Preamp modification to add line in with 10dB pad

[RishiRishiRishi]

Hi,

I want to build the attached preamp but also want to add an unbalanced Line In jack with a 10dB pad.

I've attached my proposed zoomed in schematic of the pre amp section showing an unbalanced Line In jack feeding into the SSM2017 using the same resistor network as the 10dB switch.

1. Would the proposed modification work as an unbalanced line in with a 10dB pad?
2. Are capacitors CX and CY coming off the Line In jack needed? And particular values if so?

Any help/ suggestions would be hugely appreciated!

Many thanks!

 

[ccaudle]

The values you use for the pad will result in an untypically low input impedance. Line inputs are usually around 10kOhms, the pad you have shown will be around 4k input impedance.

As shown the line input bypasses the overvoltage protection (D1-D4), so it would be a good idea to either replicate that, or arrange your line input jack so it just takes advantage of the protection diodes and pad which are already present in the design.
The design note for the That Corp. mic preamp has a discussion on adding a line input on page 5:
https://thatcorp.com/datashts/dn140.pdf

That is for a balanced line input, but you can just use a balanced input and use an adapter cable, or wire the input for unbalanced only the same as you would an adapter cable by connecting the cold input to the shield/chassis connection, and connecting the hot input to the signal connection.