Studer Balancing Unit - Input Impedance

Does anybody have a schematics for a Studer Balancing Unit? (1.915.904.81)
I only found this document:
https://www.studerundrevox.de/download/studer-balancing-unit/

What I'm trying to find out is how to raise the input impedance of the unbalanced input. In the technical data it states the input impedance is 5kOhm, which is actually pretty low. BUT it also states that higher input impedance is available upon request, that's why I'm thinking it must be possible. I'm hoping it's just a few resistors which need to be changed.

Can anybody help me out?

The schematic is in attachment.  You should rise values of R118, R121, 122, 123  in first and R218, 211, 222 and 223 in second unbalanced input to get higher input impedance.

Perfect! Thank you very much! 

One more question: what would be the highest impedance one could use in this circuit?

If you go much higher with the input impedance, I would lower the value of the 470 pF capacitor as well.
(The 470 pF would cause HF rolloff.)

Good point, thanks for the hint!

I don't have the cards, so I can't just try it. I need to make sure I can use them for my needs before I buy.
I'm planning to raise the input impedance to around 20k, would this impose any problems in this circuit (providing that I also lower the 470pF capacitor)?

> it states the input impedance is 5kOhm

*In the audio band*, it is (5k6+5k6)||100k, or 10k for any practical purpose.

Above 70KHz the impedance and the response falls. Z falls toward 5k6 and response falls toward zero.

Here's a 4X change of impedance.

Thank you very much! Actually I already wondered how they came up with 5k. Maybe there are different versions and the one in the schematic has 10k input impedance....

Any idea why they use an inverting amplifier on the input?

> Any idea why they use an inverting amplifier on the input?

Look again. It is a differential amplifier. The two 100r take the average of remote ground and local ground and subtract that from the "hot" signal.

Inverting topology naturally "protects" against dangerous inputs because there is a large resistor between jack and chip. Also some op-amps have common-mode distortion which does not matter if the summing junction stays steady (or near-steady as with this one's ground-sum).

Absolutely makes sense now, thanks for the explanation! 

The summing of the two grounds is a really interesting trick. Do the 100R resistors also help to avoid ground loops?