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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
erik_squires
15,667 posts
 
The midrange is unlikely to be the place where you blow a resistor, but I'd have to know the schematic. Much more likely to happen on the woofer, but I digress.

Put a meter on your amplifier output and measure. DC is easy for any multi-meter to read, but make sure it has a mV range. I had an electrician's meter that could only measure to 0.1V DC, not good enough.

Best,

E
herman
2,215 posts
 
I think you are looking in the wrong direction. A little bit of DC leakage is not going to blow fuses and drivers in a speaker that is designed to handle 100's of watts of power. At least I assume a $50K speaker that doesn't have a stated maximum power can handle any reasonable amount of power. The likely culprit is you listening at extremely high volume levels. Since you don't mention what you are listening to and at what volume levels that is just a guess, but much more likely scenario than the amps leaking a bit of DC
crazyeddy's avatar
crazyeddy OP
381 posts
 

Ok, to try and answer some of your responses, I do like my music fairly loud, but I think I exercise enough restraint to keep it out of the danger range. I listen to a wide variety of music. I can be listening to Holly Cole, and next album could be Robin Trower, or perhaps The Beatles or Stones. Just depends on my mood. I tend to not listen to heavy metal or very hard rock ( although I do like these genres), as the recordings tend to be too poor for my liking, so I guess what I'm saying is that nothing too bombastic is playing.

Erik, you have said that the mid range is the unlikely area where the resistors would be blowing, and I agree 100%, but the reality is that this is exactly where the damage always is. This is what has me baffled. I would expect that if I was overpowering the speakers, the tweeter resistors would go south first.

As for the actual testing with my meter, I do have a decent unit that will measure in the mv range, so no problem there. Should I jump the positive/negative with a resistor to simulate a load? If so, what wattage, and because the Wilson's are rated at nom. 4 ohms, should I use a 4 ohm value?


crazyeddy's avatar
crazyeddy OP
381 posts
 
@yogiboy  I have just recently gone from an ARC LS-17-SE, up to an ARC LS-27, and the issue has happened with both pre amps, so I would say it's fairly safe to rule that out
jea48
4,268 posts
 
I have a vintage Marantz 2252B I used to checked for DC offset at it speaker outputs. Note I have not powered up the receiver for about a month.
The 2252B has two sets of speaker outputs, system 1 & system 2. I have a a pair of speakers connected to #1 so I used #2 for my first testing. system 1 set to off.

multimeter is a Fluke87. Set to dc mv range.

I followed the procedure outline by the guy in my previous post.
Powered up the receiver and allowed it to warm up for 10 minutes.
Initial test results.
L (Left) ch 1.4mv
R (Right) ch 10.5mv

I then checked again after the receiver had been power up for 20 min.
L ch 1.2mv
R ch 9.6mv

After 30 min I checked again.
L ch 1.4mv - 1.5mv
R ch 9.2mv.

All the above measurements were taken without any load connected to the outputs. I had read on one website to get a more accurate measurement a load needed to be connected to the outputs. The guy said to use an 8 ohm resistor.
I don’t have an 8 ohm resistor for a load. I do have the speakers that are connected to speaker system 1.

With the the Marantz you have the choice to power speakers outputs 1 or 2 or both.
(By now the receiver had been powered up for about 25 - 30 min.)
Here are the test results.

L ch without connected load 1.5mv. With load 1.4mv - 1.5mv

R ch without connected load 9.2mv. with load 4.0mv

Not sure how the measurements would be using an 8 ohm resistor connected to the same output terminals as the meter test probes.

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