Baking pan tube amp sounds better than you’d expect

baking_pan_tube_amp

A few years back, [Gio] decided to try his hand at building a couple of tube amplifiers.

The first amp was more of an experiment to see how well a DIY single-ended tube amp would sound. The amp is based off the 6T9 design created by Spare Time Gizmos, and incorporates a pair of 6T9 vacuum tubes, hence the name.  He wired things up in an afternoon, then got busy drilling holes in a baking pan, where he mounted the amp. Bear with us for a second, it’s not as bad as it sounds. The amp actually looks pretty good mounted in the dark black steel, and this sort of enclosure is far cheaper than most DIY amp enclosures. He says that he was sure to be extra careful in isolating all of the electronic components from the metal chassis.

The second amp was built to test the performance differences between Pentode-mode and Ultra-Linear mode configurations. While both amps share a substantial amount of the same components, his UL amp benefits from slightly better capacitors and an uprated power supply, not to mention a more conventional case.

Both amps sound great, according to [Gio], but should be paired with efficient speakers for the best experience. He does note that the ultra-linear amp is the better choice, mounting options aside.

[Thanks, Philippe]

[via RetroThing]

Comments

  1. Pedro says:

    Metal enclosures should always be connected to earth.

    He should have mounted the whole thing on a breadboard. The kitchen kind…

    It always shocks me how expensive amplifiers are, even DIY ones.

  2. hospadar says:

    Apmlifiers aren’t expensive! I just used a $12 Class-D amp to build a boom box inside a pvc pipe. AND IT SOUNDS DOPE

  3. Bill says:

    Baking pans and kitchen breadboards are classic tube era DIY.

  4. NatureTM says:

    I think the baking pan looks great. Nice idea.

    I have a question about tubes in general:
    It’s my understanding that tubes actually have less accurate sound reproduction compared to cheaper, modern silicon alternatives. Unless you’re listening to something that was produced on tubes, you’ll get more “correct” audio with silicon. Audiophiles just like the “warm” colored distortion tubes produce.

    Is this true? I would expect audiophiles to try and maximize fidelity.

  5. StacyD says:

    Its because high end audio demands really high quality parts. Tube designs need HV tansformers with dual voltages and parts that can handle the voltage. If making a smaller and low power amp, you can generate the HV using a timer chip and clever circuitry.

    The point is that most stuff now have gone to amp-on-a-chip systems which has revolutionized portability. However, most only give at best a class AB performance which is plenty for 95% of the speakers out there. Some of us are eco-hating Class-A people willing to make massive heat-sinks and overbuilt power supplies for the sake of not loosing a tiny little bit.

    It is a nice little amp that would be suitable for smaller speakers. I have a little push-pull amp that does 9W of the old kind of watts (uses 12au7 for phase splitting and independent 12au7 triodes per channel and pentrode power tubes for each channel, cool old piece of kit!). Honestly, building from scratch is too expensive for some of these. you are better to buy busted old ones and then either fix them up or harvest for parts. I know that the 12au7 tubes I use run $40 each NOS and when you have three of those and then two pentrodes, it is nutty. The Chinese tubes are getting to be extraordinary from some makers, but it is very hard to replicate an old bugle boy or GE or electro-harmonix tube in character. I have special output transformers to use these with headphones and the original ones to drive my desk speakers.

    The Aikido tube kits are probably the best value out there in terms of tube kit ATM. The stuff from china can be either really good or really not. They tend to be built around “diagram” rather than a real world “circuit” (usually you find stuff oscillating and ground loops, etc.).

    This is a simple kit and easy enough to do PTP wiring which is great for experimenting with tubes. You will soon find that tubes do not have rules, more like guidelines!

  6. Max says:

    @hospadar, That’s a real “Ghetto Blaster”

  7. Sanjay says:

    Steel is a very bad choice for tube audio work – the metal is magnetic and spreads hum from the transformers to the rest of the circuit. It also causes unwanted coupling between the transformers.

    I didn’t know about this and it made my life hell when I built my first amp. Luckily an old-timer put me on the right track just when I was about to throw away the tubes!

    Always use non-magnetic materials for this kind of work.

  8. AnarKIT says:

    NatureTM

    Tubes tend to introduce 2nd harmonic distortion into the signals, especially when they’re clipping. That harmonic is where the warm sound comes from.

