Audio Systems: turntable hum, numark mixer, input cables

Question
QUESTION: Cleggsan, I recently read your response to someone with a grounding hum problem. You were very helpful, and you asked followup questions. The original posyter never replied so I guess the problem was fixed. I can't get rid of my turntable hum however.

here is the setup:

Numark tt200 turntable going into
Numark DM1835x mixer going into
Hafler pro 1200 amp sending signal to
Niles OS-10 speakers
The outlet everything is plugged into is grounded (light comes on the outlet adpater to let me know)

(not super high fi, but it's what I use to make radio plays)

The grounding issue only arose within a month or so, and when i turn the turntable to adjust the grounding, i can get it so the ground at the back of the turntable is quiet, but then when i place it down on its stand, it comes back on. It only affects the right channel.

Hum does not increase when you place your hand near the phono pickup.  
Hum gets huge and crackles loudly when I disconnect the ground wire  
The hum goes completely away when the volume control is reduced to zero.
I don't know if the hum is 60hz or 120hz, though I think it's 60.

Thanks for any help!

ANSWER: Very good.  You are a good student.

We need to do some further checking:

ONE: Disconnect the TT input cables.  Do you get hum when the volume is up?

TWO: Connect only the good channel.  Any hum?

THREE:  Connect the bad channel only.  Hum?

FOUR: If hum now on the bad channel, does connecting the other, good channel change the hum level in any way?

FIVE:  Try unplugging the power cord of the TT only. Any change?

SIX:  Are there other inputs on the Numark mixer you can try?

The object of the obove checks is to determine if the problem is in the TT, the cables, the I/O interfacing, the mixer, etc.

Then let me know if you need more help analyzing what is going on.

C



---------- FOLLOW-UP ----------

QUESTION: Thanks for your quick and clear f/up.

In the meantime I messed with my speaker connections and realized that one wasn't hooked up propoerly, so after I got it back propoer, I found out that there was hum in both channels.

So first I unplugged the TT inputs cables - still had hum.

(Hum on each channel separately as well)

Then unplugged the power cord w/ TT cables back in - no more hum! (I basically untangled the power cord from all the speaker cables and other power cables etc and wove it back through trying to keep it as far away from any RCA or 1/4" cables)

So that worked to get rid of the hum!

But get this: now my left speaker is only blasting at 15%, while my right side is full.

There is a dual CD player also hooked up through this mixer, and both channels work with both decks of that.
And there is also a voice processor (TC Helicon voicelive) that goes into the mixer, and both channels work with that.
And I have an output from my computer also, and both channels work with that.

I have checked and rechecked and jiggled the TT cables, but they are both securely attached to both the TT and the mixer.

So thanks for helping me troubleshoot the hum - what do you think about this channel disappearance? Cable issue?

Big thanks,
AC  

Answer
Interchange the patch cords from the output of the mixer to the hafler amp; this will tell you if the level imbalance is in the power amp or the mixer.  Do the same with the outputs of the TT and cd player to determine if imbalance is with input device or inside the mixer.

With the above trials you will determine the place that needs attention. If it turns out to be the mixer you may have to get it serviced OR there may be some setting that is not the same on both channels. I am not familiar with your Numark but many mixers have a 20db pad that is switched in and out.

Let me know next.............

C