Hi,
It's easy, since they made the connection for us already...
On the back of the SP-600 are some brown terminal strips with screws. The terminals on the top left side are marked, "Diode Output." I used those terminals as a connection into my audio equalizer which then drives a standard audio amplifier into hi-fi speakers.
BTW, I just set up my SP-600 again and found I did NOT need the FET buffer after the detector output. The signal stays clean when driving the EQ directly.
Hope this helps.
Tom, K1JJ