1M vs 2M2 when using as a Pulldown Resistor. (Some say use 1M some say use 2M2.)

[formerMember1]

HI,

I was wondering why some recommend a 1M and some recommend a 2M2 for pulldown resistors in effects pedals? 

Take the Fuzzface for example, would I benefit more from using a 2M2 at the input becuase it is a "high gain" effect, and the 2M2 will help stop more pops then a 1M would?   Or... 

thanks

[Mark Hammer]

The pulldown resistor tries to strike a balance between being high enough resistance to not load down the input (with "infinite" resistance or open circuit being "best"), and being low enough resistance to let the input cap drain fully when not in use. If the input cap gets all charged up then you step on the stompswitch - effectively isolating one end of the input cap - the cap will not drain off properly and you'll hear a "pop" next time you switch the effect on.....unless you have a pulldown resistor to drain it off by providing a path to ground.

The more charge can be stored in the cap, the bigger a pop you will hear if it goes undrained.  The bigger the cap (in uf), the less leaky the cap, and the higher the resistance the path to ground, the more charge will be left undrained.

So, of the range of possible resistor values you could use, you can err on the side of higher values when the input cap is a low value (e.g., .01 and less).  A 2M2 resistor will make it harder for that charge to drain off in a hurry, but if there wasn't much charge stored with to begin with (because of low capacitance), not a problem.  Conversely, if the input cap if somewhat larger in value (say, 100nf or larger), you will probably want to go with something like a 1M value since you want to speed up that charge draining a little faster with a lower value resistor.

So, they are both "correct" values.  It's just that one is a little more appropriate in one context than the other.  Keep in mind that the appriopriateness of the lower value will also depend on what you are feeding it with.  If the input is coming from another pedal with a low output impedance (say 1k), then even a 470k resistor might be fine.  Don't be shy about subbing a 1M5 or 820k or something similar when in a pinch.  Similarly, a 2M, 2M4, 2M7 or even 3M3 can be perfectly acceptable....if the cap value is a reasonable match to it.

Make sense?

[lessthanmatt88]

I may be wrong here but I don't think it really matters (high gain or not) as long as the value is high enough (1M being high enough).  The pulldown resistor is outside the actual effect and will not affect the sound, and I don't think using a higher value will stop more pops becuase the only purpose is that it brings the voltage level on the outside of the effect up to that of the voltage on the inside thus eliminating that small period of time the input capacitor recovers from leakage (becuase no cap is perfect) when the effect is disengaged.

I haven't tried using different values for this so there may be a difference but I imagine it can't be great; prehaps someone who has can chime in on this.

[petemoore]

  1m's are what I use.
  Since many of these circuits have gain stages, and the 1m's are 'outside' the bypasses, even if you have many effects on at once, the divided resistance to ground sprinkled through the signal chain probably has negligible to no discernable influence or loading...[?]...!
  Say you had 3 effects on at once, all bearing 1megs on input and output...that'd be 6 x 1meg resistors paralleled to ground, I don't know...would it 'matter'?, if it did, perhaps trying 2m2's for no pop cap draining would be worth the troubles.

[petemoore]

  I'm no 'master' so I'm kind of half asking whether each circuit, say it's a gain stage, buffer or maybe a phaser...if each circuit 'negates' the loading that a previous circuits pulldowns impart.

[formerMember1]

thanks Mark and others.  After reading through it a few times i understand it now. 

Lower resistor for the bigger the input cap is.  and vice versa.  But you don't want to go too low a resistance if not need be.

I guess this is where the benefits of true bypass grounded input wiring comes into play,...

thanks again   

[petemoore]

  If the cap in question was 'perfect' ie no leaky at all, 20meg or even no pulldown should work.
  I've left pulldowns off of boxes and had problems, and..not had problems popping when switched, I just routinely two 'big' ones together and install their open ends to the circuits SSwitch I/O lugs, and run a ground to where I tied them. Leaving some room to solder wires to their leads makes for easy [also low switch temperature] IN/Out wiring.

