Does this happen on that one left speaker only and is that speaker always connected to the same monoblock?
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.
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I found this link doing a search. http://audiokarma.org/forums/index.php?threads/amplifier-distortion-dc-offset-and-you.5634/ As a semi-poll, I’d like to see those on this board whip out their multimeters and take a look at the DC that is being presented to the speakers. This means..Note, I personally do not know if the guy's method for testing for DC offset at an amp's output/s is correct or not. . One obvious question, how loud are you listening to your music? |
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 |
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 |
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? |
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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Does your meter have a max hold function? If so you could get an idea of what kind of peaks you are hitting. With a monster amp and great speakers you may be be playing much louder than you realize according to Stereophile your problem may be the impedance dip causing a current surge, especially since your amps look like they are capable of arc welding able to deliver 20 amps into 4 ohms "The speaker's impedance (fig.1) reveals it to be a demanding load for the partnering amplifier, with a value ranging from 4 to 6 ohms for most of the audioband and dropping to a minimum of 2.25 ohms at 240Hz. The combination of 3.8 ohms and a 33.4º capacitive phase angle at 162Hz, a frequency where music has considerable energy, will also demand an amplifier that can source a good amount of current, as MF found in his auditioning." Read more at https://www.stereophile.com/content/wilson-audio-specialties-maxx2-loudspeaker-measurements#qh7WDRg2... |
@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 ? |
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? |
@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 :) |
@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 |
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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@crazyeddy, What are you feeding the two amps with? I assume each amp has its’ own 120V 20 amp dedicated branch circuit. Roughly what is the distance, length, of the wiring from the electrical panel to the wall receptacle outlets? What are you using for receptacles? Did you use #12awg bare minimum wire size or increase the wire size to #10awg? Are you using the stock power cords that came with the amps? If you are using after marker power cords, made by who, model? It would appear these amps need plenty of mains power if you are pushing them hard. If the mains voltage is dropping due to VD (Voltage Drop) on the branch circuit wiring it may be having an impact on the amp’s ability to deliver their full rated power. You can check the mains voltage at the wall receptacle with your multimeter. First check the voltage with the amps at idle. Check the voltage again playing music at a moderate level. Check the voltage playing music loud at the level you like to hear it at. Is the voltage lower but holds steady? Is the voltage lower, bouncing around, with the beat of the music? Specs http://bryston.com/PDF/brochures/SST2_875HT_BROCHURE.pdf https://www.youtube.com/watch?v=TOof73b60E0 Review https://www.stereophile.com/solidpoweramps/108bry/index.html You may find these posts of Charles Hansen of interest concerning "fully balanced from input to output". https://www.audioasylum.com/forums/amp/messages/22/220523.html Jim |
hey Ralph, I agree about the DC, but he has monoblocks that will do 1800 watts into 4 ohms so I don't think bigger amps is the answer Could it be the line sags so low they clip? Seems like the lights would dim if that happens but never seen that so don't know. my theory is it is playing much louder than he realizes |
@jea48 I do indeed have 2 dedicated 20 amp circuits, and am fortunate to have my main panel box in my listening room, in fact, from my panel box to my outlets is a direct vertical drop of approx. 4 ft , and yes #10 awg. None of my power cords are stock. All are AudioQuest NRG-4's, and all are either 3 ft, or 6 ft. Power out of my conditioners is a rock solid 120v, although I have not checked this under high volume conditions. It all just may be that I am the culprit here. Herman makes a good point, that it may be louder than I realize. After so many years of working in extremely loud industrial settings (metal fabrication) and add to that my years as a hard rock guitarist, some hearing loss may be more than I realise. I do have a SPL meter, but haven't used it in years. Perhaps I will dig it out of storage and see what actual values are being presented in my room. Will follow up with these readings tomorrow. Thanks all....... |
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Problem Solved folks. As some suspected I am the problem. Confirmed last night with my Watt Puppys in play, and my SPL meter. @atmasphere Many thanks to you Ralph for taking the time out of your busy schedule over the past couple of days to sort this out with me. I appreciate all of the others who have posted advice here. This is what our community is all about. Cheers to you all !!! :) |