Hafler 9505 turn off thump

hey Guys

Has anyone here any experience with the Hafler 9505? I got ahold of one for basically free and it's has a rather obnoxious turn off thump. I'm wired directly to sixteen 12 inch woofers in an all-active system and the thump is a bit like a shotgun going off due to the high sensitivity of everything. Even if I'm ready for it, it's a shocker. :D

Anyway, I've had thump issues with other high current amps (B&K in particular) so it's not like a big surprise, but it's been with passive speakers witth sub-90db sensitivities. I went ahead and tried something to see if I could at least dampen it and added 1600uf caps in series with the driver arrays. The large uf was just to get below the driver rolloffs (4ohms at ~40hz) and it helped - it now sounds like a .22 at 30 feet. :D

Here's what's interesting though. When I watch the drivers at power down, they move inward first now with the caps in the signal path and then seem to oscillate for a split second. Without the caps, they just move outward with a healthy bang. Is it possible to have some kind of negative DC going on here? I'm pretty much electrically illiterate, so my understanding of what's happening here is minimal.

Any thoughts? Anyone else with Hafler 9505 amps having turn off thumps? Should I have it serviced?

Sorry - wrong info

Sorry - I was checking too many things at once and got dyslexic. The direction of cone motion is simply related to the polarity - if I have a driver POS to POS with the amp, the cone moves inward; POS to NEG from amp to driver, cones move outward. Either way, the caps don't dampen the turn off thump completely. There's still a healthy pop and the oscillation of the cone for a split second.

I looked at all the possible input related options as well - I tried balanced, unbalanced, mono, stereo, HP filter on/off, ground lift on/off - always the same result. And I've tried finding more information on this from other Hafler owners and haven't been able to confirm whether it's "normal" for the 9505 or not. I wouldn't think it is.
Thanks Dexter. You may be of some help. There's a local technician that used to be a Hafler dealer and he said if I can find a schematic (I just found one) he'll take a look at it. I'll relay what you mentioned to him.

It's a nice amp, and if I can get rid of the thump for a hundred bucks, it'll be worth keeping.


I own two Hafler TRM8.1 reference monitors, (one of which is needing repair). The one that works has always had an audible thump when I turn it off. The one that is not working never did that when it was operating properly. I have posted questions on the repair of this active monitor found under solid state. I find the thump annoying, but it has not been so annoying that I have gone through the trouble of havingm it repaired.
Many amplifier have this problems. I turn off the input stage current when the AC power is turned OFF. Many of my designs have a SLOW ON and FAST OFF circuity the control the input current source. The Hafler has SLOW ON and no FAST OFF. I suggest getting the service manual for BGW 8000 and read the details of the circuity and the schematic.