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Thread: The truth about caps

  1. #36
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    Quote Originally Posted by Chuck H View Post
    Dai, if you go to The Amg Garage "Trainwreck Discussion" and search "sozo" you'll find some stuff. Most of it is Geetarpicker (Glen Kuykendal, is that how it's spelled?) pioneering the mods and discussions following. I didn't find the exact thread I remember, but I found enough similar info to fill in. It is definitely eluded to that the higher ESR is thought to sound better in the Express clones.
    thx Chuck. Maybe I misunderstood, but did you mean (mods to) the Trainwreck amp to get "the old sound" or the alu electros themselves? Also, I thought I saw some thread there (amp garage) where it was mentioned that low ESR (maybe just for some of the positions?) was the key rather than the opposite (perhaps high is better for some while low is better for other positions?).

    I also searched the archives for your old thread but came up dry.
    had it saved, and here is the part where R.G. mentions the specific value range to simulate higher ESR:

    From: R.G.
    Date: 6/24/2005 11:57 PM
    Subject: Re: Filter caps in series Q?

    Yep - sometimes a direct spec of ESR is hard to find. Makers really don't want to be testing that, so they're goosy about specifying it.

    ESR is one of those things that is usually not needed in today's low voltage high current supplies. The frequency is too low. I suspect that if you look at capacitors specified for high frequency switching power supply outputs they may spec ESR.

    Here's what to do about ESR: Go get an ESR meter kit. There's an outfit in Australia (Oz to you that live there 8-) )that sells the kit, about US$80. It reads ESR directly. Dick Smith, as I remember.

    You can repair a lot of equipment by reading the ESR on the caps and replacing the suspicious ones.

    As to how much resistance is enough, the only good way to do that is to measure the ESR of the old, bad cap, then stick in a resistor of that value.

    They're usually in the low ohms range, under 100 for certain. OK, I found the manual.

    Yep, the ESR for a good cap is always under 20 ohms, and that's for a 1uF cap at 250V rating. There's a table of ESR values for good caps of various sizes and voltages.

    The manual says that ESR typically needs to get to 10-30 times the "good" value to cause trouble. Obviously, it can go as high as infinitely more than the normal (in an open cap) but somewhere in the 10-50 range is going to cause problems. So an old, bummer 22uF/250V cap that's about to die might have an ESR of 90 ohms. I'd measure the ESR, you could experiment with 5-50 ohms.
    I also searched the archives for your old thread but came up dry. I did find a thread where you put R.G.'s 47ohm series resistor to the test. You reported a definite difference there. Both better and worse depending on what a player might want.
    hmmm... guess my brain is headed down the early senility chute...

  2. #37
    Supporting Member Chuck H's Avatar
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    Quote Originally Posted by dai h. View Post
    hmmm... guess my brain is headed down the early senility chute...

    Chuck said: "I also found a lot of stuff where you and I are discussing ESR, series vs. parallel caps, etc. It seems we've been down this road before. Too bad my memory can't reach all the way back to the last epoch when it all happened."

    Your not alone brother.

    Chuck

  3. #38
    Senior Member guitician's Avatar
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    I wonder if the audio industry will ever develop something akin to the mass spectrograph? Something that analyzes every possible thing occurring in a stream of waveforms. Like a Cat scanner for the instantaneous position of the electrons/air molecules as they change in every 1/3 octave. I guess hype is just a cheaper way to go...lol

  4. #39
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    Just to clarify,I am not speaking of coupling caps,tone caps or bypass caps.I am not an audiophile and dont buy into any of the hype about Sozo's or Auracaps or the like.Those are for the Hi-Fi guys.There are some differences in the type of caps used as coupling caps in guitar amps,the most noticeable being a Mallory 150 type compared to the orange drops,but even that gets too much attention for my concerns.I am not an engineer or pretend to be,have no interest in debating ESR or any specs.I only speak from what I know from my own experiences in the amps I have worked on and the symptoms and cures I have found.cminor9,I am confident that if you take those xicons out and replace them with the aforementioned Sprague or F&T's your 6G3 will sound better.Will it make it absolutely 100% sound like an original 6G3?Absolutely not,but better,yes.Oh,and I did cut open a Sprague 30uf/500v that I had in my "to be discarded"pile of crap that came out of an early '80's Boogie amp and compard it to one I purchased in the last batch I ordered and found them to be the same,no sneaky smaller cap in a larger can.I still cant imagine why they would put a smaller cap in a larger can.

