Modules without protection resistor

For those of you who’ve built DIY stuffs…

Have you encountered modules in which an op-amp output is connected straight to the output jack - without the small resistor (usually 1k)?

I’ve found an occurrence of this in a commercial module and would like to see if it’s widespread or not.

Not to my knowledge.

Could have missed one or two though, but I doubt it.

Why do you ask?


I’m considering an improvement for new modules that could cause issues with modules with odd output stages like this (no protection resistor + no bandwidth control).

What effect does it have? the resistor limits current output?

yes and thus it also limits bandwidth.

I’ve read that befaco recommend to remove the protection resistor for their slew module (there were complaints about tuning issues when using it for pitch CV).

What problem is to be expected on downstream modules?

I guess a jumper could be fitted to bypass or not the resistor. I know jumpers can be annoying and you get questions with the inevitable reply of “read the manual, there’s s jumper” etc.

The protection resistor limits the amount of current the op-amp can be asked to source/sink. This can prevent the op-amp from being destroyed if, by accident, two outputs are connected together.

There are two configurations for the output resistor: one that causes slight voltage drops and results in pitch inaccuracies (used for most of my modules which are unlikely to process pitch CVs), and a more sophisticated one that does not cause voltage drops (which I use in modules generating/processing pitch CVs like Yarns). The configuration that causes pitch inaccuracies has a slight positive side-effect, though: it allows passive summing of outputs.

Bypassing the protection resistor has two negative side effects:

  • More risks of damaging the op-amp through incorrect patching or ESD, since the output pin is directly “visible” from the outside world.
  • If the circuit surrounding the op-amp does not attenuate high frequencies (for example, for example, in an inverting amplifier, no additional capacitor between V- and Vo), you can get ugly ringing and self-oscillation when long lengths of cables are patched to the output and/or when you connect the output to modules that have additional filtering or ESD protection on their inputs.
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FWIW, I think some of the ghetto quantizers do this. Certainly Barton DNQ, IIRC.

I have an older design VCO from nonlinearcircuits that produced a pitch change when the pulse width was changed (if I remember correctly). This is a design that has been around for a long time. Removing the input resistors was the fix for the issue.

Someone should make some patch cables with the resistor built in, then you can choose to have it or not.