General question on op-amp output resistors

[Woops is my middle name]

I have seen many pedals using op-amp output buffers that include a resistor to ground after the output coupling cap, often 100k, and sometimes also with a small value resistor in the audio output path, like the 470 ohm below.

I'm trying to understand the purpose of these - I guess the 100k ohm would make a high-pass filter with the 1uf cap, and the 470 ohm does.... what? I get having the cap there to isolate the op-amp DC from the audio path, but not sure why you'd need a high pass filter, as opposed to just doing a 1uf cap without the resistor.

If anyone has any insight, that'd be great! This is from the Sea Horse, if that has anything to do with anything.

 

[Robert]

R19 is an output pulldown resistor. It discharges any DC potential that might build up across C18.

 

[Woops is my middle name]

Ooook, I was wondering if it was a DC drain or something like that, cool. I guess the 470 ohm resistor is there just to prevent that leak from making it out to the cable?

Thanks!

 

[darwin999]

One role that R20 serves is that it safely limits the output current of IC1.3 in the event of a short circuited output

 

[Woops is my middle name]

Great, thanks for the explanations! It seemed like a best practice sort of thing so I figured it was for a reason...

 

[dcfvgb]

R19 also an output impedance to help signal transfer to whatever it plugs into
General rule of thumb: High input impedance, low output impedance.

 

[giovanni]

R19 also an output impedance to help signal transfer to whatever it plugs into
General rule of thumb: High input impedance, low output impedance.

The opamp typically has a low output impedance, which in parallel with a high resistor remains a low impedance so it shouldn’t affect the pedal’s output impedance substantially.

 

[JTEX]

R20 is mostly there to isolate the op amp output from the effect of a highly capacitive load, usually due to a long cable. Too much capacitance directly across the output can cause many op amps to become unstable (oscillate). R20 also allows passively summing the output with possibly another output coming from some other op-amp. Without series resistors on their outputs, you can't directly tie together two op-amp outputs - they would just fight and load one another.

 

[JTEX]

The opamp typically has a low output impedance, which in parallel with a high resistor remains a low impedance so it shouldn’t affect the pedal’s output impedance substantially.

Indeed. R19 has virtually nothing to do with the output impedance. Its only purpose is to gently drain any DC present on the right side of C20 to ground. Without it, DC would build up and cause a loud pop if the cap is suddenly discharged, for example through a bypass switch. In AC, the extra 100k load makes no difference to the opamp or the following stage.

 

[Woops is my middle name]

Brilliant. Thanks everyone for the input!