    Transistorized amplifiers are much easier to get very accurate, however when they distort, it’s harsh 3rd harmonic distortion that sounds good to no one.

    There are other factors, but it would be correct to say that semiconductor devices can be made to be more accurate, especially over time.

    This is a very pretty project, I love cases made out of items that wouldn’t normally be used to house electronics.

  9. Aussitech says:

    @Sanjay – “Steel is a very bad choice for tube audio work – the metal is magnetic and spreads hum from the transformers to the rest of the circuit.”

    And yet another audio myth is born.

    See also;

    http://www.ozvalveamps.org/ava100/ava101lamington.htm

  10. Miroslav says:

    @NatureTM
    In regards to “accuracy” (or fidelity) between tubes and solid state question, an ideal amp should simply do “linear” transformation of the input signal (as in Vout = const * Vinput).

    With transistors (plus some magic called feedback), it’s easier to get closer to above. In other words, when you take a look and transistor curves (wow, that was some scientific lingo there), for each delta (increase) in the input voltage, output gets changes in equal proportions along the input range (deltaVo = const*deltaVin).

    With tubes, there are significant (?) components that are proportional to the square of the input signal. This generates the second harmonic (double the frequency), but also some other components that corespond to sums and differencies of all frequencies in the signal.

    If you search HaD for fairly recent post with 555 as a radio transmiter, you’ll find comemnts that describe this effect a bit more.

    In other words, tubes add more music then you have paid for. In some cases that actually sounds good, but can be called an accurate reproduction.

    There is more, some sware that the pure principle behind tubes (electrons squizing between metal plates (?), other electrical fields ecting on electrons flying through the tube, unwanted modulation due to the heating filament’s characteristic), but I would not account that for much of what you get (at least in higher class amps).

    All that “fidelity” problems aside, I still miss a good vinil album when listening to CD :)

  11. Miroslav says:

    @StacyD

    Class A vs. AB vs. B vs. C is a story of ugly byproducts when you leave the active state of your (mostly output) transistors. To some extent, as far as you go from always on (class A), to mostly closed (C), distortion (non linear) ramps up while efficiency goes up.

    But, as in some magic, when you get to Class D (only on or off, never in a linear state), you can actualy ripe both benefits – close to ideal linear transfer curve + no losses (well, close to it).

    It comes with some design problems, but nothing that can’t be resolved fairly easy.

    In this class, the main problem becomes (kind of) A/D conversion – how to transfer teh analog input signal to stream of on and off’s at your end state, but some smart people have packed all that in a single chip, so you can find 4 channel, 100+ wats, ultra low THD (distortion that you care off) in a chip size of a dime.

    And you just need a small heat sink, nice rectifier, and solid speakers to rock the house :)

  12. Nikita says:

    What is the rated power of these amps in watts?

  13. jimmy says:

    Err, why would I expect it to sound bad?

    The baking pan idea isn’t new. It’s at least as effective as some commercial chassis.

    FYI, some designers claim to be able to hear the difference between steel and aluminum chassis. MAYBE there’s some subtle nuance, but I doubt it. Steel was the standard for many years; it works fine…

  14. lwatcdr says:

    @NatureTM Audiophiles are insane. It is as simple as that. They buy CD demagnetizers and speaker cables made from virgin gold that is only mined at the equator so that is has the least paramagnetic distorsion. If you think monster cables are expensive you should take a look at what audiophiles pay for super hi fi digital cables.
    However this project is actually very cool. Looks great and is well made. Kind of like someone building a prefect replica of a WACO biplane.
    Oh and as to the quality of the sound? I bet it sounds great. Of course that will depend a lot on your source and your speakers.

  15. NatureTM says:

    Thanks for the responses!

    Part of me still wants to ask, if these harmonics sound better, shouldn’t the producers just run their music through tube modeling software or actual tubes during production? I have a feeling that will boil down to a matter of taste or opinion, however. I guess if I really want to know what’s up, I’ll need to listen to a side-by-side comparison of tubes vs transistors someday.

    @lwatcdr specifically on hi-fi digital cables:
    I hate when stores like Best Buy try to sell people really really expensive digital cables. Are the 0’s and 1’s really going to show up any different on the decoder? It’s really taking advantage of people’s ignorance.

