Shunt resistor load in guitar pedal's output

[amihol]

Hello,

currently I'm building my own delay effect, but don't know what resistor to use in pedal's output.
I have seen different values for this resistor in different pedals. For example:
1.   10k: TS 808,
2.   100k: TS9, CE-2, DM-2, DD-2/3,
3.   A100k volume pot: Proco rat,
4.   150k: phase90,
5.   1M: PT-80 digital delay and AD 3208 (analog delay) by Scott Swartz, and SHEcho project by Dean Hazelwanter - all of this by General guitar gadgets.

As I can read in R. G. Keen's article "The Technology of the Tubescreamer", this output shunt resistor reduces output impedance.
But doesn't it lower input impedance of the next stage (another pedal or amp) too? It seems to me, that this shunt resistor is in parallel with input impedance of the next stage, isn't it??

In my delay project, last stage is op-amp (4558 or TL072) mixer, similar to that in Boss pedals (dm-2, dd-2/3, ce-2). Its output (pin 7) goes to 1uF capacitor and then comes shunt resistor.
Can you recommend me, what resistor should I use in my pedal?

Thank you very much,
Regards,
Ales.

[anchovie]

I'll let someone else explain the science if they want to, but if your output stage is similar to a CE-2/DM-2/DD-2 and they use 100k then I'd reckon you can use 100k too. I doubt Boss have been doing something terribly wrong all those years!

I'd expect 10k would work fine too. If you're really curious, try both and see if your ears can spot any difference or not.

[tubegeek]

It seems to me, that this shunt resistor is in parallel with input impedance of the next stage, isn't it??

An excellent observation, and one that will make RG kvell when he reads it.

In my delay project, last stage is op-amp (4558 or TL072) mixer, similar to that in Boss pedals (dm-2, dd-2/3, ce-2). Its output (pin 7) goes to 1uF capacitor and then comes shunt resistor.
Can you recommend me, what resistor should I use in my pedal?

The larger the resistor, the less current your mixer op amp will have to source. The purpose of the resistor is to "pull down" the output of the effect to 0V for DC with the audio AC signal riding on that center voltage. It will reduce/eliminate "popping" when you switch the effect in and out.

10K is fine (the op amp can drive it) 100K is fine too, which is why you see both those values come up.

Note that at AC, we can ignore the cap, and we can see that the output impedance is the low impedance of the op amp in parallel with the pull-down resistor. So driving the next item in the signal chain is not a problem either.

You will lose a little more signal voltage with the 10K as compared to the 100K, but basically the difference between the two should be pretty small in every way.

[PRR]

> output shunt resistor reduces output impedance.

Maybe in filter and loss networks.

> In my delay project, last stage is op-amp

Then output impedance is essentially "zero". Adding a to-ground resistor won't change that.

However you very often have an output DC-block Capacitor. Since it blocks DC, the far end could charge to "any" voltage. When you switch or plug the output, that stored charge will discharge into the next input, POP! Harmless (usually), but annoying.

A to-ground resistor will bleed the cap towards ground so there is a neglible DC voltage. The resistor value depends on the capacitor charge and leakage. Big caps (10uFd) may need small resistor (10K) to drain leakage. Small caps (0.05uFd) have small leakage so you can use a larger resistor (1Meg).

Also note that a small cap with a small resistor is a Bass-Cut. If you put 10K on a 0.01u cap you would cut-off nearly all the guitar fundamentals.

But the value is rarely critical. Often "anything with an orange stripe" will do.

If you like math, aim for about a 1 Second time-constant. Inversely, a 0.2Hz bass-cut. That will bleed-off pretty fast yet won't dent audio bass.

[tubegeek]

(I was going with the existing 1 uF cap, PRR included the what-ifs when the cap is different.)

[amihol]

I think, if this output shunt resistor don't affects input impedance of the next stage (another pedal or amp), then probably we may use any of "standard" values (10k, 100k) without doing a big mistake.
But, if it affects next device's input impedance, then is probably good to use 1M or even higher. Probably, Scott Swartz thought in this sense when he designed his delay circuit variation.

In true-bypass CE-2 clone pedal I've used 2M2 input pull-down resistor and it works just fine. Without this resistor the popping was audible only for the first time when I turned on the effect. I didn't hear popping when I turned on the effect second, third... time. Original pedal don't need this resistor because it has flip-flop silent switching.

Regards
Ales.