Grounding Question

 can you elaborate on your last statement bout two pronged older equipment? I have an amp when you turn it on you get a ground loop hum hd it goes away when the preamp fully turns on. 

@dinov That doesn't sound like a ground loop at all.

Normally the procedure for turning on any audio system is to power up the preamp (or in pro audio systems, the mixer) first. After it has stabilized then you power up the amplifier. In this way you avoid buzzes, thumps and the like as the preamp turns on.

When the preamp is off, it can act like an antenna for buzz and hum. When its turned on, its a much lower impedance so the antenna thing goes away.

@chayro Lifting the AC ground is a thing that people do to eliminate ground loop buzz. Its an indication that the grounding scheme of one or more components in the system isn't well thought out.

You can test for this sort of thing using a Digital Voltmeter set to the Ohms scale. The test applies to equipment having a metal chassis.

You connect one probe to the RCA ground connection and the other to the chassis. If the reading is the same as when you connect the two probes to each other then the equipment you are testing has a grounding problem.

Older equipment like the Dynaco ST70 didn't have a 3-prong AC cord. So on account of being vintage it gets a Murphy. But for newer equipment with a 3-prong AC connection there really isn't any excuse.