Purpose of 68r resistor on SMR4 Mk II filter board?


Just when I thought I had this licked, I’ve got another strange issue.

I get a perfect signal on pin 7 of the TL072, as per the test points on the SMR4 Mk II build page, but following the 68r resistor, I get a very faint signal and not a lot else.

I’ve checked the soldering on this component, checked the bridge where the output gain pot can go, reflowed solder over all these connections etc etc. I measure resistance from the tip of the output Neutrik socket to pin 7 of the TL072 (so through the whole circuit which contains the 68r resistor) and I get 68.2 ohms, so I don’t think there’s any problem with this part.

I’m keen to know what this resistor does. Can I just jumper it and go without, since when I probe directly before it the signal sounds good and is at a decent level…?


If there’s a part, there’s a good reason for it!

This resistor prevents the destruction of the TL072 in one of the following situations:

  • Temporary short circuit when the jack on the other end is inserted in an input (happens with some types of connectors, like badly designed 3.5mm jacks).
  • Accidental short circuit when you are connecting by mistake a signal into the Shruthi output.
  • Accidental current overload when you are connecting by mistake the output to a super low impedance device (like headphones).

Post a picture of your board and describe to what you connect the output jack…