  5. #40
    Supporting Member Chuck H's Avatar
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    Quote Originally Posted by stokes View Post
    Oh,and I did cut open a Sprague 30uf/500v that I had in my "to be discarded"pile of crap that came out of an early '80's Boogie amp and compard it to one I purchased in the last batch I ordered and found them to be the same,no sneaky smaller cap in a larger can.
    Hmmm... The one I cut open was also a 30uf/500v, keeping accuracy and all (I accidentally tortured it in such a way that I didn't plan to reuse it ). It was from about two years ago, purchased from Mouser. And I did find a smaller cap inside a larger can just as the earlier photo shows. I wonder why some would be like that and others not!?! And I wonder if there is any difference in performance between the two. But I can assure you that it's true.

    Chuck

  6. #41
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    will this sound better than brand "X" capacitors?

    submitted for your entertainment

    Aaudio Imports: Product

  7. #42
    Old Timer defaced's Avatar
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    Why, do tell, would someone cut open one of those caps in the first place?
    Simple: it was dented. I'm not putting 250-300v across a dented cap.
    -Mike

  8. #43
    Senior Member cminor9's Avatar
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    Quote Originally Posted by rudutch View Post
    submitted for your entertainment

    Aaudio Imports: Product
    from the page, "each and every fuse has also been accurately measured and checked in order to bring your equipment to the highest standard."

    LOL! Measured? How!?
    In the future I invented time travel.

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    Quote Originally Posted by cminor9 View Post
    from the page, "each and every fuse has also been accurately measured and checked in order to bring your equipment to the highest standard."

    LOL! Measured? How!?
    I asked that very question to a Bussman fuse rep.... It blows when you test it and then they are hard to sell, right?

    his answer was "ohms law and a micro ohm meter."

    Yup, he war right and I learned something

  10. #45
    Senior Member guitician's Avatar
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    What, even fuses are being hyped......

  11. #46
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    Quote Originally Posted by rudutch View Post
    will this sound better than brand "X" capacitors?

    submitted for your entertainment

    Aaudio Imports: Product
    I love this quote from their "specifications":
    Fuses always carry high electric current thereby causing metal fatigue. This would then adversely alter the conductivity behavior of the fuse element and hence the performance of the equipment.

    Well, I'm a metallurgist, and I was never taught that electricity causes metal fatigue, and I never read about that possibility before. So, I guess we should also tear out the wiring inside our house after some period of time because the copper has been fatigued by the electrical current. I think I'll go purchase some stock in the copper mining industry. Maybe we should ask them why their gold plated fuses aren't fatigued by the electrical currents. This is fascinating science...

  12. #47
    Senior Member guitician's Avatar
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    You have to ask yourself: What makes the fuse blow?

    The thinness of the wire in the fuse! If your house was wired with 24 AWG, there would be some fatigue for sure. In the way of a fire....lol

  13. #48
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    $39 for a fuse!!!Its gotta sound better,no?What was it that guy said...theres one born every minute.Maybe for $39 it never blows,just repairs whatever would have caused those cheap fuses to blow.Never realized that slow blow was responsible for "best sound quality"

  14. #49
    Noodle of Reality Steve Conner's Avatar
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    Quote Originally Posted by guitician View Post
    I wonder if the audio industry will ever develop something akin to the mass spectrograph? Something that analyzes every possible thing occurring in a stream of waveforms. Like a Cat scanner for the instantaneous position of the electrons/air molecules as they change in every 1/3 octave. I guess hype is just a cheaper way to go...lol
    Ha! This is in fact the manifesto of the Audio Objectivists. They say that if you have an oscilloscope and a spectrum analyzer, those analyze every possible thing that can occur in an audio circuit, exactly as you said. So as an objectivist myself I say we've already got this thing.

