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Hum mystery to me

Calling all Hum guru solvers!

A while back, I started getting a hum out of my right speaker when playing a record on one of my 2 turntables. When I played the 2nd turntable, there was no hum.

On table 1, I am using an MC cartridge which runs into an Audio Valve phono pre-amp and from there into the system preamp.

Table 2 is plugged directly into the phono input on the system preamp.

 Since the tables have different phono pre-amps, I thought it was the Audio Valve phono pre-amp and took it to the shop for repair. After getting it repaired, I hooked it up and now I am getting a hum out of the right channel. So, I took it back to the shop. We hooked it up to a turntable, played and there was NO hum at the shop. I will say that they had a MM cartridge on their test turntable, but do not know if that makes a difference.

This is a mystery to me and if anyone has any suggestion to try, please chime in,

These are the trouble shooting tests I have done so far:

  1. I played a side on turntable 2. No hum. So that should rule out the amp or pre-amp.
  2. The table 1 phono pre is connected to a DVD input on the system preamp. I changed the input to another line level input on the system preamp. Hum continued to be produced. So, I concluded that the inputs on the system preamp are not the problem.
  3. I disconnected the ground wire on the tonearm—hum persisted.
  4. I hooked up a cheap NAD PP2 phone pre-amp to turntable one and there was NO hum.
  5. I tried a different interconnect from the Audio Valve phono pre-amp to the system pre-amp and the hum persisted. I conclude it is not the interconnect.
dmgrant1
atmasphere's avatar
atmasphere
11,810 posts
 

@dmgrant1 Whenever you have a problem in one channel only, to find out what's up you swap cables (at one end only) left for right to see if the problem moves.

1) Since the hum is associated with the outboard phono section, swap its output cables left for right at the input of the system preamp. Don't do it at the output of the phono section just yet.

2) If the problem moved, then swap the cables at the output of the phono section. If the problem does not move you have a bad cable. If it does move, swap the inputs coming from the tonearm.

3) If the problem moves from one channel to the other, the phono section is off the hook and you have something going on with the turntable. It might be a loose connection where the tonearm connects to the tonearm interconnect cable.

If the problem did not move then it makes a difference that you are using a LOMC cartridge and the preamp still has an issue.

 

dmgrant1 OP
55 posts
 

I have run all the tests that have been suggested and it all traces back to the phono pre-amp.  Am taking it back to the shop and hope they will honor their labor warranty.  I also hooked the offending phono pre to a head phone amp and got the hum out of the left head phone speaker.  Done trouble shooting with the aid of the good fellas on Audiogon.  Thanks so much for your time and input.

hagtech
175 posts
 

 

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