Rock Solid PowerBass 8" Active Subwoofer

[Raj Neel]

Hi Folks
I picked up a vintage 8" PowerBass Active Subwoofer made by Rock Solid (part of the Bowers & Wilkins stable) last week. Given it's size, specifications and wattage, I am surprised and delighted with how well it fits into my set-up and room configuration. It is being fed a line level 'sub out' signal from my receiver. The only other wire to the unit is the mains power line with what looks like an in-line ferrite filter. As it seems to stay on all the time I am turning the power off when not in use. There is a thump when I power on and off even with the sub volume turned all the way down. This is even when the receiver is not connected so not coming from there.

Question:
1. Does this sub have a 'stand by' mode that it should drop into when not in use?
2. If not, can it be left on all the time? Or ist it OK to let it thump and carry on powering on / off?
3. Is the thump normal? Or is there something I can do to stop it?

Many thanks for any suggestions.
Raj

 

[GordonW]

A slight turn-on/turn-off thump isn't uncommon on some amplifier designs.

OTOH- if it's enough to visibly make the woofer cone move- that may well be a problem. Could be either a bad protection relay (stuck on), or an imbalance between the two "rails" of the power amp (there's usually a side of the amp powered by a positive supply voltage, and another powered by a negative supply voltage, working in "push pull". If one of the two rails is coming up or going down faster than the other (positive vs. negative), that momentary DC offset can make a significant noise, depending on the severity of the imbalance.

I dealt with this on an original Harman-Kardon Citation 12 power amp. Those didn't have protection relays, so as soon as the amp was turned on, the speaker was being powered. I had to very carefully match all the driver and differential amp transistors in both channels of the amp, to get the thump to go away.

Regards,
Gordon.