Connecting Laptop HDMI to RCA on Amplifier

[Nordo]

In the past, when running REW on my laptop, the output to the amplifier/speakers was always with a HDMI cable from the laptop to my AV receiver.
However I now have a different situation where the amplifier will be an old stereo Cambridge A60 that only has analogue RCA inputs.

Any ideas how to to connect the laptop to the amp?
The laptop does have a headphone socket, but I don't think that would make a good match to the amp.

In searching for a HDMI to RCA connect cable, most seem to incorporate an electronic box that must be doing something (and not cheap).
All I need is the analogue audio output signal from the laptop.

EDIT : I should have mentioned that I will be using REW to test some speakers, not a room.
I need the SPL (at 1m, in an open area), as well as the in- cabinet impedance curves, for crossover design.

 

[sm52]

External sound card with two inputs and two outputs, which is connected to the USB port of the laptop. But it's not cheap either.

 

[Nordo]

External sound card with two inputs and two outputs, which is connected to the USB port of the laptop. But it's not cheap either.

I used to have an external sound card, but it died on me.
I solved all my problems by buying a UMIK-1 usb plug-in mic (which I still have).
The imput into the laptop is not the problem.
It's sending the test signal to the speakers.

 

[John Mulcahy]

The headphone output is absolutely fine for that, no problem at all to connect that to an amp input.

 

[Nordo]

The headphone output is absolutely fine for that, no problem at all to connect that to an amp input.

Thanks John.
Yet again, you come to my rescue.

 

[Papioaf]

Apologies up front for my ignorance, but does one need to connect to both the right and left amplifier RCA inputs (e.g., computer headphone jack out via a Y cord RCA cable)?

 

[John Mulcahy]

Yes, so that you can measure left and right separately by choosing the output channel in REW.