[gooberdude]

I keep my sub set to about 40Hz, but just noticed that when i rotate the x-over close to 60 Hz a loud hum
is audible.  is this normal?   the hum continues throughout the knobs rotation, it doesn't become inaudible unless the x-over knob is rotated to below 55 Hz or so....

This is not an issue for me, just wondering if its normal.  the hum disapears 100% when the knob is set below 60 Hz.

There's a bluejeans cable LC-1 interconnect from the preamp to the sub's rca input, and a Flavor 2 power cord.

[JoshK]

Could be quite a number of things causing this.  But if it doesn't bother you then ignorance is bliss. 

[Daryl]

changing the corner frequency of the subs lowpass filter is going to effect the level of noise who's energy is near the upper end of the subs range if it is entering the system before the LP filter stage.

Certain quirks sometimes exist in the design of a circuit where it will pick up noise the level being dependant upon the position of a control even if that control isn't one which you might think would have direct bearing upon that noise.

Poor power supply rejection could be to blame.

More likely it is a ground loop issue.

A subwoofer is a componet which will often be located and plugged in a considerable distance from the rest of your equipment and speacial care should be taken in the design of their electronics (actually any componet but especially subs).

The greater the distance the greater potential for common mode power line noise between locations which can become noise between the grounds of two connected componets if they are not properly designed.

The primary/secondary capacitance of the power transformer should be low as possible (split bobbin?) which will minimize the power lines influence over the ground potential of the componets electronics and allow it to 'float' to the same potential as a connected componet with the least current possible.

The inputs should be floating/balanced/differential with a decent CMRR (Common Mode Rejection Ratio) which will reject noise present between the ground potentials of two connected componets.

Also the distance between source componet and subwoofer can make the system susceptable to electromagnetic and electrostatic fields.

The long interconnect cable and the componets chassis can act as antennae for these fields.

You might try turning your system on (source componet and sub) and set it so some noise is present.

Now disconnect the interconnect from the source componet at the source componet end....noise still there?

Now disconnect the interconnect from the sub at the sub end....noise still there?

Now move the sub right next to the source componet, plug them both into the same outlet and connect them with a short interconnect....noise still there?

Daryl

[gooberdude]

Thanks to both you for your replies...i'll definitely try your ideas Daryl.

One thing i had not thought about is that the power cable is plugged into the wall outlet, not into the power strip that feeds all the other gear.   The power cable itself has only a pos & neutral cables, however there are 3 prongs in the male plug - its just not attached to a ground wire.  Not sure if this can cause a ground loop issue, but tonight i'll going to plug it into the main strip and see what happens.

My sub is only about 4' from my rack and the rest of the set-up... i can place it elsewhere, but it doesn't sound as good.  lots of re-inforcement from the room where it sits now.

Would a shielded power cord help maybe?   i could easily buy another length of Flavor 2, but this time get the length with the braided shield on it...

[gooberdude]

I think the problem is solved.    this sub is still new to me, but as i was listening
for the hum issue it became apparent that the x-over needed to be over 60 hz.  its been at about 40hz for weeks.  lots more music at about 65 hz, and there's no boom or overhang and its omni-directional.  at about 70 hz its easy to 'know' where the sub is located in the room.

i unplugged the pc from the wall outlet and into the power strip with everything else...it was a ground loop issue.

odd enough, with the pc plugged into the wall the 60hz hum was only audible with the phono selected on the preamp.