Input impedance resistor needed without pot?
Feb 27, 2006 at 10:33 AM Thread Starter Post #1 of 4

Ikon

100+ Head-Fier
Joined
Nov 21, 2004
Posts
269
Likes
11
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?
 
Feb 27, 2006 at 10:41 AM Post #2 of 4

amb

Member of the Trade: AMB Laboratories
Joined
Apr 1, 2004
Posts
4,933
Likes
41
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.
 
Feb 27, 2006 at 10:53 AM Post #3 of 4

Ikon

100+ Head-Fier
Joined
Nov 21, 2004
Posts
269
Likes
11
Thank you
etysmile.gif
 
Feb 27, 2006 at 10:58 AM Post #4 of 4

amb

Member of the Trade: AMB Laboratories
Joined
Apr 1, 2004
Posts
4,933
Likes
41
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.
 

Users who are viewing this thread

Top