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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
herman
2,215 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
5,486 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 :)


crazyeddy's avatar
crazyeddy OP
381 posts
 

@gs5556  I may need to add that I never power down any of my solid state gear. Only when I go away on extended holidays. My tube gear goes off after every listening session, and upon powering up, is given at least 45 minutes to warm up.

The Brystons really help keep my heating bill down in the winter!!! LOL Great for toasting marshmallows too :) LOL

atmasphere's avatar
atmasphere
11,615 posts
 
This problem is **not** caused by DC!!

The speaker has crossovers that prevent the midrange and tweeter from seeing any DC- even if the amp had a serious failure that put out so much DC that the woofer was fried, the midrange and tweeter would still be OK.

The woofer is always the part that fails due to DC, but a few millivolts is not going to harm it in any way. It has to handle a lot of power from the amp and a few millivolts of DC is nothing. IOW, that's not the problem.

What is going on is that the amplifier is not making enough power to make the desired volume. So as a result, it is being overdriven pretty hard, and the harmonics of the distorted bass notes are at frequencies that the crossover allows into the midrange and tweeter. Since these drivers normally don't have to handle such large amounts of power, they get burned up.

The solution is either get a bigger amp (start out with at least twice as much power) or put fuses in series with the midrange and tweeter drivers to prevent damage. Or don't turn it up so loud, or get a speaker that is more efficient, so you don't have to clip the amp.


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