A good way to visualize the difference between even and odd-order distortion harmonics is to imagine a sine wave ... a perfect, happy little sine wave. That's the original signal.
* Now, use a single diode to clip one side of it ... say, the positive side. In addition to generating a DC offset, if you run a spectral analysis of it, you'll see a series of harmonics ... 2nd, 4th, 6th, etc. etc. If you look at the "transfer curve" ... a curve mapping the input/ratio at different levels ... you'll see a diagonal line that is perfectly straight (the linear portion) that also has a sharp bend in it at the top, with a flat-topped region beyond the bend. The transfer curve is actually the true distortion mechanism; the spectral analysis of it is an indirect indicator that is (relatively) easy to measure.
* Let's use two diodes to clip the top and bottom sides, both positive and negative. This is known as symmetric clipping. If the flat-topping is at exactly matched levels, there will no DC offset. Similarly, if the clipping is precisely symmetric, the spectral analysis now shows no even-order terms (2nd, 4th, 6th, etc) but only odd-order (3rd, 5th, 7th, etc.). The transfer curve now has TWO kinks in it, at matching plus and minus signal levels. If the levels precisely match, there will be no even-order terms, but odd-order terms are abundant.
The diodes create hard clipping. Vacuum tubes, properly biased, create a softer "knee" region, but rest assured distortion is still there, just not as much, and with less high-order content. The property of symmetric circuits is they cancel transfer curves that are precisely opposite in shape ... a "C" shape that is inverted in the other phase of the circuit. But that depends on symmetry and precise phasing that tracks as levels go up and down.
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@curiousjim
The speakers appear to be 8 ohm 90 dB from a stereophile review. They dip to 3.2 ohms, which isn't bad. The amps would drive them with no trouble in any reasonable sized room. I cannot comment on pricing until Spatial Audio figures out all of their costs. I would expect $15,000 - 20,000 per pair with premium tubes, but that might be off a bit. There will be an announcement after the Seattle show in the spatial audio lab website once it is all figured out.
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Hi, CuriousJim!
I am kind of dubious about any KEF speaker being really 91 dB/meter/watt. That’s almost 1% conversion efficiency, and believe it or not, that’s quite high for the mainstream market, and especially KEF. True efficiencies between 85 and 88 dB are much more common. Efficiencies in that range need 100 to 200 watt amplifiers, which is very large for tube amps. When people say XYZ speaker needs 200 watts to "come alive", they are not joking.
1% efficiency is 92 dB/meter/watt. 0.5% is 89 dB/meter/watt. 0.25% is 86 dB/meter/watt. This raises the question ... where does all that power go, if not making sound?
It heats the voice coil, which eventually radiates its heat to the magnet structure, which radiates its heat into the cabinet. Copper increases its resistance with temperature, which leads to a type of dynamic compression as the voice coil heats and cools. In addition, voice coil heating eventually damages the cylindrical former it is wound on, leading to driver failure over time. That’s why Nomex and other fire-resistant materials are commonly used for VC formers.
This is the charm of high efficiency speakers: for a given SPL, much less power is wasted in heating the voice coil. More is turned into sound, which is the goal.
P.S. I agree, it is somewhat annoying to realize that 99%, or more, of the expensive watts we buy do nothing more than heat a voice coil, but that’s what’s really happening. It’s kind of shocking: hundreds of watts from the AC wall socket end up as milliwatts of acoustic power. The rest ends up as heat, and not in a great place, either.
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@atmasphere
I have to admit that the conclusion about SE+PP leads to emphasis on 5th harmonic is a bit counter-intuitive to me from pure math perspective, but I haven’t read his paper and perhaps there are certain situations that contribute to that conclusion (?).
Many report excellent result using a tube pre with a solid state or push-pull tube power amp though. In many cases, such combination includes some SE stages with at least one PP stage. So, SE+PP is not necessarily bad empirically :-)
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It is difficult to predict real-world distortion from idealized triode or pentode models. The models assume tubes with perfect physical assembly and ideal emission characteristics. In practice, grid windings are not evenly spaced, grids are tilted a little bit, coatings on the cathode are not perfectly uniform, and there is always just a bit of residual contamination. Tubes are not built by robots, but skilled technicians, and as a result, they are all a little different from each other. By looking at spectral distortion measurements, patterns that are unique to each manufacturer emerge, and none conform exactly to the tube model. (The map is not the territory.)
Successive stages multiply distortion terms as more and more kinks end up in the transfer curve. Of course, this applies to the entire transmission chain from microphone to loudspeaker, with everything in-between.