    Cminor9: I don't buy the "Xicons= butt tone" theory. It'll be something else (that makes more than 3dB of a difference to something! ) If I were troubleshooting this, I'd hook a scope up to the amp and look for some kind of waveform that appeared at the same time as the "butt tone" was evident. From my experience of tweaking audio circuits, I have a fair idea of the relationship between waveforms seen on a scope, and sounds. In this way I'd eventually figure out what part of the circuit was faulty or in need of modifying.

    Do you only get the unobtanium nut on 10-top PRSs? Mine doesn't have one
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

  15. #50
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    Quote Originally Posted by guitician View Post
    You have to ask yourself: What makes the fuse blow?

    The thinness of the wire in the fuse! If your house was wired with 24 AWG, there would be some fatigue for sure. In the way of a fire....lol
    That's not fatigue though. Fatigue is deterioration of metal's properties (usually mechanical strength and ductility) due to repeated long term application/release of stress. A fuse blowing is just increasing it's temperature (due to increased current) to the point where the material melts/explodes/whatever - it's basically uncontrolled welding. It's an instantaneous event, which is not fatigue.

    Fuse fatigue, untill someone shows me micros of the material's structural change as a function of cyclic loading along with how that correlates to its electrical properties, is BS.
    Last edited by defaced; 11-27-2009 at 07:17 AM. Reason: spelling
    -Mike

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    I happy you all found it amusing, I was amazed at the marketing. I wonder how may people actually bought these and were then convinced they could 'hear the difference'?

  17. #52
    Senior Member cminor9's Avatar
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    Quote Originally Posted by Steve Conner View Post
    If I were troubleshooting this, I'd hook a scope up to the amp and look for some kind of waveform that appeared at the same time as the "butt tone" was evident. From my experience of tweaking audio circuits, I have a fair idea of the relationship between waveforms seen on a scope, and sounds. In this way I'd eventually figure out what part of the circuit was faulty or in need of modifying.
    Hooking the amp up to a scope; now that would be ideal. But I don't have one. I don't have hundreds of dollars to invest in a new one. Maybe I could find an old one for cheap somewhere. Until then...

    I want the xicons to be ok; $1 for a xicon cap vs $7 for a sprague vs $5 for an F&T. But there has to be some common variable between the two amps. But correlation does not equal causality, I know.
    In the future I invented time travel.

  18. #53
    Old Timer J M Fahey's Avatar
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    who can measure 6800uf anyhow to know if it's off? I can't do it with my DMM.
    You can, as anybody else.
    Just drive your amp to clipping .
    It's very easy to measure, hear and specially see, if you have a scope, the difference between, say, 6 Vpp ripple and 2 Vpp, which come from the difference between 2200uF and 6800uF caps.
    We're talking a 3:1 difference in ripple/hum content mixed with your clipped musical signal, or 10dB, way over the usually accepted 3dB threshold of audibility.
    Of course, I'm talking about capacitance differences here, the only significative parameter.
    All others pale by comparison.
    Modern components are usually *very* good and consistent, much more so than 20 to 40 years old components, with the exception, perhaps, of tubes.
    The market is *very* competitive, we're talking billions of parts made, in a fully robotised way.
    Just there's not space for a backyard, improvised factory, simply because its poor products will be more expensive than good generic ones.
    Of course, fakes as those shown above, are a whole different game.
    You don't even need to open the cans to spot such gross fakes as above:
    1) they weigh less
    2) they feel very different in your hand.
    Those smaller caps floating , suspended by two puny legs inside an empty bigger can, vibrate physically at a couple Hertz frequency, will feel very funny when you move them, or when you put them on any hard surface.
    The cheap fakers, might at least have used some plastic foam, hot glue, or even a cardboard ring to keep the whole mess steady.
    Even more, they should have pulled the inner , self-incriminating plastic sleeve.
    That alone, makes me think that this particular photo is a fake or a joke.
    As a side comment, caps *have* been shrinking steadily for the last 40 years.
    I still have, somewhere, a couple Campbell Soup can size electrolytics from the late 60's, Spragues or Mallorys, which are twice (or more) the diameter and twice or more the height of modern ones, we're talking an 8:1 size and weight difference.
    Do they sound bluesier/warmer/better/britisher/soulful/transparent/heavenly/..../etc. than new, "inferior" ones? In fact, not, specially because they are probably half-dry by now.
    Of course , I don't build "Audiophile grade" equipment, so maybe I'm not qualified to post these, admittedly, personal opinions.
    Peace.

