Welcome to Audioholics
.
I am a new/1st-Time owner of a "modern" A/V receiver ...
I now have a 5.2 receiver and would like to access the "ZoneB" functionality ...
I am looking at Phono PreAmps ~
which I would connect to (some of) the now unused PC-Speakers I have ...
I would use an RCA(dual male)-to-Stereo(female) adapter ...
If I understand you correctly, you wish to attach a phono preamp to the Zone B output jacks on a AV receiver. And you plan on attaching PC speakers to the phono preamp. Correct? Are these PC speakers self-powered?
On most AV receivers, using Zone B outputs require a separate 2-channel amplifier with enough power to drive the speakers for Zone B. Because you plan on using self-powered speakers for Zone B, you should be able to directly hook up your self-powered PC speakers to the Zone B output jacks on your AV receiver. (I am assuming your PC speakers accept RCA type plugs to allow a direct connection. If not, it should be a simple matter to find an appropriate converter plug, or cable with RCA plugs on one end and something appropriate for your PC speakers on the other end.)
A phono preamp cannot work for this, as it is not at all the same as an amp or a preamp. They are meant only to equalize the output from a phonograph pick up, and amplify it's signal from a very low level of ~3-5 mV, to ~100-150 mV, similar to the analog output level from most other audio sound sources that might be connected to a receiver. The voltage level of most Zone B outputs are about the same level as a preamp, roughly 1 Volt, much higher than the output from a phono preamp.