Models are useful for finding bias points and the expected high-frequency response, but predictions of high-order distortion can be way off from the tubes you can actually buy. Low-order terms like 2nd and 3rd harmonic distortion may conform to the model, but I wouldn’t trust it further than that, not with real tubes. Think of the models as first-order approximations.
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The speakers appear to be 8 ohm 90 dB from a stereophile review. They dip to 3.2 ohms, which isn’t bad. The amps would drive them with no trouble in any reasonable sized room. I cannot comment on pricing until Spatial Audio figures out all of their costs. I would expect $15,000 - 20,000 per pair with premium tubes, but that might be off a bit. There will be an announcement after the Seattle show in the spatial audio lab website once it is all figured out.
I don’t know Don’s power supply design, although with Lynn’s collaboration, I’m sure some sort of split supply has been implemented - whether via isolation or regulation. It’s one area Don and I haven’t discussed.
Lynn’s original Karna concept (which I’m employing in my NiWatts) runs a separate separate B+ supply for the output section. We’ve already discussed how the sound of a 300B is the sound of the driver.
With a dedicated power supply to run the input and driver sections (separate power transformers and supply circuitry in the NiWatts - 4 transformers in total per channel), an inefficient speaker may whip the output tube’s power supply into submission, while the input and driver stages are just coasting along.
The perceived effect is that of much more power than the specs would lead you to expect.
Lynn was at a session where I drove a pair of Von Schweikerts with my puny, single-ended NiWatts. Until things got rock concert loud (I put in my musicians’ ear plugs - these guys were nuts), the NiWatts were breezing along.
... Thom @ Galibier
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I was there. Thom’s amps did not use interstage coupling (at the time), so 8 watts is pretty much all she wrote. Frankly, I was kind of surprised ... I knew Thom’s NiWatts were maxed out for all they were worth, but audibility was surprisingly low. No flabby bass. No clipping as such, just really loud sound from an absurdly low-efficiency speaker. No screech, but not dull or muffled, either. A shocking amount of punch and bass slam. We both knew in advance that the combination of no feedback (thus no hard-clipping or saturation) and the split supply would protect the amp from things getting really out of hand.
Thom kind of did it on a dare, trying to get the fuse to blow. We both wanted to hear what the NiWatts, still in rough prototype form, did when given an impossible situation. The split power supply really, really came through. The 300B was getting hammered while the rest of the amp just sailed right through. It acts like an 8-watt compressor with surprisingly subtle action.
It reminded me of a little practice guitar amp, which if you have heard one, can get insanely loud, and retain its basic character.
P.S. Yes, Karna Mk II, or Statements, have independent regulated B+ supplies for input+driver and output section, with massive overdrive capabilities. That’s something the Mark I’s, the Statements, and the NiWatts all share. It makes them sound many times more powerful than the nominal wattage rating would suggest. Surprisingly, the 300B survives this gross abuse with no apparent damage, not something I expected.
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@lynn_olson
Surprisingly, the 300B survives this gross abuse with no apparent damage, not something I expected.
Well as you said earlier,
Bell Laboratory and Western Electric knew what they were doing.
90 years later and it's still very much admired, appreciated and enjoyed by discerning music lovers. Quite the testament.
Charles
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As Lynn said, the amps have dual regulated supplies, one for the 300b and one for the input and driver tubes. IF the specs on that KEF speaker above are really what the review said, then the amp would drive them. Also, as I said far above somewhere, the first stereo prototype of this circuit with only one power supply per channel drove a pair of experimental Spatial Audio Labs X4 prototype speakers that were approximately 87 dB 4 ohm to insane levels, much as what Thom was describing above. The monos have dual supplies in each amp, so would be even more capable. All that said, I would stick to speakers that are 88+ dB and very well behaved. The spatial audio X4 current model is 88 dB 4 ohm (and very well behaved) and the amps will happily drive them to ear splitting levels in reasonable room.
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@donsachs
I would stick to speakers that are 88+ dB and very well behaved. The spatial audio X4 current model is 88 dB 4 ohm (and very well behaved) and the amps will happily drive them to ear splitting levels in reasonable room
As @atmasphere has made the point eloquently on past occasions, easy load impedance (Higher ohms and less fluctuations ) speakers are less stressful on amplifiers. In addition he has pointed out that they are lower in distortion compared with low impedance speakers.
Charles
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You don't have to convince me, my speakers are 97 dB 8 ohm:)
I wouldn't own an inefficient speaker because I would be doomed to SS amps, and I hate the sound of solid state devices.