  19. #54
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    Quote Originally Posted by Diablo View Post
    I love this quote from their "specifications":
    Fuses always carry high electric current thereby causing metal fatigue. This would then adversely alter the conductivity behavior of the fuse element and hence the performance of the equipment.

    Well, I'm a metallurgist, and I was never taught that electricity causes metal fatigue, and I never read about that possibility before. So, I guess we should also tear out the wiring inside our house after some period of time because the copper has been fatigued by the electrical current. I think I'll go purchase some stock in the copper mining industry. Maybe we should ask them why their gold plated fuses aren't fatigued by the electrical currents. This is fascinating science...
    Their explanation/advertisement is indeed pure male bovine excrement. However, fuses DO fatigue and DO fail because if it. It's just not the electricity (directly at least) that does it. As a metallurgist, you're familiar with the physical characteristics of low-melting-point metals, I'm sure. The thing about fuses is that they are designed to hold their specified current forever, but to open in about a minute, depending on the time rating of the fuse, on a 100% overload. As you could guess, this requires some fancy calculations of the resistivity of the metal, the temperature coefficient of the resistivity, and the melting point of the metal and how those affect the fuse element when it operates near the melting point of the metal.

    But think about this: if you run a fuse up to 100% of its rating, it gets hotter, right? And all the fancy alloying in the world can't guarantee that a special melting point type alloy also has a zero thermal coefficient of expansion, right? For clear glass fuses, you can see the element expand into a curve instead of a straight line as it gets to its rating, and it will visibly wag when overloads push it over 100%, even if the overloads are so short they don't make it actually clear.

    This mechanically flexes the ends of the fuse which are held fast to the metal and heat-sinking caps, which keep some of it cool and therefore not annealed by the heating. Any surface defect on the cooler ends can and does snap under fatigue cycling from near or just-over currents. And old fuses do die this way. Bending, spiraling, etc. the element helps and this is why many fuse element are formed this way.

    But you're right - you can't HEAR the stressed metal in the fuses.

  20. #55
    Noodle of Reality Steve Conner's Avatar
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    You can measure it, though.

    Douglas Self once published plots of THD generated by speaker fuses near the point of blowing. They produced a big rise in distortion at low frequencies. I think it was due to temperature coefficient of resistance, and the temperature varying wildly on the timescale of the signal.

    I also remember once testing a brand new power amp at work, just out of the box. One of the channels sounded weak and crappy. I opened it up, and the speaker fuse for that channel was loose in its holder.

    In either case, the solution is not to use speaker fuses. An electronic current limit is better anyway. The fuses should be in the DC rails as backup, to prevent a fire hazard if the output transistors do fail. And in the DC rails, you won't hear them because the power amp's negative feedback will get rid of any distortion they might introduce.

