The 'Mysteries' of SET Amplifiers Exposed!

Discussion in 'Amps, Pre-Pros & Receivers' started by ski2xblack, Jan 27, 2011.

  1. ski2xblack Audioholic General

    ski2xblack
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    Hello folks-

    I was going to post this in another thread, but it got kind of long, and given the recent apparent interest in all things tube related, I thought that this might be helpful to the newbs. I hope to hear from the old hats, to correct me where I'm wrong and share their input. I don't mean for this to be a defense of SETs, just an exposition of what is actually going on.

    So we occasionally hear from someone about how great SETs sound, how they do this and that better than ss, etc. But what is really going on? Are those guys crazy? In need of an audiologist? To figure out what is really going on here, it's important to know some general basics about tube vs ss circuit behavior. Here is a good article from an engineering perspective and citing reputable primary sources:

    http://www.theaudioarchive.com/TAA_Resources_Tubes_versus_Solid_State.htm

    At the end where they're wrapping it all together, and particularly regarding the behavior of tube circuits when overloaded, they write:

    Now, compared to any other amplifier, one of these low watt SET amps is almost always operating in this 'overload' area. High amounts of measurable distortion. But the SET's distortion pattern is also quite different than the other approaches. Push pull tube amps (and SS for that matter), squelch 2nd order harmonic distortion but also squish the distortion out to higher orders at much lower levels. (None of us would run our amps into clipping anyway, would we, so it shouldn't even matter.) SETs do not filter 2nd order distortion, with a heavy bias towards lower orders in general.

    One huge factor which allows it to misbehave is called 'perceptual masking'. This is simply the phenomenon of lower harmonic components being overshadowed by the fundamental. 2nd order harmonic distortion is virtually indistinguishable from the signal as 'distortion' until it gets quite high (you are much more tolerant to it than you would think). As the harmonic content extends away from the fundamental tone, it is masked less, and becomes objectionable at a lower level.

    Also keep in mind what the rising 'edge harmonics' referred to in the article are doing as well. They add subjective loudness, but not necessarily perceived as distortion, even though it is.

    'So what' you might be thinking. It's still a non-linear amp with low power. It doesn't make rational sense to even try, over conventional gear. Is it a good amplifier? Technically, no. It's a low power, signal-mangling nightmare. But it also can defy rational expectations.

    Perceptual masking allows SETs to get away with more distortion. The overload characteristics of SETs has them constantly embellishing the signal with harmonic content, which, as stated goes from unnoticable to gross clipping over a pretty wide range. Within that range, the subjective impression of the overload characteristics is uncompressed headroom, not distortion. As the reviewer of the Decware amp in that cnet review might have said, it puts the meat on the bones.

    The problem is, if you don't know this stuff before hearing one, their sonic difference is misleading. Counterintuitive doesn't begin to cover it. Compared to solid state, they sound much richer and more well rounded, even if it's harmonic embellishment. Your ears don't recognize it as such. It's mind boggling that such puny power can sound so good to the uninitiated.

    None of this makes SETs good amplifiers, but I think this does explain why they can cast a spell. It's tenderized sound which some find irresistably pleasant. Some folks write it off completely without hearing it. I can respect that, SETs are strictly for the hobbyists. Some folks try it and don't like it's limitations. Some folks try it and say 'This is what I've been seeking all along' and live happily ever after. It's not the magic nonsense the uninformed would lead you to believe, nor is is as bad as some of the more objectivist types would lead you to believe. It's just another way to make noise in your house.

    I could be completely off my rocker with this, so feel free to tear this to shreds.
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  2. ski2xblack Audioholic General

    ski2xblack
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    As an addendum, I have to admit that I'm one who finds their sound irresistable. (But then again, I'm an audioholic and find most gear that recreates music irresistable.)

    Anyone curious about them should know just what their getting into, but I would suggest not ruling them out before actually hearing one first. They really are pretty neat.
  3. TLS Guy Audioholic Overlord

    TLS Guy
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    A good try at explaining away pure lunacy.

    A bad amp is a bad amp period. SETs are bad amps.

    Now an awful lot of speakers, probably the majority espoused by the high end are perfectly dreadful speakers. The fact is these amps roll off their ruff edges.

    I can absolutely guarantee you that if you connected SETs to my rig or any really superior speaker system they would sound awful.

    I will make a categorical statement if a speaker does sound better driven by a SET it is a really awful speaker. Unfortunately the world is awash in really awful speakers even at extravagant prices.

    You can not improve on clipping. This rig here will not clip before you go deaf.

    Solid state amps can be built powerful enough not to clip. I'm sorry but I don't want my sound altered by distortion of any stripe. All distortion is a problem in audio systems and is to be cut to the absolute minimum.

    Tubes hate varying impedance loads and big phase angles between voltage and current. That includes an awful lot of loads.

    This whole thesis is harmful and wastes resources and gets us further away from the "closest approach to the original sound." That is my pursuit always has been and always will be. SETs set us back an awful long way from that goal.
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  4. GranteedEV Audioholic Ninja

    GranteedEV
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    My opinions on SETs is that they're novelties. Enjoy them if you like, but only for the sound they make. Don't treat it as reproduction gear. If the first time you hear a CD, you heard it through a SET, then I don't believe you can truly tell yourself that you've even heard the contents of the CD.

