Parasound JC 3+ Phono Preamp Hum

I believe that I have a ground loop.

In a power supply there is +V, -V, Ground or 0V, and earth ground.  A ground loop can form in a system when there is more than one path to earth ground.  Earth Ground comes from the third outlet prong, through the power cord, into device 1, to device 1 signal Ground or 0v, passes through the signal interconnect cable, into device 2 signal Ground or 0v, into device 2 earth ground, through device 2 power cord, and back to the AC outlet third prong.

To test for possible ground loops.

1 Turn off each device

2 Use an ohm meter to measure the conductivity between the signal connector ground, the outside of a RCA connector, and the third prong on the device AC power inlet.  If the reading is then < 5 ohm, the signal ground is connected to earth ground.

To remove ground loops

1 Pick one device and make it the conductor of earth ground, typically the largest power draw or the amplifier.

2 Put all other devices connected by signal interconnects, that are not fiber optic, on a 3 to 2 prong adapter.  This will allow earth ground to still provide protection against electrical shock, via the signal interconnects, while only providing 1 path to ground.

3 Power up only 2 devices at a time, CD player and amp for example, and see if a difference is made in the hum.  If not then power down and remove the 3 to 2 prong adapter.  Continue with all devices till the hum is minimized.

I believe my turntable phono ground is not correct.

Phonos are by nature high gain devices and therefore will amplify any electrical interference or ground loops.  In general connect your grounds as follows.

1 The tone arm cable and tone arm ground should be connected to the phono ground binding post. The phono can be on a 3-2 prong adapter and get its earth ground via the preamp or amp provided that is third prong is connected to its RCA or balanced connector ground.  The tone arm and tone arm cable MUST be shielded for best performance with a low output cartridge.

2 The table or motor ground connector should connect to the ground from the turntable power supply which is connected to earth ground.  This assumes that the table or motor ground is not connected to the tone arm / cable / phono ground.

I believe the hum is coming from my phono or other device

Hum generated internally is caused by internal ground loops or poor power supply noise rejection, PSSR < 80dB for phonos and < 65dB for amps.  To test a device for hum.

1 Turn off all devices

2 Disconnect the input cable to the device in question, for example the tone arm cable from the phono.

3 Use shorting RCA plugs to short the signal inputs, in the in this case phono.  This will ground the input signal into the device thereby ensuring the noise is not from the outside of the unit.  You can also test the device without shorting plugs but that assumes the device was designed properly with “pull down resistors” to ground or in a phono your loading.

4 Turn on the phono, preamp, and amplifer

5 If there is hum then it is the phono or preamp or amp

6 To test the whole system, start by shorting the inputs to the amp and then add one device at a time until you have tested all devices with their inputs shorted

7 For step up transformers you may need to ground both sides of the transformer designated as ground. A phono cable by nature does not have a ground as no part of the phono cartridge is connected by ground. It only gets grounded when connected to the phono preamp

I believe my device placement is incorrect.

Good phono setup is as follows:

1 Phono is located within 6 inches of the tone arm

2 The turnable power supply and motor are as far as possible from the phono preamp,  2 to 3 feet for the power supply.  The motor is what it is.

3 There are no other devices or power cords within 1 foot of the phono preamp either on the sides, above, or below.  This includes wireless phones, networks, computers, and blue tooth

4 The cables into the phono preamp are shielded as well as the tonearm

5 The cable from the phono preamp runs unimpeded to the preamp / amplifier with no crossing of AC power lines.

6  Ideally the tone arm cable and the phono preamp to amplifier / preamp are only 3 feet in length.  This provides the best cable capacitance per foot.

7  Twisted cables perform better then solid core in that they provide a bit of natural noise rejection.

I believe I am getting noise from my power cables

Power cables can radiate noise and this noise has the most affect on low level signals such as a tone arm / phono cable.  If this is the case, then use a shielded power cable from Mouser, part number 686-17742,or Digikey.  This will require that the third prong is connected to earth ground, to 3 to 2 adapter.  These cable differ from other power cords in that there is a shield to keep the AC noise in.  Other cords rely only on the twisting of the cable.

I am getting a echo or bump when I set the tone arm down in the cradle.

This would indicate that shock through the cartridge is large enough to be picked up and amplified.  There are three possible causes:

1 The tone arm is being dropped into the cradle with too much force

2 The phono preamp has too much gain

3 The suspension in the phono cartridge is not sufficiently compliant