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DC leakage from power amps

I am hoping to get some advice on how to check for dc leakage from my power amps. I have a pair of Bryston 28B SST2 mono blocks, powering a pair of Wilson Maxx2's. The Wilsons drivers are protected by resistors, acting as fuses. I have been blowing quite a few of the mid driver resistors, valued at 5.8 ohms. Last night I lost another mid resistor cluster on my left channel ( there are 4 wired in a series/parallel configuration), and what I thought was a tweeter resistor, but upon replacing that single resistor (4.2 ohm) with a fresh one, still no tweeter. I was fed up, and did not investigate further, but fear that I may have a dead tweeter, or worse, crossover issues. I am no tech expert, but am concerned that I may have DC leakage from my Brystons. I live about a 4 hour round trip to the Bryston facilty in Peterborough Ontario, and have lots of warranty left, but don't want to pack up the beasts and have them inspected if I can confirm on my own, if there is a DC issue. How do I go about checking the amps at home, and confidently knowing if there are amp issues, or not. If the amps are faulty, I will return them for repairs, but would like to eliminate/confirm the issue at home. Any help will be greatly appreciated.

128x128crazyeddy
gs5556
1,234 posts
 
If this happens on both speakers, then I agree with Herman it's volume setting/listening levels. The chances of both monoblocks passing through excessive DC on startup is minimal, especially before this happened you didn't hear any thumps on amplifier turn on.



crazyeddy's avatar
crazyeddy OP
381 posts
 

@herman  If I remember correctly, my meter does indeed have a max hold function, but as I am not at home at the moment, can't check for sure. As the owner of a welding company, your analogy of my amps being capable of arc welding made me chuckle. Good one!!

Thanks you for sending me the link to the Stereophile page, but I must admit that it is a little beyond my technical understanding.

I'm hoping to get home at a decent hour tonight and do some testing. What level of output do you think I should be running my pre-amp at, for safe, yet accurate measurements ?


herman
2,256 posts
 
I would hook to the output of the  amps and play something you typically listen to at what you would consider a moderate to slightly loud volume level and see what your peak hold AC meter captures. Power is voltage squared divide by impedance so if you assume you are at the dip of 3 ohms the calculation is simple.

if you get 20V then you have 100W (20 times 20 = 400 divided by  = 133W)

if you get 30V you have 300W
40V = 533W
etc

bear in mind your meter probably measures RMS voltage so the actual peak is appreciably higher

turn it up until it is loud. You may be surprised how much power you are pumping into the speakers

that should give you an idea of where you stand. Have you contacted Wilson to see what they think?
yogiboy's avatar
yogiboy
5,665 posts
 
@crazyeddy  
As a shot in the dark it might be the Croft or the SUT passing DC!
  
crazyeddy's avatar
crazyeddy OP
381 posts
 

@herman  I'm reluctant to do this, as I have a possible left speaker tweeter issue, or perhaps crossover issue. I have removed the tweeter, and have disassembled it. Tonight I will do some testing on it as well. The voice coil looks sound, but I really need to test the integrity of the leads to the binding terminals. So the Maxx's are out of commission for the time being. I do have Watt Puppys as well (yup, I like Wilson) but am reluctant to power them up for fear of damage to them as well. I am more inclined to do a test of the outputs of my amps, to eliminate/confirm dc leakage.

I will be speaking with Jerron at Wilson next week, and also Mike at Bryston. I have had conversations with both guys over the years, and trust their advise. Unfortunately, this being a long weekend, I'll have to wait till Tuesday. I was hoping to get some diagnostics done before next week. I'd really like to get the Puppys into play, if I can determine that the amps are sound. I have sever withdrawal issues when I have no music!!

Your math is appreciated by the way. Pretty simple, and easy to calculate once I get things into play. Perhaps I may surprise myself on what actual power is being applied. 

FYI, the final test inspection spec sheets from Bryston for my amps were both in and around 1247 watts before clipping, into 8 ohm load. A welding we will go LOL!! 

 I can look at all this with a certain amount of levity. as in the big scheme of things, this is small potatoes!!


Thanks for all the advice folks, enjoy your evenings :)


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