  16. StacyD says:

    I will be the first to say that some audiophiles really are so serious it can kill. Most do not have any sort of electrical skill or math/science. The point is audiophile stuff is about art more than science. You want the distortion low, high fidelity/detail, not loss of certain ranges, but that is the easy stuff. The art of matching equipment (and impedances), presentation, emphasis, color, tone and all the other junk is the hard part. The 32-bit audio DAC’s are a bit of a scam though and it is the latest thing raging through the audio crowd.

    Truth is that there are some things lost in 16-bit encoding (detail and sound wise that the human ear can hear and feel). 24 bit covers all of those bases and then some, beyond what we can hear or feel. Plus, when you start talking about top chips from Analog Devices, Cirrus Logic, and Wolfson there really isn’t much to loose from the DAC. You do need to worry about getting good clocks and timings more than X-number of bits. It does not matter how much resolution you have if the bits are flipped or just wrong because of your crazy baud rate and cheap clocks/crystals(big culprit of many USB to I2S converters).

    I know I am nuts, but at least I know it and do not take myself seriously. If it sounds better, than it is better. They try to make science out of art and there is only so much of that which can be done!

  17. Space says:

    @NatureTM Tube audio is.. specific. An constructor with limited skill might crate an amplifier with excellent quality of sound using minimal number of parts. That makes it very attractive for DIY.

    Tube audio amplifiers, especially single ended A class amplifiers sound much better when used at low power than any semiconductor based technology regardless of its output stage operating class. That is mostly because single ended A class tube amps like this one have no crossover region distortion. Lack of crossover distortion makes sound rich in details even when listened on low power.

    Yes, audio producers like tube preamplifiers, mostly because their low noise at lower frequencies, high input impedance, excellent linearity and headroom.

    The rest of it: wooden knobs, gold plated connectors, CD demagnetizers, non-metallic wires, Wookiee fur enchanted bass traps are part of audiophile folklore.

  18. Miroslav says:

    @Space,

    Sound quality and a “sound better” should not confused.

    “Tube audio” might be more pleasing, but if you compare apples for apples (like in vacuum apples with silicon apples), especially if you compare the same class of devices (as in high end vs high end, or low vs. low), tubes do not have any technical advantages.

    Let’s compare what you have mentioned:

    – minimal part numbers:
    * silicon based for sure have simpler (and less dangerous power supply). And cheaper.
    * (some) integrated audio amps really don’t require almost any components. And even include all kinds of protection. Tube amps don’t
    * the only advantage with tubes is the coolness factor + no need for PCB :)

    – cross over distortions
    * either silicon or tube class A is class A – work point is in the middle of the linear region, so now crossover distortions on either.
    * what makes silicon “more accurate” is that transfer curve for transistor is more linear then the equivalent one for tube. And that non’linearity with tubes actually makes them “sound better” – they “color” the sound.

    – “rich in details even when listened on low power” – I can only guess that what you meant here is the old fashioned effect of “loudness button” – intentional distortion of the transfer curve that compensates for the nonlinear characteristic of human ear at low volumes. Tube possibly does something similar (like enhances low frequency range more) that sounds “richer”. Or not. But anyway, decibel for decibel, Hertz for Hertz, especially at low volumes, silicon is more linear (accurate) then tube.

    -“low noise at lower frequencies” is also not an accurate statement. Dollar for dollar, tube is noisier then transistor.

    -“higher input impedance” is also a myth. If you really desire high impedance, go FET or MOSFET. But the driving principle is to MATCH impedance of your source and your preamp. Most sources are not high impedance devices, even the ones that are can be easily matched with silicon based components

    -“excellent linearity” – easier/cheaper with silicon

    -the “headroom” – here I have to admit, overloading the tube produces more pleasant results. Even though I would not call that a head room, but once you hit the silicon’s boundaries, resulting clipping creates abundance of 3rd harmonics that sound “ugly” and “cold”. In such cases, tube produces much more 2nd and 4th order components that give that nice, warm, and fuzzy feeling. But on the other side, I do NOT want my amp to go in overdrive ever :)

    The last one is why actually rock music owns thanks to tubes. When you hit that guitar hard, you don’t want your sound to sound harsh, and that’s where silicon fails miserably.