    About the photo I posted: As far as I know, it's an example of some particularly nasty Chinese fakery. Or maybe it is a joke, you never can tell on the internet.
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

  21. #56
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    Quote Originally Posted by Steve Conner View Post
    I also remember once testing a brand new power amp at work, just out of the box. One of the channels sounded weak and crappy. I opened it up, and the speaker fuse for that channel was loose in its holder.
    Yep. Loose contacts, especially on mechanically-shaken thing like a guitar amp will do you in.

    In either case, the solution is not to use speaker fuses. An electronic current limit is better anyway. The fuses should be in the DC rails as backup, to prevent a fire hazard if the output transistors do fail. And in the DC rails, you won't hear them because the power amp's negative feedback will get rid of any distortion they might introduce.
    Fuses in the DC rails are problematical. You have to ensure that both rails are interrupted within the time that would burn out a speaker if only one is blown. One obvious approach is to fire off a thyristor between the two rails when a defect is detected. This was once a popular protection scheme, but the brutal turn on of a thyristor creates a new set of problems, especially if the fuse either doesn't clear or arcs over.

    My favorite scheme is to put high current arrays of MOSFETs in the + and - rails, clamping them saturated-on for normal operation and turning them off when the fault detector circuit goes off. I thought this was an obvious application, but I've never seen anyone else do it, even after I posted it on my web site. A first impression would be that it's no better than having the output transistors, which are after all in series. However, these devices are always saturated on, not active until tripped. So they stay (potentially, if you do it right) cooler and less stressed by normal activity. High current ratings are cheap in MOSFETs too, so one could over-design these MOSFET current switches to be massively better at carrying and interrupting the power rails than the output devices, which have other considerations to act on. If you use P-channels for the + rail and N-channels for the - rail, you can get by without high-side drivers, although LED/photovoltaic MOSFET drivers will let you do both sides with the same activate/trip circuit, no high side drivers.

    Once you have rail-interruptors, you can gang up fault sensors and clean up some other issues. DC offset on the output? Fire the fault detector. Overheat on the output heat sink? Fire the fault detector. Overheat on the drivers? Same remedy. Want a remote enable? Hmmm...

    I'm in the middle of doing the work on one of these for an amp I'm refitting. The nice thing if you use both N and P channels for switches is that you can bolt them directly to two strips of aluminum, which becomes a current buss bar for supplying the output stages or the entire amplifier. The inputs are on the sources, the outputs on the drains. The body diodes let reverse currents pass back into the filter caps. And the aluminum buss strips form all the heat sinking the MOSFETs need.

    About the only drawback is that you have to use multiple parallel MOSFETs for the job, because getting really low channel resistance to keep any audio artifacts from being caused by the MOSFETs requires you to get the MOSFETs down to about 1/10 or less of the emitter resistors on the output stage. So you have to use several devices, more than you would think from casual examination of the power to be handled.

  22. #57
    Old Timer Gtr_tech's Avatar
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    I got scammed on some caps a while back. I ordered 22u and 33u @450v. The 22s read ok on my cap checker, but the 33s read around 20. I got suspicious of the double rubber ends/double crimps like a non polar has, so I ripped one apart. Found this:
    fake caps on Flickr - Photo Sharing!
    DSC02035 on Flickr - Photo Sharing!
    radial on Flickr - Photo Sharing!
    DSC02036 on Flickr - Photo Sharing!

    And one was even kinda smashed under the re-shrink....there's the original shrink under that. I suspect they were "recycled" pulls.

    They were all 22u 400v parts.

    Seems like you can't buy *any* electronic parts without getting fakes, unless you buy from a distributor you can trust. Fake STK chips....why would they even bother? Most of the larger/older ones aren't a big ticket item since they were used in 20-30 year old designs and not in high demand. As some of these parts get harder to find at the large distros, its gonna get real hairy for the tech who has to track down replacements.
    The farmer takes a wife, the barber takes a pole....

