Strange Thumping Sound From AU-6500

[Jace]

When the TT is on, but idle, I hear (best description I can think of)a dull thumping stuttering heartbeat sound from my speakers. It doesn't happen when in tuner or aux modes. I don't think it's the TT, since I changed my TT a while ago, and it did it before.
I don't have any testing equipment. Any ideas.

 

[dr*audio]

This phenomenon is known as "motorboating." It's a type of feedback, or instability in the phono preamp stage. The instability could be caused by a ground loop, poor ground or bad cable, or faulty component in the phono preamp.
1. Disconnect the TT, does it still make the sound? If so, does this amp have a phono 2 input? If it does, it should have shorting plugs on the unused input. If they are missing, you will need to find some or make some.
2. If the sound does go away when you unplug the TT, try different cables. try unplugging the AC cord to the TT. Let us know what you find.
3. What kind of TT is it, brand and model?

 

[Jace]

Thanks Warren
The au-6500 does have 2 phone inputs.
I'll try these when I get home and let you know.
The TT is a Hitachi PS 48.

Jace

 

[Jace]

motorboating

I put shorting plugs in. That helped. After plugging and unplugging stuff I pinned it down a cheap cable between the pre-amp out line and a sub I've got hooked up.