Re: Distortion and source impedance in JFET-input op amps
Posted: Thu Jan 15, 2015 8:12 am
I'll have a stab at this from an analysis point of view. Mind you I don't believe I have any practical experience with issues that may necessitate the use of R8.
I read the previous analysis regarding breakthrough and I am not arguing that side when the pot wiper is at ground.
What I see is a DC compliance issue or spike potential when connecting or disconnecting equipment to the input when the pot wiper is at terminal 3, i.e maximum volume. Without R8 there is potential for large currents to flow, from stored charge on C46 , if the connecting equipment has a low output impedance. This could result in a spike on the +ve input that may cause damaging clipping of the IP to the power supply rails (depending on the IP stucture of the op-amp) and/or latch-up condition.
Of course if the preceding stage to the input is part of a composite circuit and therefore always connected then compliance issues would unlikely ever occur.
I read the previous analysis regarding breakthrough and I am not arguing that side when the pot wiper is at ground.
What I see is a DC compliance issue or spike potential when connecting or disconnecting equipment to the input when the pot wiper is at terminal 3, i.e maximum volume. Without R8 there is potential for large currents to flow, from stored charge on C46 , if the connecting equipment has a low output impedance. This could result in a spike on the +ve input that may cause damaging clipping of the IP to the power supply rails (depending on the IP stucture of the op-amp) and/or latch-up condition.
Of course if the preceding stage to the input is part of a composite circuit and therefore always connected then compliance issues would unlikely ever occur.