  23. #58
    Supporting Member Chuck H's Avatar
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    Quote Originally Posted by Gtr_tech View Post
    Seems like you can't buy *any* electronic parts without getting fakes, unless you buy from a distributor you can trust.
    I got my 30UF 500V caps from Mouser Electronics. Trusted and used by almost everyone U.S. on this forum. Yet I got a "small cap in a big can" thing. I'm more inclined to go with R.G.'s explaination of the whole thing. They're not "fake" caps, just smaller ones. The bigger can is what the MFG has been selling to their distributors so Sprague just made it work... Performance differance???

    Chuck

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    Quote Originally Posted by defaced View Post
    That's not fatigue though. Fatigue is deterioration of metal's properties (usually mechanical strength and ductility) due to repeated long term application/release of stress. A fuse blowing is just increasing it's temperature (due to increased current) to the point where the material melts/explodes/whatever - it's basically uncontrolled welding. It's an instantaneous event, which is not fatigue.

    Fuse fatigue, untill someone shows me micros of the material's structural change as a function of cyclic loading along with how that correlates to its electrical properties, is BS.
    Fuse fatigue is certainly not BS- fuses break all the time, without blowing. In solid state amps the fuses pass enormous ripple current, which causes the wire to continually flex (from the force exerted by the magnetic field). Also, when you switch on, the inrush current will cause the fuse to visually jump. Over time this will lead to a fuse breaking of course, without blowing.
    Even in a valve amp the inrush will eventually fatigue a fuse, although the ripple current is obviously lower.

    While I agree that gold-plated fuses are unlikely to have any audibly effect on an amp, especially a valve amp, fuses certainly do have a measurable effect on amp performance, at least where powerful SS amps are concerned. See page 248 of Self's book:
    Audio power amplifier design handbook - Google Books

    Given the relevance to this thread, it is also worth reading the section about electrolytic LF distortion, which I believe manifests in guitar amps when small cathode bypass caps are used:
    http://books.google.com/books?id=BRQ...age&q=&f=false

    Page 44 also showed some differences between ordinary caps and an audiophile cap:
    http://books.google.com/books?id=BRQ...age&q=&f=false
    Last edited by Merlinb; 11-29-2009 at 02:27 PM.

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    I hadn't really thought about the service conditions of a fuse. I didn't realize they are operating normally so close to their melting point. I never watched them expand and contract in response to the current. Under these conditions, thermal fatigue would be the failure mechanism. Gold plating the ends surely won't make a difference to the thermal fatigue of the fuse filament wire.

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    Quote Originally Posted by Chuck H View Post
    I got my 30UF 500V caps from Mouser Electronics. Trusted and used by almost everyone U.S. on this forum. Yet I got a "small cap in a big can" thing. I'm more inclined to go with R.G.'s explaination of the whole thing. They're not "fake" caps, just smaller ones. The bigger can is what the MFG has been selling to their distributors so Sprague just made it work...
    I'm kind of torn on this one. All electro cap innards are smaller than the outer can by a notable amount. Cornell Dubilier has a diagram in some of their tech lit about this. The wound cap pellet is stuck into the outer can with either pitch or a mechanical holding arrangement. But there is no internal can nor any internal shrink wrap. That would impede heat flow out of the can, and that's a big consideration in power applications.

    The ones with internal heat shrink cans are definitely fakes, IMHO. Just a smaller wound bundle inside is the way they're made properly.

  27. #62
    Noodle of Reality Steve Conner's Avatar
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    Quote Originally Posted by R.G. View Post
    My favorite scheme is to put high current arrays of MOSFETs in the + and - rails, clamping them saturated-on for normal operation and turning them off when the fault detector circuit goes off. I thought this was an obvious application, but I've never seen anyone else do it, even after I posted it on my web site.
    I'd guess that's because it does less than current limiting on the output stage, combined with rail fuses and a speaker relay, but costs more.

    If either rail fuse blows, the DC offset detector will trip the speaker relay. And a speaker relay gives you free anti-thump.

