Pure Acoustics Subwoofer faulty

nuryev

Member
2007-08-11 10:02 am
I have had a Pure Acoustices XP12- 200 (powered sub) for about five years and now it has developed a fault. The sub was used in a 7.1 surround setup driven by an Onkyo 807 amp.

When the amp is powered on, the sub will generate a motor-boat-like sound, even when no audio is present. After about a minute the thumping fades to a hum. I have switched connections - secured ground wires attached to both sub and amp but still no improvement.

Any adjustment to the sub's controls only increases the thumping sound. I would like to try a diy repair before relegating to the rubbish tip.

Besides saving a few hundred dollars, recycling saves the environment and hopefully, I will learn something from the diy experience.

Thanks
 

nuryev

Member
2007-08-11 10:02 am
Thanks for the suggestion - I tried with input disconnected and still get the rumbling sounds.

What I also noticed is that when I physically touch the GAIN knob with my finger to adjust the GAIN control, the noise increases??? The subwoofer is grounded and so is the sub cable.

Would a failing GAIN potentiometer produce such symptoms ?? - just a shot in the dark.
 

nuryev

Member
2007-08-11 10:02 am
Thank you all for suggestions - I removed the plate amp and psu from the unit: Sadly, the quality of workmanship and components is very poor. I looked at the underside of the psu pcb and found some signs of heat stress.

Removed caps and resistors and tested them using dvm and found intermittent readings significantly below or above spec. Some ceramic resistors had their coatings so badly degraded they appeared to be disintegrating - As a last resort I decided to price the components and search through my spare parts bin. I have no idea what a repair would cost but the parts cost me less than $10.

Found two 3500uf 63v psu caps a few 2k2 and 1k1 resistors and replaced those in the power supply.

Powered it up and was greeted by with an extremely muted 'thud'. Left it for a while and an hour later - decided to connect to av system.

Decided Pearl Harbour would be a good dvd to test the LFE - long story short.....The Sub-Woofer is now working again. Saved from the tip - another win for the environment! :)

Unfortunately I did not take any pics of this project.
 
Does anyone have a service manual or link to the appropriate service manual for this subwoofer?
Thanks
I also want to know this. From the manufacturer I don't get reply. I have the model "Dream Sub" - go to
http://www.pure-acoustics.com/design/manuals/manual_dream_sub.pdf
with similar unwanted effects. I have reduce the background hum. Therefore I have connected the backplate with the input-GND at the female cinch connector socket.
With help of the schematic it must be possible to reduce the background hum additional to get lowest values.
 
I have fixed the issue. The reason for the residual hum-noise is the fact, that the stereo-volume-control (gain potentiometer) is allocated between the input socket and the summing amp. this means, the background noise will not be attenuated by reducing the so called gain level.

I have placed the potentiometer now between low pass output and power amplifier input as mono configuration (second half not connected).
Now no audible hum is present by select normal level. After converting this small enclosure from a vented box to a closed box (due to high distortion arround 40Hz) the subwoofer modul works fine.

From my view the developer of the circuit and the cabinet of this model was unqualified and thus his results don't provide satisfactory acoustical results.
 

nuryev

Member
2007-08-11 10:02 am
Hi guys,

Update on the old Pure Acoustics XP12-200 subwoofer.
Old faithful has had a change of capacitors and resistors on PS board and has had pots replaced. A new symptom has developed i.e. it plays very well but after 20-30 minutes the output volume drops drastically until it is barely audible. I am guessing it could be transistors but would need to do some tests to confirm. I will attach some pics as soon as I can work out the process and look forward to any comments and suggestions.
Thanks