Turntable Hum Problem

[Hal Rockwell]

A little off topic but DIY project non the less. I have an AKAI AP-B21 old turntable. It comes without an integrated preamp so I managed to squeeze one in. I took apart a Behringer PP400 phono preamp with its 12VDC power supply and inserted them into the body of the AKAI. Everything works fine except that this mod introduced a great amount of hum into the system. I grounded the preamp and the core of the power supply but it changed nothing. Anyone has any ideas how to fix it?
 

 

 

 

 

 

 

[liamstrain]

See if there is another ground from the tonearm or other part of the phonostage. 

 

[Hal Rockwell]

Can it be that the power supply is the one to blame for the hum. Can it be the one that is introducing the interference? Will a switching power supply be any better?

 

[Uncle Erik]

I'd blame the power supply. A switching supply would just make it worse.

If possible, take out the phonostage's power supply and use a 12V wallwart to power it. That should get the noise out.

 

[FrankCooter]

Quote:

I'd blame the power supply. A switching supply would just make it worse.
If possible, take out the phonostage's power supply and use a 12V wallwart to power it. That should get the noise out.

X2.
 
If you really must have everything in one chassis, use an SLA battery and charger. Wire the charger so it's turned off when the turntable is turned on. 
 

 

[fzman]

x3 -- it's the transformer from the phono pre's psu - looks like it's right under the arm, and the field is inducing a voltage directly into the cartridge's coils!  external ac wall wart will help

 

[Hal Rockwell]

I found the real cause of the hum. I used regular speaker cable for the wiring of preamp. I replaced it with a shielded microphone cable and it took the hum almost entirely away. It becomes audible only when I turn my powered speakers almost all the way up which I never do.
 
Thanks to all for your help. If anyone has an idea on how to shield the entire preamp section, I'll be more than glad to read about it.
 
 

 

[headfi_calderon]

ERS paper... be careful cause its conductive.