That cable simply converts TRS 2.5mm to TRS 3.5mm.
The TRRS 2.5mm in the BTR5
has a ring between the ring and the sleeve. The moment you plug
this cable in that second ring and (shorter) sleeve, which both have an amplifier section connected to it INSIDE the BTR5 are
shorted.
So do NOT use this cable on the BTR5 !
These cables are intended for devices having a 2.5mm TRS SE output so you can connect a 3.5mm TRS plug to said output.
So do I need a separate balanced cable with 2,5 mm male to use the 2,5 mm output of BTR5. Is that correct?
Both the Sundara and HD560S come with the regular TRS plugs.
In order to use these headphones balanced you will have to buy
balanced aftermarket cables.
Meaning the Sundara cable will have 3.5mm TRS (or TS) jacks on the headphone side and 1 TRRS 2.5mm on the amplifier side.
For the Sundara these are easily found, for the Sennheiser you may have to look carefully of what is offered.
The HD560S has a propriatary locking 2.5mm TRRS plug which can be used balanced. The cable that comes with it has 4 wires in it where the 2 ' - ' wires are connected to the sleeve in the provided cable.
Either one removes that TRS plug and solders on a 2.5mm TRRS plug instead OR you need to find an aftermarket cable that has a 2.5mm TRRS plug on both ends where one plug is either so small in diameter it fits in the headphone or has the locking mechanism.
The cable mentioned by Paolo is a cable that can be used but won't lock. There are probably manufacturers making balanced cables for it.
Myself I don't use the HD560S balanced (there is no need for it really when you have some decent SE source.