    Nothings wrong with toys though. For example a SET might be fun to put onto the speaker on your home phone lol.

    Kind of like breathing in helium. Sure it sounds cool, but you don't want to people to perceive you as "the guy with the chipmunk voice". It's not you, but it's amusing once in a while, I guess. Until your lungs collapse.

    Likewise, I would never spend my money on this sort of thing unless we're talking like 25 dollars.
    Last edited: Jan 27, 2011
  5. KEW Audioholic Spartan

    KEW
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    That was the impression I have developed trying to understand why they have a following among people who are more than casual about their sound.
    I have always wanted to know what an acoustic instrument which I know well sounds like when played to clipping through it.
    Does it sound like a different instrument?
    Maybe like the same instrument in some type of resonant hall or chamber?
    Or like the instrument is being played through some type of synthesizer?
    KEW,
  6. zumbo Audioholic Spartan

    zumbo
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    I honestly don't understand the negative opinions purpose for even posting. It's not like this is a Bose thread, and misinformed consumers are being informed to throw money away.

    There are people here who are interested in this. If you are not, then simply don't participate.

    It's not like people are recommending tubes to people looking for amps, and making any claims about them in terms of. I believe it's something that some of us just want to talk about.
  7. ski2xblack Audioholic General

    ski2xblack
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    Thanks for stopping by Dr. Mark. I appreciate your input, as always. I was merely trying to strip away some of the confusion about what such devices do and how you can be fooled, so please don't take it the wrong way. If I was factually incorrect, please let me know how so.

    Thanks, I guess. :rolleyes:

    I believe I said that in the original post.

    I would agree with this.

    SETs would NOT drive your speakers, or the vast majority of any others, very well at all. Disregarding the output impedance and other flaws, they simply lack the power for 99% of speakers out there. Toys, remember. Maybe good for a office or bedroom with some fostex full rangers as a curiosity.

    Funny you mention this. The harmonic 'processing' comes through with all speakers, and it can make some old crappy vintage speakers with higher sensitivity sound much more, uh, interesting than they otherwise would. But they would still be really awful speakers.

    Don't ever change, Dr. Mark! You're a rationalist warrior fighting the mediocrity and decay of a postmodern world! We rely on you to keep us young impressionable folks honest.
  8. ski2xblack Audioholic General

    ski2xblack
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    Good question. I tried to answer in the original post about how the distortion is to a large part masked because SET distortion is primarily 2nd and 3rd order, with gradually increasing edge harmonics as the tubes overload producing a false sense of added spl to the listener. It does not sound synthesized; if anything, it is the embodiment of tube smoothness taken to the extreme. It really is a tube harmonic synthesizer in essence, but the sound is quite natural and inoffensive.

    Subjectively, it really doesn's alter the timbre as you would expect. My piano sounds like my piano when reproduced by SETs. Ditto for my guitars. If anything, it's more convincing at low volumes than the rendition of my ss systems, where the tubey harmonics kind of act like a loudness control. Thats the rub. Read the cnet article carefully, and this is pretty much what the author is saying, more or less. The distortion results in a dynamic punch at low levels which the author found pleasing. It's highly distorted but doesn't really sound like it.

    It's confounding, and anyone seduced by the sound will quickly run into the severe shortcomings that are part of the SET package (e.g. no listening at high volumes with such a device, unless you're talking horn loaded compression drivers). For some, the limitations are not as much of a concern.

    SETs do seem to enhance the recorded space in such a way. It's the 'holographic imaging' thing as some may describe, and it is for real. (One of the few of SET's redeeming charms, and it's a good one. Still artificial strictly speaking, but many find it very pleasant.)
    Last edited: Jan 27, 2011
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  9. GranteedEV Audioholic Ninja

    GranteedEV
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    Definitely! The types of music I wouldn't be bothered to listen to probably sound better distorted!

    You know you walked right into that.
  10. ski2xblack Audioholic General

    ski2xblack
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    Dr. Mark, if you would indulge me, I would like your thoughts on a specific area of SET mythology.

    Many SET folks claim that the fact that they are inherently class A and thus have zero crossover distortion is partially to account for their unusual qualities. I'm not sold on this, but on the face of it seems slightly reasonable. As one who prefers Quads and has touted their class A output, what are your thoughts? Anything to that, or more audiophool mumbo jumbo?

    (Please try to resist the urge to categorically dismiss the question; I am genuinely curious, as you are one of the few who actually knows what he's talking about and can back it up with facts.)
  11. TLS Guy Audioholic Overlord

    TLS Guy
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    Since SETS only have a single output device it has to carry both the positive and negative parts of the wave form. Since there is no handover between devices there can not be crossover distortion. Crossover distortion occurs at the zero crossing in push pull amps.

    Class A is a different issue and has to do with biasing. In class A anode to cathode current is constant and independent of power output.

