Crackling: 2*5*2*0 mic to line

I've just about completed my mic pres but have one issue to resolve and hope someone has a "simple" solution for my problem. The mic pre is Jensen JE-115K input, 2*5*2*0 opamp and A*P*I output transformer. The input to the opamp can be switched between mic and line with a Shadow type switch. This switch injects the line input through a blocking cap to a 100k termination resistor at the non-inverting input to ground. Now the problem. When depressing this switch, momemtarily, there is lots of crackling until the switch is fully depressed and everthing settles down. I'm sure it's caused by the changing offset at the input of the opamp. The JE-115K uses the typical termination network (150k termination resistor only). Any ideas on how to reduce this problem without re-design?

By the way, this unit sounds very good and works well (less the mic/line switching issue). The bottom end is noticeably rich. I will build 7 more units, some with Jensen output transformers and hope to have sound clips for comparisons. Once I've solved this problem I can finish the other 7 I'm building so, I am anxious to get this fixed.

Any ideas?

Thanks,
Jeff

Do you use a shorting switch? That's a switch which makes the line-connection before it breaks the mic connection.

Samuel

It's the cap charging and dis-charging. My guess at least.

I'm using the typical Shadow type switches found on most of the old API consoles and mic pres. I'm fairly sure they are break before make. I agree it's the line input cap that's probably the culprit. I've switched the input with no line input and it still crackles. The 100k resistor is tied to ground and the input cap at all times so it will likely be discharged if not used for awhile. This problem is a speaker blower if I don't do something to fix it!

Regards,
Jeff

Just for the heck of it, put a 100k across the cap and see if it goes away. If it does, try a 1 meg. If that works, leave the one meg in there. I do not think it will hurt anything, maybe the CMR will drop a bit, but better than a blown speak.