[5875] Brian Says: 11:46 am, February 4th, 2014

So the pulldown on the input side of the circuit affects the input impedance correct? For the circuit in your example, does the input impedance become 1M || 1M || Rpd, where Rpd is the pulldown resistor value?




 

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