    And that’s why audio studios (engineers) prefer tube preamps – they can handle wider range without killing the mood. But if you can keep all your inputs under control (as in you know what to expect so you bring appropriate tools of the trade), transistor rules (as in keeps the original signal accurately preserved)

  19. DarkFader says:

    those ferrite beads on the cable actually do something?

  20. Ken says:

    Space is plainly wrong in asserting that great class A tube amps sound better than any transistor amp. Bob Carver proved this in the 1980s in a series of challenges to major audiophile publications. The audiophile press was forced to admit that their favorite tubes amps were indistinguishable from the solid state models carver designed (and matched to those tube amps) in blind listening sessions.

    Carver even sold an amp which sounded just like the highly regarded (and very expensive) Mark Levinson ML-2. Carver’s amp offered ten times the power at one tenth the cost, or thereabouts.

    That doesn’t mean *your* transistor amp will sound just like a tube amp. Only that transistors can definitely get the job done.

    There is one obvious example of where tubes really shine, and indeed are used in recordings to get that fat warm sound: Electric guitar amplifiers. Overdriven tube amps with saggy power supplies and nonlinear speakers ROCK!

  21. lwatcdr says:

    @NatureTM BestBuy? Monster? Hah they are value brands compared to what some audiophiles pay Try http://www.musicdirect.com/category/43
    130 dollars for an HDMI cable. I am sure I could find worse if I spent the time.

  22. Aussitech says:

    @DarkFader – “those ferrite beads on the cable actually do something?”

    They are intended to prevent RF pickup from getting into the preamp and being rectified/detected, ‘tho they are more normally applied to the signal live within the chassis. (not Kozmo-Fi but real audio engineering)

  23. Garbz says:

    @lwatcdr You didn’t try very hard either clearly, get “Spread Spectrum Technology” in your cabling here: http://www.noiseaddicts.com/2008/11/most-expensive-speaker-cable-world-audioquest-audiophile/ $21000 for 3m

  24. Space says:

    @Ken Hi-Fi died in mid 80s. Low THD designs were replaced by models advertised in audiophile press as better. Most of them were cheap units using single hybrid chip as output stage. Some of those models can be found in working conditions even today. Sound of such units is nothing to be proud of.

    @Miroslav
    I’d like you to compare any silicon apple with no negative feedback with any vacuum apple with no negative feedback.

    – minimal part numbers:
    * power supply units are almost the same.
    * unlike wimpy silicon tech vacuum tubes are resilient: they need fuse, nothing more.
    * (many) vacuum tube power amps have less parts
    * chassis is necessity: tubes on pcb pick up too much vibrations (ask around for Peavey Classic 30)

    – cross over distortions:
    * Single ended A class vacuum tube power amplifiers HAVE NO CROSSOVER DISTORTION. THD levels given for such amplifiers are at single % digits levels with no feedback loop at full output power (worst case). That speaks MUCH about linearity of vacuum tubes.
    * “loudness button”? On tube amps? louder == better? rly? All known constructions with semiconductors have crappy THD levels at low volume because of crossover distortion.

    – ”low noise at lower frequencies”
    my bad. I meant low I/F corner. Better?

    – ”higher input impedance”
    * FET = useful
    * MOSFET = overrated religious nonsense in audio. Input impedance of MOSFET: too much input capacitance, thousands times more than any vacuum tube.
    * impedance matching – useful but usually impedance of sink must be 5 to 10 times greater than of the source. <- disclaimer: in audio only.

    – ”excellent linearity”
    * meant superior compared to semiconductor systems with no negative feedback loop.

    – "here I have to admit, overloading the tube produces more pleasant results"
    * because tubes have no saturation and related recovery effects unlike semiconductors. Irrelevant to headroom.

    – " ..and that’s where silicon fails miserably."
    * not all silicon. Transistor based guitar amps are usually made to be cheap, not to sound good.

  25. Miroslav says:

    @Space said:

    “…silicon apple with no negative feedback with any vacuum apple with no negative feedback.”

    But why would I compare two technologies at the lowest level possible? I never said that single transistor beats pentode. I said that dollar for dollar, silicon beats vacuum :)

    “power supply units are almost the same”

    No they are not. Tubes require higher voltage, often dangerous. Watt for watt (output), tubes often require more juice.

