JBL TLX-PS10/12 Subwoofer power on thump. Normal?

[thekid72]

I picked up a JBL TLX-PS10 powered subwoofer locally on the cheap, seller upgraded to a true home theater subwoofer and this fits my need for adding extension to bookshelf speakers just fine. It does have one possible issue, and that is when it cycles on (automatic signal sensing with no bypass) there is a strong DC offset "thump".

Does anyone have experience with this or the 12" version (same basic amp according to the schematic) and is this normal? If not, I'd rather be pro-active and dig into it as I know the quality is not there - yet as I said above, it works for what I use it for.

Thanks!

 

[llanger]

Thumps are normal when turning on powered subwoofers.

 

[thekid72]

Maybe I should clarify.. this is quite a bit stronger than any other subwoofer I have worked with. Also it is not just a little bit of a DC offset that comes as a "thump", but more of a "pop" that is irritating.

I know this is a cheap subwoofer, and don't have the highest respect for JBL products of late... but I still have to wonder if this is normal. An opinion from someone who has one or has worked on /with one would be great.

 

[larryderouin]

Any driver that is powered from a amplified source that DOES NOT have a protection circuit (start up relay, and dc protection while running) will THUMP or POP due to the voltage surge that is put thru the circuit until the voltage stabilizes in the capacitors. This is common in amps and receivers that are cap coupled and the caps are weak or direct coupled without a startup delay/protection circuit. You can have a Thump/pop in a spkr even if the startup delay/protection circuit is in there, if the circuit is either weak, or damaged. It's not the speakers fault, it's the design of the amplifier. The schematic shows that it has some kind of protection circuit, but it looks like it's a overdrive protection circuit rather than a delay type. Check C31 on the amp board for leakage and value. replace as necessary. And troubleshoot the protection circuit.

 

[thekid72]

Thanks for your input Larry. Good call on the overload protection - indeed the relay is closed at power-up, which I suspect was to minimize delay. My experience with standby power subwoofers has been that effort was made to minimize offset and allow nearly immediate audio coming out of standby. This one just isn't too happy as it is the worst I've ever dealt with.

I'll dig into the amp and check DC +/- during power up as I suspect this one is getting out of spec due to Cheap caps.

Thanks again!

Alan