question on pulldown resistors and impedance

[Marcos - Munky]

Does it make any difference on the impedance if the input pulldown resistor is before or after the input cap? And what about the output pulldown resistor, does it make any difference if the resistor is before or after a master volume pot?

I ask this because I made a tube preamp board, but by mistake I put the input pulldown resistor after the input cap. Also, I didn't added a master volume and may have to do it, but there's already a pulldown resistor after the output cap. Easy things to fix, but if it doesn't make any differences I'll keep them this way .

[idy]

For what you are asking, I think, It isn't a pulldown unless it is outside the caps. Inside they are part of biasing arrangements, outside they serve to drain a leaky cap.(?) And you don't need one after a volume pot because the pot is a path to ground. Unless it is a buffered volume pot, and then you may add one after the buffer.  And yes they affect the input impedence, they are parallel to the "natural" impedence of the circuit, which is why they are generally big.

[GibsonGM]

What IDY said.

[bushidov]

What IDY said.

The pull down resistor after the input capacitor may create a high pass filter that you didn't intend to create, though (RC filter). Assuming your input cap is 100nF, 1 / (2 x Pi x R x C) = 1 / (2 x 3.14159 x 1,000,000 x 0.0000001) = 1.6Hz, which means you are filtering out sounds so low, Barry White may blush. So, yeah, probably not a big deal.

[amptramp]

The pulldown resistor is there to avoid a pop when the unit is switched from active to bypass and vice versa.  It has to come ahead of the capacitor on the input.  It has to be a high value because the inductance of the pickup (if it is the first pedal in the chain) works to cause the high-frequency output to be reduced, known as "tone sucking".  The typical value is 1 megohm but if you are using a film capacitor for the input, you can easily increase it up to 10 megohms.

[GibsonGM]

Right. So, if the 'pulldown' is after the input cap, all it is pulling down is your DC bias voltage...not doing a thing for any popping or whatnot.  The input cap isolates it.

[Marcos - Munky]

Thanks for all the replies. So I'll remove it and put it in the correct place.

[antonis]

Why you don't establish the habbit to wire your 3PDT switches with IN (and OUT, in case of Vol/Level pot absence) grounded when effect by-passed..??

[Marcos - Munky]

I do wire my 3pdt switches with in grounded when the effect is bypassed. But since this is a preamp, it doesn't have a switch . I'm building it to be part of a tube amp, but with preamp and power amp on 2 separated boards.

[antonis]

But since this is a preamp, it doesn't have a switch . I'm building it to be part of a tube amp, but with preamp and power amp on 2 separated boards.

Then, there is no need for pull-down resistor(s)..!!

Tube amps need more time to get warm than the time needed for input cap voltage "equalization"..

[Marcos - Munky]

Yeah, it's a little habit I got from making layouts for guitar effects. I put a pulldown resistor on both input and output of everything I build . Still didn't got time to work on this preamp, so I'll just clip off those misplaced resistors.

[bluelagoon]

on this FA-1 circuit, I were considering placing a pulldown resistor on the input. But did not really want to change the impedance too much. So were wondering if I replaced original circuit value R2, 3M3 resistor for a 6M2 resistor and placed a 6M2 pulldown resistor at the input, would this mess up the existing relationship of R2 biased voltage onto Q1 Gate? or would it suffice to still be same compatible circuit with aprox. 3M1 input impedance?, Please see pics. Thanks.

[fryingpan]

The pulldown resistor is there to avoid a pop when the unit is switched from active to bypass and vice versa.  It has to come ahead of the capacitor on the input.  It has to be a high value because the inductance of the pickup (if it is the first pedal in the chain) works to cause the high-frequency output to be reduced, known as "tone sucking".  The typical value is 1 megohm but if you are using a film capacitor for the input, you can easily increase it up to 10 megohms.

On the other hand, (I'll repeat what my electronics professor told me) the pull-down resistor should be as small as possible because of both noise and the fact that large resistances may also cause currents to "get stuck" and potentially cause a voltage difference to arise at the two ends of the resistor (although this should only matter with really large resistances, in the order of gigaohms). The current should be able to "drain".

[GibsonGM]

I would just use a "large" value for the pulldown and call it good.  1M, whatever, see how it sounds.  2.2M if you 'hear' loss of 'chime'...

"The typical value is 1 megohm but if you are using a film capacitor for the input, you can easily increase it up to 10 megohms."

So go ahead. Let your ears be your guide.  Trying it across the input jack you won't have to worry about bugging up the PCB or anything.

[bluelagoon]

Yeah but effectively if I add 1M as a pulldown resistor the impedance then goes down to 760k, which isn't my ambition, Was wanting to keep the impedance close to original circuit spec.

So my main question is, if I change up R2 from 3.3M to 6.2M, would this in effect mess up the bias onto Q1 gate and compromise somehow the integrity of the original design.? I know you can trial and error and most things will be near enough good enough a lot of the time, but really just wanted verification on changing out the value of R2 of what significance if any it might make? Thanks

[amptramp]

The pulldown resistor is on the input side of the coupling capacitor, so it has no effect on the FET bias because the capacitor does not conduct DC.

[GibsonGM]

^ This.   And any possible change in 'tonal response' is extremely unlikely to be audible. 

[MrStab]

Biasing resistors are technically pulldown resistors for waveform peaks and pullup resistors for troughs!
*Gets punched in face*

[antonis]

On the other hand, (I'll repeat what my electronics professor told me) the pull-down resistor should be as small as possible because of both noise and the fact that large resistances may also cause currents to "get stuck" and potentially cause a voltage difference to arise at the two ends of the resistor (although this should only matter with really large resistances, in the order of gigaohms). The current should be able to "drain".

Pull down resistor "noise" (either voltage or current) is the least contribution in total noise figure of the stage..

[bluelagoon]

Biasing resistors are technically pulldown resistors for waveform peaks and pullup resistors for troughs!

So then if I changed out the R2 biasing resistor from 3,3M to 6.6M would this affect the circuit dynamics from its original design?

You see I had in mind to place a 6.6M pulldown resistor after the input or possibly after R1, 10k and then change out the R2 from 3.3M to 6,6M, This then in affect should still allow me the same input impedance as the original circuit design of 3.3M ohm.

But my concern was as mentioned had not wanted to disrupt the integrity of the original circuit dynamics if it were that changing out R2 to 6,6M was going to have an adverse affect to original design dynamics?? Any thoughts?? Thanks