question on pulldown resistors and impedance

[MrStab]

Please ignore my comment, I was being silly.

Unless using piezo pickups, using needlessly high biasing resistors is trading placebo benefit for thermal noise.

I don't know where dynamics come into it - they don't. Unless you mean characteristics in some other way.

[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.

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".

Your professor was wrong.  If you have two resistors in parallel, the noise output of the combined resistors will be that of a single resistor with the value of the parallel resistance.

The noise of a resistor is not as important as it seems in a lot of cases because if you have a situation like a guitar where you have a pickup with approximately a 15 K resistance (usually less) in parallel with a tone control of 500 K and a volume control of 500 K, these resistances are all in parallel at frequencies where you can ignore the capacitive reactance of the tone capacitor and the inductive reactance of the pickup.  This would be the midband of guitar response.  If you turn the guitar volume control down, which adds resistance in series with the signal, you still have less than the 500 K in parallel with a 1 megohm input resistor.  If you turn the tone control down, this further reduces the input impedance because this resistance is in parallel with the input resistor.

If you have a 1 megohm resistor at 25°C with a bandwidth of 10000 Hz, the thermal noise output is 12.832 µV.  There is a calculator at:

https://www.omnicalculator.com/physics/resistor-noise

If you have a design (like a Tillman amplifier) where there is no input capacitor, the effect of parallel resistance goes down to 0 Hz for the guitar volume control and pickup winding resistance, where the 15 K pickup dominates.  In the case of a 15000 ohm resistor, the noise is 1.5716 µV.  This isn't much noise even after a lot of amplification.  Where you have capacitive coupling, this effect remains throughout the passband of the coupling cap, so an input cap doesn't isolate the noise from a resistor, it just isolates it below the coupling frequency, which is relatively low because it must pass the 82 Hz open low E string signal.

[bluelagoon]

Yeah Characteristics then. whaever.  I really just wanted to know if upping the value of R2 the Q1 bias resistor in the FA-1 circuit from 3M3 to 6.6M, at same time as placing a 6.6M pulldown resistor in parralel to achieve same impedance of 3M3 as original Boss FA-1 design.
Does this then alter the characteristics of the circuit by messing the bias to Q1??
I'll figure it eventually or forget about it, meantime open to any insight into the matter. Cheers

[antonis]

Does this then alter the characteristics of the circuit by messing the bias to Q1??

So, you want us to estimate Q1's leakage current and calculate the DC bias drift for the double the bias resistor value..??

P.S.
In case you're OK with C1/R2 HPF corner frequency, make C1 4.7nF for R2 6M6 ..

[bluelagoon]

Thanks Antonis, So if I have the Q1 leakage current figure, then what is the Formula to calculate the DC Bias drift for the larger value Bias resister ?

[antonis]

Bias drift = I_G X (Rnew - Rold) =  I_G X 3M3..

Considering I_G in the order of 10^-9, you'll result into a couple of mV drift..
As long as your circuit dosn't work in elevated temperatures, don't bother with bias resistor value..

[bluelagoon]

Thanks Antonis