[Ikon]

I was just asking myself if the impedance resistor in tangent's designs serves any purpose when I leave out the pot and the input cap.

I'm talking about the resistor that connects between each input and the ground and is usually in the 100KOhm-1MOhm range (mostly named R2).

Shouldn't the input impedance of the op-amp itself be sufficiently high/low?

 

[amb]

Yes, it is required. The opamp's input transistor would lose its bias path without it, if the input cable is disconnected (whether intentionally or by accident). Without bias, the output DC offset will likely go awry and damage headphones.

 

[Ikon]

Thank you

 

[amb]

You're welcome.

I should also mention that without that resistor (and with the input cable connected), the loss of bias path could also occur if the source output is capacitor-coupled and it has no resistor at the output side going to ground. All the more reason for not omitting the resistor.