The 520 would have to be from 78 as the 820S came out about that time. i have one of each but have not put the 520 on the air yet after buying it this past summer. I use a Kenwood MC-50 microphone on the 820S. As for the amplified D104 IIRC the amp has an output impedance of about 5000 ohms according to my Astatic info. The 520 and 820 like to see about 50,000 ohms. Mismatched impedances will result in a difference in available drive levels from the mic as well as the frequency response will be altered. My MC-50 is dual impedance 600/50K. Either will work but the settings have different sounds. The d104 has lots of drive so you can insert a series resistor to help with impedance matching and you should have enough level left for proper operation.
