Halo P3 Phono Hum

Try a 'cheater' plug (disconnects the ground) for the preamp.

First of all, if you want to lift the ground, you don't need a cheater plug because the P3 has a ground lifter switch on the back. Second of all, DON'T lift the preamp ground, it's the ONE component that should be grounded!

Next, are you using Hero XLRs between your TT and your preamp (I didn't think so). So what cables are you using? (If they have arrows on them, make sure they are pointed to the preamp) And what kind of cartridge? It better be a moving magnet.

Are your phono cables connected to Aux/Phono inputs and is the Aux/Select toggle switch pointed to those inputs? Good. BTW, you'll almost always get hum through "Phono" if the inputs or ICs aren't connected to anything.

If all the above is in order, check the cartridge clips. Make sure they're all connected and to the proper pins -- especially important w/ MM carts because they have a common ground.
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Thank you very much for the detail in your suggestions. My P3 has no lifter switch. I called Parasound about this. It's been phased out. I have an independent ground from outlet to earth anyway.

The XLR connection is strictly between preamp and pwr amp. TT to preamp connect is through RCA interconnects. I've used Audioquest Copperhead, CircuitCity Monster Cable, RadioShack's 'premium' cabling, and sweathog zipcord quality RCA interconnects and there was NO difference in the hum. I am using the P3's phono stage and the only way to get at it is to use the jacks so labelled on the backplane and flip the toggle. My Audioquest cables had direction labels and they did point to the preamp, the other ICs had no such labelling. Again, no difference in hum with any of the cables.

Cartridge is a Grado Gold MM. I just dbl checked cartridge cabling. Red, Green, Blue and White all connected properly. The black ground comes out of the tonearm and screws to it. I've run continuity between that contact point and the grounding post on TT and there is a connection. I see no specific grounding connection between the cartridge itself and the arm though. There are only 4 pins out the back of the Grado. I am expecting the cartridge connection to be through the mount, but that's plastic.

I gave the rundown to Parasound in an email and am hoping for a perspective from them.

It sounds to me like you may not have finished grounding your TA/TT to the preamp.

First of all, your cartridge ground(s), blue and green, should not connect to any EARTH ground, neither must the cartridge body be grounded (even if it was metal.)

So, the black ground wire comes out of the tonearm wand and screws to the tonearm base which is electrically connected to the TT ground lug, is that correct? If so, you then need to run a small insulated wire from the TT ground lug to the ground screw on the back of the preamp (near the phono inputs.)

Use the AQ cables, and don't lift/cheat the ground on the preamp. You might want to lift the ground on the amp, though it's probably not necessary since it's connected to the pre with XLRs which are hum cancelling anyway.
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I suggest you investigate the Grado as being the cause of the hum. (If you do a bit of searching, you will find that others have had hum difficulties with their Grado cartridges as well.)

Just a thought.