    “unlike wimpy silicon tech vacuum tubes are resilient: they need fuse, nothing more”

    Unlike wimpy tubes, not so modern audio amp ICs have internal protection and do NOT require fuse :)

    “chassis is necessity: tubes on pcb pick up too much vibrations (ask around for Peavey Classic 30)”

    With silicon, no need for chassis. You can even skip the PCB if one insists.

    “Single ended A class vacuum tube power amplifiers HAVE NO CROSSOVER DISTORTION”

    And I never said that they do. But that’s the same with transistors in A class. I only said that tubes are less linear then transistors. Even without the feedback.

    “THD levels given for such amplifiers are at single % digits levels with no feedback loop at full output power (worst case). That speaks MUCH about linearity of vacuum tubes.”

    Do you really call a single digit THD a success? A dollar worth audio ICs can easily beat that. I would not them drive at full power, but would spend another buck, buy a bigger one, run it at the half power. And all that without the fuse that tube requires :)

    ““loudness button”? On tube amps?”

    I misspoke – what I wanted to say is that tube possibly inserts distortion that has effect like “loudness” – like in “due to imperfection of a human ear, “an ideal” low volume reproductions sound worst then mid/hi volume, but if you “tweak” the original, low can sound better”

    “louder == better? rly?”

    Yes. Ear has nonlinear frequency curve at different volumes. Actual, the curve is different at different volumes.

    “All known constructions with semiconductors have crappy THD levels at low volume because of crossover distortion.”

    And then you’ve got your facts confused. Cross over distortion happens ONLY when you leave the linear region, and that would be the same if you drive a tubes in class B. If you run your transistor in class A, and your tube in class A, neither one would have cross over distortion.

    Then again, I have never compared transistor and tube, but if you insist, even though either class A is free of cross over byproducts, tubes (especially triodes) suffer from nonlinear distortion that causes THD (non linear means that the transfer curve has products equal to ^2 (power 2), ^3, etc, that cause not only harmonics, but other byproducts that are + and – of involved frequencies).

    And that’s what actually makes tube amp sound better – it’s “more colorful”, and “deeper”, and “spatial” (whatever you might consider that being), but it’s simply distortion.

    Good silicon based are “clean”, “sterile”, “hollow” – all to the extent that the original was having those same attributes (as in fidelity in reproduction).

    “I meant low I/F corner. Better?”

    Sorry, not sure what that might be.

    “MOSFET = overrated religious nonsense in audio.”

    Up to now, our discussion was trying to use technical merits. You are now going where I don’t want to go. Some will say the same for tube followers, I don;t think that’s a valid statement in a technical discussion. Put some facts out, and I’ll comment. Don’t have any facts to say if tubes are more or less “overrated religious nonsense” then MOSFET, can only take your word for it.

    “too much input capacitance, thousands times more than any vacuum tube.”

    Great! Back on technical merits. Just for fun, I picked the firs “small signal MOSFET” that google has offered. BS170. Input capacitance at 1MHz is 60pF. Even (much) less at lower frequencies. Hope you can provide the same for your tube of choice, but regardless, even if you find one that’s 60fF (femto, 1000 times less then pico), 60pF so small in audio circuits that it does not matter at all. And if it does, just for fun, I’d switch to some of HF MOSFETs (BF981 comes to mind) with under 3pF.

    “useful but usually impedance of sink must be 5 to 10 times greater than of the source. <- disclaimer: in audio only."

    Agree, some very special cases come to mind, but you are right. Still, any decent FET, MOSFET transistor or el chipo OpAmp, can match any source that comes to mind.

    "- ”excellent linearity”
    * meant superior compared to semiconductor systems with no negative feedback loop."

    And tubes are even better when you compare them with germanium transistor. Not to say diodes (?!?). As I said, I have never said transistor is better then penthode. And I have never said it's not. Just have never compared them in that way.

    "- "here I have to admit, overloading the tube produces more pleasant results"
    * because tubes have no saturation and related recovery effects unlike semiconductors. Irrelevant to headroom."

    Overloading means putting more load then supposed to. Or running component out of their linear range. If you say that tubes can not be run out of the linear range, we can readdress this one, as I strongly disagree (just for a hack, how do you call trying to get 100W out of tube amp designed to do 1W, and not getting more then 1.5W or so?).

