[el34]

Hi all,

Vintage Vandersteen owner here.  I've got 80s or so 2c's with 2w sub.  Sub has some hum noticeable when music isn't playing.  Reducing the wall voltage helps, but I'm wondering if it's worth rebuilding the amplifier, so I'm checking here to see if anyone else has worked on this sub.  I'm considering a recap, and/or moving the power transformer out of the box.  Yes, I know the factory can rebuild it, but given these were a used purchase, and shipping costs will be prohibitive, I'd like to give it a try myself.  Anyone have any experience here?

Thanks

[jeffreybehr]

Hmm...

"Reducing the wall voltage helps...".  How do you accomplish that?

Is this hum the output of the drivers or is it mechanical?  If it's the former, I'd first try floating the (3rd) groundpin of the powercord.

[el34]

Thanks for replying..

The hum is apparent in the speaker, If I put my ear up to the slot on the lower front, its very apparent.  I have a large transformer from a computer UPS that has a tap on the primary, so connecting the primary only as an autoformer to the typically 125v line gives me about 109v.  By the way, the input plate on the sub indicates 110v line nominal.  Lifting the ground does not help.  Isolating the input (removing from the amp/main speakers and shorting the inputs) does not eliminate the hum.  I think that leaves 3 possibilities:
- power supply caps in sub amp going bad
- power transformer in sub amp loosening up and radiating magnetic field
- DC offset on power line saturating transformer, also causing radiated magnetic field

[el34]

Ok, I hooked up my breadboarded DC blocker circuit (as described at Elliot Sound Products site) that maybe helps a tiny bit, but not much.  I think my only options left are to recap, and to replace the power transformer, siting it outside the sub box.  Last time I had the plate amp removed from the sub, it appeared the power TX was bolted AND glued in place, making removal impossible.

[el34]

I unhooked the power transformer, and used the secondary of the UPS tranformer, which is massive, but a little low in voltage output.
Hum gone! (99% anyway) So, I'm guessing the sub's power TX is degraded and radiating a field into the amp circuit (which it's right next to.) I guess my options are to buy a suitable transformer to put in an external box, or try to shield the internal transformer.  Note that this is an early 2w (low 2000's SN) likely different from what's made now..