1. I tried powering my preamp from one circuit and my turntable from another (had to use an extension cord), not from the same as I did before. No difference. I kept it connected separately until the end of this series of experiments.

2. I tried disconnecting the cartridge. No difference. Then I connected it back.

3. In addition to the hum, I noticed some quiet irregular sounds, short buzzes in particular.

4. Before grounding the turntable, I would get a slight decrease in hum intensity when I touch the platform or the tonearm. Now, having it grounded, it makes no difference whether I touch them or not.

5. Having everything connected I tried touching the RCA plugs with my hands. This did make a difference: the hum intensity decreased slightly.

6. I tried unplugging the turntable out of the power outlet. This did make a difference: the hum intensity decreased a little more than in # 5 and those sounds (see # 3) disappeared.

7. I tried unplugging the turntable's RCA cables out of the preamp. This did make a difference: the hum disappeared completely. (Wow! Now I can really enjoy the music!)

8. The most interesting one! I unplugged the turntable out of the power outlet and began touching the RCA connectors (without disconnection them) with the turntable's power plug prongs. The hum disappeared ALMOST completely. Then I touched the RCA connectors with the grounding prong and the hum disappeared completely.

Sad and funny. Now I am trying to come up with a more precise idea of what might be wrong...