[Greg]

Hi guys,

My project .... 4 channel 50w bi amp / pre amp / active X/o

Almost completed but has hum

Altronics K5130 Stereo module x 2
(LM3886 IC power supply on PCB with +/-15v aux for pre amp etc)

Jaycar    KC5343 3 way active cross over using 2 outputs 1 left for sub ?
Jaycar    AA0315 Pre amp (treble, mid, bass, balance, volume)
Toroidal  30 0 30 300Va with transformer to pull voltage from 46v to 41v
All in the same 2U rack case
So all PCB run from 1 toroidal via aux from amp modules  
Earth the mains to case only, no earth to PCB except via input voltage from transformer (+/-41v then via amp PCB aux out +/-15v)
The amps are located in the back 1/2 of case, pre amp & x/over in the front 1/2, transformer in rear corner

Has very little hum (ear on spk) with no input to amps (no lead on input post and not using ground post on amp as pre there instructions)
As soon as you connect pre amp or X/o hums
Tried x/o with power pack to isolate power supply still same hum
Pre amp only no x/o hums
Using double shielded input cable (centre core plastic cover foil then metal braid then outer cover)
Tried my MP3 player via RCA inputs hums
Tried MP3 player direct to amp inputs OK not as good as no input
I'am thinking the greater the input signal strength to the amp the greater the hum ??

Any suggrestions ??

Thks Greg ...

Perth W.A.

[Redbone]

Interesting...I am gradually switching over to an active crossover and have also experienced an intermittent hum.  It is definately the XO as the system is dead quite without it.  I have found that the input cables to the XO are very sensitive to placement and proximity to speaker cables.  But that is not the complete solution.  

Fortunately (or not) the hum is intermittent, so at least I know it can go away.  There are three fixes I am going to try :

1) "Plug" all unused RCA inputs with RCA stubs.

2) Run a ground wire from the preamp (CD Palyer) to the XO.

3) Get the Preamp, XO and Amps on separate AC lines.

I will try one fix at a time to see if I can isolate the problem.  Also, are you using a "power line conditioner" ?  I am using a computer surge protector, and this will have to go too.  Let me know if you find your source of trouble, thanks.