    Also, the "saturation" – if what you have meant is characteristic that you find in transistor datasheets (Ic < hFE * Ib range, where Uce gets stuck at 0.2V or so), you have the same with tubes. If all that you wanted to say is that tube recovers faster, I have to ask, why in good mind would you ever need to run component in saturation. Not happy with the 5W amp? Why didn't you get 10W one? Like to run your amp to the max, cool, but that's what I'm saying – distorted signal leaving the tube amp is more pleasant then distorted leaving silicon, but still distorted.

    Now, let's just imagine that this is used at all steps – artist records his voice under the above conditions (and it sounds great, even better with guitar, I've been on many rock concerts). Then you take that to studio, run through another great tube amp run at it's max and over, put on the vinyl (good old times), then you take it home and run through another similar setup. Do you think you would even guess who's singing?

    New repeat the same with "oversized" (never running over designed limits) solid state amps. I bet you'll be able to say who's singing if you know the artist :)

    "- " ..and that’s where silicon fails miserably."
    * not all silicon. Transistor based guitar amps are usually made to be cheap, not to sound good."

    We just stepped into bizzaro world – you sing praises to silicon, and I oppose :)

    By definition (by design), silicon transistors have an ugly habit to over produce 3rd harmonic, and if one does not intentionally adds some "measures" to artificially add more of 2nd harmonics, regardless of how cheap of expensive you go, it sounds bad.

    So, to add to my comparison criteria, in addition to not limiting myself to a single transistor, in all above I assumed that we are using semi conductor based audio amplifier, designed with linearity in mind, so that excludes all "good sounding, cheap, guitar amps". By the way, those usually rely on well matched speakers, that are no good for anything else.

    Would be glad to continue this nice discussion, but I still think that dollar for dollar, pound for pound, W for W, vacuum tube based amp cant beat solid state based one if fidelity (accuracy) is criteria.

  26. Miroslav says:

    @HaD

    Is there a contest or prize for the longest reply?

    [Space] and me might win it with a few more iterations :)

  27. Ken says:

    Space: Check out this reprint about Carver’s challenge [http://www.stereophile.com/content/carver-challenge]. You might learn something. Wikipedia claims the reference amp was a Conrad-Johnson Premier 5, which is about as far from the kind of “single hybrid chip output” equipment you referred to as it gets.

  28. Miroslav says:

    @Ken
    That was a good reading, can’t say a real eye opener as I used to tweek some of parameters to achieve desired effects.
    Have to admit, that was SO MUCH less scientific and technical, that I should not be comparing with what Carver did at all :)

    Extent was to make solid state guitar amp sound more like tube ones. And that was before I “disovered” the secret recipee of effect that otherwise speaker can do :)

    So, to me, it was more like adding sound efects to otherwise nice transfer function, but to my “customers” it was making it “sound real” :)

  29. Space says:

    @Ken 700$ is big pile of money for an amplifier, even bigger if the factory is purchasing parts. It is contest designed to fit the needs, not to prove anything. I would like mr Bob to make another challenge, this time with the speakers, and to try to fit in the same pile of money.

  30. Miroslav says:

    @Ken
    The test was performed almost 30 years ago. $700 was even more then today, but also, electronic components were more expensive too. And this was not a mass market amp made somewhere in China.

    On the other side, comparison was with another amp that was probably 15 times more expensive (they are 5-15k today, I can only guess they were pricey then too).

    And the contest actually was intended to prove that that amp can be retrofitted to sound identical to the expensive one (in other words, to prove that medium priced solid state can sound like high priced tube amp), so not the need, but the bragging rights.

    Knowing some sworn audiophiles (read deep pocket audio enthusiasts), I’m sure that they would never buy equally sounding “cheep” amp, when there are 10kUS$ “equivalents” available. A simple justification, why would they have an amp cheaper then cables they use to connect it to speakers :)

    In regards to comparison of speakers, probably some other HaD’s article, this discussion was solid state vs. vaccum :)

  31. Space says:

    @Miroslav: I see intention to ward away any technical explanation or reference.

    Related to my post on high input impedances: BS170, an switching low power MOSFET has input capacitance like some kilowatt output RF tetrode.
    BF981 is in 3pF range, like so many 2-5W triodes. However, BF981 is mixing dual gate MOSFET, so it should be compared to mixing heptodes. Mixing vacuum tubes have input capacitances usually less than 0.1pF. So I guess there is no really high input impedance with silicon unless tricks are used. Same tricks can be used with tubes.