    In pure class B current follows output and quiescent current is low. However since the output devices are switching on and off there is non linearity at the zero crossing.

    Since tubes are high impedance devices, biasing class A has an insignificant effect on power consumption and device life.

    In solid state amps the situation is different. Pure class A biasing leads to high quiescent current and therefore continuous heating of the output devices. This requires big power supplies and lots of output devices in parallel. Even then failure rate over time is high and the cost of the amps is high.

    So class AB biasing is usually used where the devices are never allowed to switch off and quiescent current is increased over class B. With skillful design crossover distortion can be brought down to low levels. However you can't get greedy and bias excessively towards class B operation.

    Now crossover distortion increases as volume is turned down, which is a big problem. It gets very much hidden in conventional amplifier tests.

    I would like to have a chance to get receivers on test. I only encounter them when visiting friends. I had most experience with a Pioneer Elite on a visit of about a week. Speakers were Paradigm, and that was probably some of the problem. However I thought the receiver was contributing a nasty grainy sound, and I was convinced it sounded worse as the volume was turned down. My hunch is that crossover distortion is under recognized in these cheaper receivers, but I can't be sure.

    Peter Walker solved the problem by a cunning feed forward design, using a very small but high quality class A amp to correct the AB output stage. He showed mathematically that the performance was true class A. I can confirm personally that this approach is free from all trace of crossover distortion.

    I hope that answers your question.
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  12. GranteedEV Audioholic Ninja

    GranteedEV
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    Interesting you should say that, because I read this one time with regards to crossover distortion:

  13. ski2xblack Audioholic General

    ski2xblack
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    Yes, that is exactly the type of response I was looking for. Thanks. (And as soon as one of my solid state amps dies, the amps you have recommended are at the top of my list.)

    So being class A is an illogical argument in favor of SETs, since most conventional amps address it adequately, without the compromises. Some less expensive conventional gear may overlook it's importance based on your post. If that's an accurate assessment, I think we agree on this.

    Having no crossover distortion certainly can't hurt, particularly at the low volumes SET amps are actually able to attain without gross clipping. I suspect the complete lack of crossover distortion is responsible for their reputed 'purity' (but it still comes with the harmonic processing aspect of SETs, which again, may or may not be discernable as distortion per se). Based on the half dozen or so SETs I've heard, the apparent transparency and purity, particularly in the midrange, is a common feature. It's really about the only thing they're good at, but it does go a long way. Between that and the harmonic coloration, I think a lot of the suspicious audiophool hyperbole is the direct result ('air', imaging prowess and such). Trickery or not, SETs garner such descriptions because they actually do accomplish that one task very well.

    Some of what I've heard did in fact sound like crap, and a few were undeniably impressive. SET sound is not one unique thing, but a continuum of different recipes by their respective designers, some more syrupy and embellished, some more sterile and dry.
  14. mtrycrafts Audioholic Slumlord

    mtrycrafts
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    One has to be careful how much helium they breath. It can kill.:D
  15. mtrycrafts Audioholic Slumlord

    mtrycrafts
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    Well, Bose is also something that someone might be interested here, like a lurker?
    Some do like to make claims about SETs though. ;):D no, I don't have a reference to one;)
  16. mtrycrafts Audioholic Slumlord

    mtrycrafts
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    I am lost to understand what he means by that. Unamped, natural music is not distorted at any level, period. Maybe he is confusing the harmonics of the natural music with harmonic distortion?
  17. highfigh Audioholic Warlord

    highfigh
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    Out of curiosity, which tubes did you use when you were working with SET amps? Were they lower output, or something like EL34 or KT66/KT77/KT88?
  18. TLS Guy Audioholic Overlord

    TLS Guy
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    I was about 7 or 8 years old when I built it. I was pretty sure it was an EL 84 and it was when I checked. It was a version of this 3 watt amp.

    [​IMG]

    It was a prettier version than that with a dark mauve and gold perspex panel. It was a version offered by Sterns radio as one of their Mullard kits. A few years later I progressed to building the stereo Mullard 10/10 (10 watts per channel push pull), with Gilson transformers. It is still in my possession and I should get around to restoring it.
  19. ski2xblack Audioholic General

    ski2xblack
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    Mtry, SETs may be an ***-backwards way to make music, but I hardly think those who inquire about Bose are the same ones who inquire about esoteric tube amps.

    If there are any lurkers, I hope they take a few things away from this. SETs are the definition of 'bad amps'. They don't really amplify that much, and they add tons of distortion. They're toys, not real hi-fi equipment.

    Despite that, they do sound far better than you would reasonably expect. My post, a continuation of the other thread, was just an attempt to explain how the trickery works. While simply toys, they're quite fun for the more relaxed, lighthearted types.

    It's all academic until you hear one for yourself, which I would encourage the curious to do. Only then will you know if it warrants a 'meh' or a 'holy cow that's amazing' response.
  20. AcuDefTechGuy Seriously, I have no life.

    AcuDefTechGuy
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    Two Wrongs make a Right, right?:D

    Bad Amps + Bad Speakers = Good Sound?:eek::D

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