"pull down" resistors....could I get a little working theory?

[plexi12000]

hey dudes.....i was looking at some ciruits of popular pedals.....and different biz's that make PCB's.

some will use a 1m, or 1.5m ....others will use something larger like a 2.2m.  -for the same effect pedal.

I guess it's not a huge deal. but, is it something to take into consideration?  Thanks!

[dbp512]

Mr. Black has a wonderful article that goes into detail about pulldown resistors and much more. Very much worth the read.

http://www.mrblackpedals.com/blogs/straight-jive/6629778-what-really-causes-switch-pop

[induction]

Too big and they don't cure the pops, too small and they suck tone. The goldilocks zone tends to be pretty wide on most circuits. Not something to sweat too much about, usually.

[PBE6]

There are two main points of interest when choosing a pull down resistor:

1. Input impedance

The pull down resistor will present an AC load in parallel with the input impedance of the stage following the input capacitor. A buffer, a very common choice for a stompbox input stage, will present a very high (1M or higher) input impedance to minimize loading on the guitar and cable being plugged in. Since the pull down resistor is in parallel with this impedance, small values will drag down the overall input impedance (e.g. a 100k pull down resistor will drag a 1M buffer down to 1/(1/100k+1/1M)=91k). In order to keep the input impedance high, pull down resistor values are usually in the Mohm range.

2. Discharge time

A pull down resistor allows any excess voltage on the input capacitor to bleed off in a controlled fashion. This minimizes the "pop" you get when switching effects in and out. The time it takes to discharge the capacitor is usually given as 5*RC, where R is the value of the pull down resistor in Ohms and C is the value of the capacitor in Farads. If the input capacitor is 100nF and the pull down resistor is 1M, the discharge time is about 0.5 seconds. This may be fine if the effect stays on or off for a long time before being switched, but it may be too long if the effect will be switched in and out rapidly.

[GibsonGM]

Nice presentation, PBE6.

** RC can also be calculated using Megohms and Microfarads = Seconds.  Easier units to work with   

[ashcat_lt]

The way we normally have pull-down resistors connected, they are always connected between the "outside" of the cap and ground.  Shouldn't be pretty much constantly doing its thing and bleeding off charge and pretty much preventing that node from floating up too far from ground?  Some of the discussion above, and the general understanding I've gotten over the years is that it somehow starts discharging when the switch is flipped, but I don't think that's really true, is it?

[PRR]

Aim R-C for about 1Hz.

Or 1 Second if that is easier.

Pick any resistor anywhere NEAR that size. 10:1 different should not make a difference.

Much less cuts bass.

Much more may not bleed well enough.

The "acceptable" range is VERY wide.

Panasonic film caps will typically have Insulation Resistance over 9,000 Meg. A 9Meg bleeder would cut stray voltage by 1,000:1; smaller resistors even more. In most guitar circuits, 1 Meg and even sometimes less will do no harm.

Electrolytic caps leak more but are typically larger. e-caps are common at outputs, less common at inputs. Values like 100K will generally be ample.
_____________________

> Shouldn't be pretty much constantly doing its thing

If it is there, yes.

If not, if the far end of the cap goes "nowhere" (with open switch), then we have whatever DC voltage divided by 9,000Meg:nowhere (nominally infinite impedance). The cap will self-bleed "up" to whatever DC is connected to it. When you flip the switch which connects it to a "somewhere", that DC voltage discharges and POPS.

Ask yourself what is the DC voltage at both ends of the cap in all switch positions. If you find the end is sometimes connected to "nothing", it will leak-up to whatever DC is on the other end, and then POP when you connect it.
Pick any resistor anywhere NEAR that size. 10:1 different should not make a difference.

[anotherjim]

I think if the pull-down is always connected on the external side of the cap, whatever the state the bypass switching is in, the only time the charge/discharge time happens is when the pedal is powered on or off.  Once powered on, the external side of the cap has charged to 0volt via the pull-down and will stay that way irrespective of the bypass state - unless you connect something that leaks DC. So if you have the pull-down fitted right and still get pops - suspect something else in the chain.

Some do have the idea of fitting the pull-down to the jack sockets. This seems a convenient place when it's added to something that lacks it and you don't want to modify the pcb, but it's wrong as it's disconnected when the pedal is bypassed (and it's no longer true-bypass is it?). You will then have the cap charging/discharging according to the bypass state. The bypass switch is a better place on the board in/out wires.

I have never had a problem with pull-down as large as 10Meg. A popular trick is to fit the input with 2.2M input impedance and with a 2.2M pull-down giving you 1.1M. If the output cap is before the volume pot, you don't need to add pull-down - the pot is it already.

[plexi12000]

excellent comments!!!  thank you fella's!  most of it over my head....but thank you! lol