    If you're concerned about contacts in the signal path, you could probably make a speaker relay from back-to-back MOSFETs powered by a photovoltaic isolator, but Douglas Self would have fits at the THD.
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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    Quote Originally Posted by Steve Conner View Post
    I'd guess that's because it does less than current limiting on the output stage, combined with rail fuses and a speaker relay, but costs more.
    Hey! kEwl! A true work of engineering excellence! More bells and whistles and less real substance!

    My reasoning is more esoteric - if perhaps more misguided. This is a fire- and speaker-saver. The presumption is that the output devices are already toast or may be assumed to be. Hence, current limit circuits on the outputs won't help. Or one rail fuse has opened by either a close encounter with its rating or metal fatigue.

    When one rail fuse blows but not the other, the output voltage goes to the opposite rail and is only current modulated by the remaining output device in the best of cases. In the worst of cases, it sticks at the opposite rail, connecting the speakers DC-wise to the rail.

    If either rail fuse blows, the DC offset detector will trip the speaker relay.
    At that point, the speaker relay is supposed to open. I've worked with relays. They work great - as long as you don't really, really need them. If they're regularly interrupting anything like their rated current, they also arc, pit, and weld. Like any mechanical device, they need maintenance. I'm not concerned at all about the sonic aspects of another contact in the audio path - but I really do want to stop a transistor failure from becoming an opportunity to replace all my speakers. A good speaker relay needs to handle transient currents up in the 10-20-30A range, depending on what speakers you have attached and what kind of transients happen. If you happen to short the output line (Hey! I can, no problem. It sez right here that the output transistors are current limited...) you can get some interesting transients through those relay contacts. They'll work great the first time, and second, and ...third? fourth?

    I guess put another way, I have this almost religious faith that if I quit driving the gate of a MOSFET, it will stop passing current. I believe that more than I believe in relays.

    And a speaker relay gives you free anti-thump.
    It does do that reliably. In my experience, thump comes from single power supply amps with capacitor outputs, and symmetrical Linn architecture amps are almost thump-free, but there's always some amp that will thump.

    If you're concerned about contacts in the signal path, you could probably make a speaker relay from back-to-back MOSFETs powered by a photovoltaic isolator, but Douglas Self would have fits at the THD.
    Yeah, I experimented with that, and decided that I could do the same job at the power supply with the same number of MOSFETs and PV isolators, and have the THD cancelled by the amp's feedback and power supply rejection. Actually, that is exactly the point where I started thinking about this. I don't mind the extra contact if thought I could actually rely on it. But I have this mental image of big relays being expensive and welding closed or open.

    Practical or not, it does work well, and sidesteps the issues of non-symmetrical fuse blowing and relay issues.
    Last edited by R.G.; 11-30-2009 at 03:25 AM. Reason: pressed the send button too fast...

  29. #64
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    Quote Originally Posted by R.G. View Post
    Hey! kEwl! A true work of engineering excellence! More bells and whistles and less real substance!

    Practical or not, it does work well, and sidesteps the issues of non-symmetrical fuse blowing and relay issues.
    I dread to think what Douglas Self would say!
    No doubt he would argue that anything standing between the PSU and the amp will usually retard the overall performance, or limit the transient current available, depending on where you put the FET relative to the reservoir caps.

    Alternatively we might suggest that the output devices be overated enough that the rail fuses blow before the output devices can fail (or you might say the the rail fuses are slightly underrated for the theoretically available output current). This amps I used to build were designed this way- the fuses would always blow before the output transistors could.

    Also, even if one fuse blows, the output offset should immediately force the remaining devices into cut-off, with no harm done to the speaker. Adding a relay would offer just a little extra help.

  30. #65
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    Quote Originally Posted by Merlinb View Post
    I dread to think what Douglas Self would say!
    No doubt he would argue that anything standing between the PSU and the amp will usually retard the overall performance, or limit the transient current available, depending on where you put the FET relative to the reservoir caps.
    No doubt. Although they do look like small-ohms resistors when they're driven on as much as possible. Most output stages have good rejection of the power rails, although that's not necessarily true of the earlier stages.