[mac]

I use the bigger resistor that eliminates the pop to avoid lowering the input impedance on high-Z circuits. In a FF this is not a problem since it has a low-Z.


mac

[Mark Hammer]

Some perspective.

Long ago, when stompswitches were pricey, simpler, and harder to find, it was not uncommon (even amongst many "name" manufacturers like MXR, Electro-Harmonix and others) for stompboxes to use SPDT switches for "bypass" purposes.  With only one set of contacts, only one "thing" could be switched, and that thing was the output.  The input of the pedal always stayed in-circuit.  If there was only one such pedal, that wasn't too much of a problem, but if there was more than one it started to turn into a problem.

Whether you had 1, 2, or 5 such pedals, and whether it was or wasn't a problem, one thing was certain.  Once you got past the initial "thump" of plugging your instrument or patch cord into each pedal, there would generally be no popping because those "hanging caps" at the input would remain in circuit, and would never be lifted or disconnected until such time as you unplugged.

The need for pull-down resistors came about largely for two reasons: 1) people started using more than a couple of pedals, and 2) people started using DPDT to take effects entirely out of circuit because they were using more than just 1.  The moment that a DPDT gets used to switch the input as well as the output, the risk of having a hanging cap that will pop when reconnected the next time you engage the effect doubles.  The outputs aren't always that big a risk because a great many effects (i.e., fuzzboxes) would have a volume pot on the output, which provided a means for the output cap to drain.  The input were more often the culprit.  The use of a 1M (or greater) input resistor to cure thumping and popping became standard.

What people tend to forget, however, is that there is this circuit in between the input of the effect and the output.  When companies used SPDT switching, the inputs of effects would always remain tied to the signal line, and having several such things in parallel would add to "tone-sucking" measurably.  Take 3 pedals, each with a 100k input impedance, and even when they are switched off, the amp sees all those loads in parallel, AND in parallel with the guitar.  Why on earth should an amp "prefer" the guitar signal over the thermal (and other) noise of those paralleled inputs?  If the input impedance of each of those effects could be increased enough, then having even a half-dozen of them in parallel and always in-circuit should probably, in theory, pose no problem: the guitar signal would always be significantly lower impedance and the "best choice" of the multiple signals the amp was seeing.

When you adopt DPDT (or 3PDT) switching, however, bypassing the effect means that it is not in circuit at all.  Moreover, whenever the effect IS in-circuit, there is always all this "other stuff" after the effect input that generally results in a low output impedance from the effect.  Hence there is no particular advantage to having the input impedances of all those pedals be so high that even in parallel they won't be a problem, because they will never be in parallel.  The parallel impedance issue is unique to the SPDT-only scenario.

Consequently, while it is generally good practice to make sure that the input impedance is at least 10 times higher than the output impedance of whatever is feeding it, once you get past the first effect in the chain, your output impedance has dropped from the 10k+ usually found in a guitar, to something around 1k coming from a pedal.  If you have a commercial pedal using FET-switching, or have some sort of "always-on" buffer of your own making, goosing the input pulldown resistors from 1M to 2M2 or higher offers no particular advantage.  Heck, even a 220k input impedance is looking good in the face of a 1k output feeding it.  Where increasing the pulldown resistor value DOES have advantage is if your entire signal path is true-bypass and any given pedal stands a reasonable chance of being the "first one in line" (i.e., everything ahead of it is completely bypassed so your guitar simply follows wire up to that pedal's input).

As long as nothing in your signal path is using SPDT switching, however, there is no strong reason to NEED to go above a 1M input pulldown resistor, because that 1M will never be in parallel with anything other than maybe the output terminating resistor of the previous pedal.

There, I hope that clears up some misconceptions and needless anxiety.

[formerMember1]

There, I hope that clears up some misconceptions and needless anxiety.

yep it does. Thanks!! 

[mac]

As long as nothing in your signal path is using SPDT switching, however, there is no strong reason to NEED to go above a 1M input pulldown resistor, because that 1M will never be in parallel with anything other than maybe the output terminating resistor of the previous pedal.
There, I hope that clears up some misconceptions and needless anxiety.

Doh! 


mac