Console mic input question. Phantom power "Pops", Coupling caps and Transformers

I've been noodling around with my Amek BCII. The mic input's were bypassed
to act as a second line input stage. Reversing it was pretty straight forward,
but a few channels are suffering from loud "pops" when the 48v is engaged. I'm a
little worried this will damage a mic or such.

What is causing this? Perhaps small variances in the 6K8 resistors? I used 1% tolerance
and matched them with a DMM as close as I could. Is this generally adequate? Perhaps
the pushbutton switches are on their way out?

Also, I've added some transformers prior to the mic input stage as outlined here in the schematic
below. The transformer effectively replaces a pair of ferret beads, but there is no additional information
regarding installing the transformers in the manual. They are installed, sound great but is it possible to now
remove the two coupling caps prior to the transformer? I don't see why not? Would the popping (possibly mismatched)
48v cause a problem with the transformer primaries?

Sammas said:

I've been noodling around with my Amek BCII. The mic input's were bypassed
to act as a second line input stage. Reversing it was pretty straight forward,
but a few channels are suffering from loud "pops" when the 48v is engaged. I'm a
little worried this will damage a mic or such.

What is causing this? Perhaps small variances in the 6K8 resistors? I used 1% tolerance
and matched them with a DMM as close as I could. Is this generally adequate? Perhaps
the pushbutton switches are on their way out?

Click to expand...

You're applying a pretty large step voltage to the circuit so that's your "pop."

Turn your monitors down/off before switching phantom on and off and you won't hear the pop (or pop your tweeters).

-a

Wire the unused second stage of the 48V switch in parallel to check if the switch is faulty. If not you cold add a simple RC timebase to get a little ramp after switching 48V. Lift the legs of both R1 and R2 at the side where they are connected together. Solder them together, then solder a 47uF ´lytic from the same node to ground and then add a 100 Ohm resistor from again that node to the spot where one of the pulled resistor-legs has been soldered to.

Thanks guys,


Just to clarify what you've said Jens,

- Lift the two 6k8 resistors on the pushbutton side,
- Solder those ends together with a 47uF electrolytic to ground
- Then re-bridge the connection from the pushbutton switch to the two 6k8 resistors with a 100ohm resistor.

?

Piece of cake. Thanks!!

Sammas said:

- Lift the two 6k8 resistors on the pushbutton side,
- Solder those ends together with a 47uF electrolytic to ground
- Then re-bridge the connection from the pushbutton switch to the two 6k8 resistors with a 100ohm resistor.

Click to expand...

exactly

Done and done.

I don't suppose anyone has some words of wisdom on the coupling caps (C7 & C8)?