    Alternatively we might suggest that the output devices be overated enough that the rail fuses blow before the output devices can fail (or you might say the the rail fuses are slightly underrated for the theoretically available output current). This amps I used to build were designed this way- the fuses would always blow before the output transistors could.
    That's one way to do it - excepting that bipolars tend to current hog, so you have to work at forcing current sharing or the problem still exists. In my experience, it's at best a statistical matter whether the fuse protects the transistor or vice versa. My presumption is that when you need to turn off the power rails, one or more output devices are already dead.
    Also, even if one fuse blows, the output offset should immediately force the remaining devices into cut-off, with no harm done to the speaker.
    It should - unless there's a signal going into the amp. Hard to say what happens in all cases.

  31. #66
    Noodle of Reality Steve Conner's Avatar
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    This is always a big worry in hi-fi amps, that can find themselves hooked up to speakers worth more than your car. There has to be a second line of defence against a failure in the amp itself that sticks the output hard against one rail: a rail fuse can't be relied on to pop in time to save the speaker.

    From a safety perspective, you must work on the basis that the transistors always give their lives to save the fuses. The fuses are just there to keep the amp from catching fire once the semiconductors have failed short.

    RG's idea of de-thumping the power amp by design, and using MOSFET rail circuit breakers instead of fuses, may well bear some consideration.


    My own experiences in this context are:

    I once had a 400 watt Class-D amp fail on me. It overheated, then one of the power MOSFETs failed short and stuck the output to the (-80V!) rail. Luckily it was on a dummy load, saving some lucky speaker from complete incineration. I had planned to use this module without a speaker relay because the maker claimed that it was "fully protected".

    The only hi-fi amp I ever built was a current-feedback MOSFET design that had the worst turn-on and turn-off thumps ever. Not so much thumps, as vicious cracks and bangs that made me fear for my tweeters.

    The design was supposed to be speaker-relay- and rail-fuse-less: the mains was supplied through a latching relay, and the protection circuitry (thermal and DC offset) just unlatched it, turning the whole amp off. There was also SOA protection that muted the audio, and if muting didn't bring the devices back within their SOA, it would fall through to the second stage of protection and again turn the amp off.

    To get rid of the thumps, I eventually fitted a speaker relay with gold contacts, salvaged from the power supply of an old TV camera.
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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    Steve, I'm probably overly affected by my experiences with switching power supplies. We were trying to get these to work reliably back when the only devices which could switch power were high-voltage bipolars that simply were not fast enough and/or did not have enough SOA to do the job. I actually had transistors fail in a way that melted holes in the tops of steel-case TO-3s. The output section of the power supply would have charred and vaporized copper tracks (some of them 0.2" wide) and dead everything, but the "protective" fuse would be intact. So I'm always thinking in terms of how many nanoseconds I have to shut down the current flow before filling the lab with noxious fumes.

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    Back to capacitors...

    Getting back on topic, Walt Jung wrote some good articles on cap distortion. You can find them here, about a third of the way down, "Picking capacitors".
    Services

    Some of it ties in with Self; polar capacitors tend to generate distortion when the AC voltage across them starts to become appreciable.

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    Quote Originally Posted by stokes View Post
    For the longest time I swore by Spragues only,but I found the F&T's live up to my expectations,so I guess there could be others,but for me right now its just those two.I wont use any others.
    Just out of curiosity, anyone tried Panasonic FC/FM series or Elna Silmic? Of course, these are often not available in high enough voltage for power amp filter caps, but they would certainly work for cathode bypasses. Those are my current default caps for lower voltage circuits, e.g., FM tube tuners.

  35. #70
    Senior Member guitician's Avatar
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    Polypropylene and Foil are great, but are pricey.... anyone try CDE 940C30S1K-F .01 3000V $1.5 and can slew 2568 volts a microsecond!

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