    – “Do you really call a single digit THD a success?”

    I have mentioned amplifiers with no negative feedback loop in order to inform you on controversial method of amplifier construction often used in tube amplifiers. It yields THD in single digits. This shows how linear vacuum tubes are.
    All amplifiers made with transistors (BJT, FET, MOSFET) are using negative feedback loops to improve linearity and extend frequency response because they have to as they all are extremely nonlinear. Without negative feedback loop the distortion levels in silicone (and germanium) amplifiers would be hard to tolerate.

    About A class operation and crossover distortion. A class allows both parts of push-pull configuration to work all the time. This in turn makes crossover transition (push to pull and reverse) smoother. Smoother transition makes less choppy negative feedback correction signal. The result is smooth correction signal and less (odd) harmonic distortion in signal.
    Sometimes all the benefits of A class output stages are negated by cheap power supply unit and intermodulation distortion caused by ripple noise.
    The worst of all is the new holy grail in audio the D class. I’m not saying D class is bad, it’s the ripple noise form chopper PSUs. The combination of D class and chopper PSUs makes more intermodulation noise than I can tolerate.

    low I/F corner – I wanted to be technical. It is specific value that represents noise current input in amplifiers at lower end of frequency response. Vacuum tubes have the lowest I/F corner noise. FET transistors are near, bipolar junction transistors are following, and the MOSFET transistors are at the last place.

    I’ll say one more time: saturation is unrelated to headroom. Tube preamps have insane headroom because they use high supply voltage. In a recording studio that might be an advantage. Tubes in preamplifier might have acceptable THD even without negative feedback loop.

    The bizzaro world..
    The “warm” sound of tube preamplifiers is caused by intentional equalization in middle and low part of audio spectrum. It is not something that tubes do by design. Germanium transistors sound even warmer, yet noone uses them. That might be because the audiophiles don’t know that there are germanium transistors like AF139 with *gold* plated leads :)

  32. Miroslav says:

    @Space,
    I admit, I was just being lazy and picked up the first MOSFET that popped up. But I really don’t think it’s relevant (as I said, you pick ANY small signal MOSFET you want, and ANY tube you wish, show that tube has 1000 times smaller input capacitance, and then show me why do you think MOSFET’s capacitance is too big for ANY audio application.)

    Just a side note, MOSFETs usually go to the output, FETs are on the input side, but that’s another discussion, here we are just comparing capacitance (don;t even remember why anymore).

    You are back to bare bones, no tricks (negative feed back, etc.) amps. Why would anyone take that as a design principle avoids me.

    Back to class A. By the (very old) definition, class A is not a push pull. AB (or let’s say B) uses push pull. Class A is when you put your “no signal” at the middle of the linear curve of a single component.

    And as in “cheap PSU”, let’s limit the scope to the same type/quality of PSU.

    And I agree that chopper PSU usually don’t jive well in audio applications. I’ve seen some designs where chopper acts as a “pre-stage” for old fashioned linear stabilizer that takes care of HF ripple.

    Now I see, I/F is actually 1/F corner. Why would I care if tube has it better (lower) if silicon gets it sufficiently low. Do you have any numbers to compare, or to show that silicon has it unacceptable high?

    Have I ever said saturation = headroom? For me, simply said, headroom is how more watts can you get after you hit the designed limit. And I just over design the amp.

    You said “use high supply voltage”, but then you don’t drive 4 ohm speaker directly, right. And then HV supply does not quite translate to HV delivered to speakers. Would not make much sense.

    And you are back to “no negative loop”, but why would I NOT use it?

    If the goal is to have fidelity (accuracy), then any intentionality (additional components that you can put with solid state), or unintentional (by devices nature) “equalization” equals to distortion (difference between input and output that’s not attributed to linear amplification (for example freq response, phase shift, noise, etc.).

    And if that “equalization” is desirable, I do admit it sound better done by tubes.

    Mentioning germanium, there is a good reason it’s not used anymore, other then when you do want to introduce “equalization” (sic) – I would never put silicon transistor for a good guitar effect, germanium ruleZ.

    So, back to original “scope” of our discussion:

    “..dollar for dollar, pound for pound, W for W, vacuum tube based amp can’t beat solid state based one if fidelity (accuracy) is criteria.”

    For any other criteria, define it, I might agree.

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