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@donsachs
I wouldn’t own an inefficient speaker because I would be doomed to SS amps, and I hate the sound of solid state devices
It seems that the latest generation class D/GAN may be addressing this. Time will tell.
Charles
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I hate the sound of solid state devices.
@donsachs There are now class D amplifiers that have a distortion spectra that if you had one on the bench you would be completely convinced you were measuring a really well-behaved triode tube amplifier.
There are three reasons for this:
1) its really easy to get really high Gain Bandwidth Product values, such as 20MHz with class D, so feedback can be supported across the entire audio band (no rising distortion with frequency- like a zero feedback amplifier in this regard)
2) the feedback node can be very linear so the feedback signal does not get distorted prior to doing its job.
3) the things that cause non-linearities in the circuit tend to generate lower ordered harmonics.
Despite inexpensive power being available, I still prefer higher efficiency loudspeakers for the reasons Lynn outlined above. In addition, the harder you make any amplifier work the higher the distortion. For this reason I feel that higher efficiency and higher load impedance is an advantage to all audiophiles.
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@atmasphere
"There are now class D amplifiers that have a distortion spectra that if you had one on the bench you would be completely convinced you were measuring a really well-behaved triode tube amplifier."
Maybe. Except that there are things we cannot measure. I prefer the sound of a good tube to any SS device I have heard to date. Personal preference. I would not be at all surprised if that Class D amp measures better than my tube amp in every regard. I bet in a blindfold test I would choose to listen to the tube amp. Again, personal preference. Other people would make other choices, as it should be.
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donsachs:
When do you think your new 300b mono-blocks will be available for order? I would want to buy them independently of a package with the Spatial Audio speakers as I have a pair of Daedalus Apollos (sensitivity about 96db) that I am super pleased with. I want to upgrade an integrated tube amp and have been patiently looking for the right 300b amp to drive them.
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I have a purifi amp and it couldn’t hold a candle to the original 300b statement prototype. I love new class D as when you get the synergy right I find it to top any SS class AB I’ve heard. But the 300b just makes everything right. I find it odd because it does even the technical attributes better. The blackground is soooooo black, the details precisely rendered, and the soundstage depth of the charts. The only system I’ve heard that could compete was a magico paired with ClassE mono blocks. But even that system while the soundstage was mind melting it was very polished sounding. The 300b statement has much more realistic tone with great texture.
I still snicker 🤭 when bringing a friend over who’s never been a big fan of tubes and putting on Pot by Tool cranking it to 11 and watching his mouth drop. The sheer power and realism the 300b’s presented!
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I’m not sure how the 300b statements could achieve such great technical performance over some of the “state of the art” amplifiers I’ve had in. Maybe Lynn, Don or Ralph could comment on that. It’s put things into perspective that maybe just maybe we don’t know every way to quantify all measurements to what we hear.
I’m convinced that no feedback plays a role as every amp I’ve heard with feedback doesn’t have that super inky back blackground or perfectly rendered transients. Although the amps I’ve heard with no feedback trade that off for less extension on both ends of the spectrum and a lack of power/effortlessness. The 300b statement is the outlier in this regard.
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@cloudsessions1
I have a purifi amp and it couldn’t hold a candle to the original 300b statement prototype. I love new class D as when you get the synergy right I find it to top any SS class AB I’ve heard. But the 300b just makes everything right
That is quite a tribute and observation.
Charles
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Maybe. Except that there are things we cannot measure.
@donsachs That was true back in the 1980s. It really isn't now. Measurement technology has really advanced in the last 40 years!
Its the distortion of any amplifier that is its 'sonic signature'.
There are many class D amps I've heard over the years that I had to struggle to take seriously. But I've heard some now that sound every bit like a very good tube amp; just like in a tube amp where arcane subtleties can make or break a design, the same is true in class D (or any design for that matter). I pointed to what is needed to make a successful amplifier (if you're going to use feedback) in my prior post. Most amps using feedback fall short of the GBP needed so distortion is much higher at higher frequencies than the THD measurement suggests!
THD by itself, if that is the only harmonic distortion metric used, allows a lot of problems to be swept under the carpet. When it is the sole metric, it leads to the myth that 'there are things we cannot measure.' The reality is if the harmonic spectra is measured at several frequencies (including above 1KHz) and if distortion is graphed vs frequency, then we start to be able to predict the 'sound' of the amplifier.
Both the 'measurement only' guys and the subjectivist guys hate this! But Daniel von Recklinghausen was right- if it measures well but sounds bad, you measured the wrong thing.
I’m convinced that no feedback plays a role as every amp I’ve heard with feedback doesn’t have that super inky back blackground or perfectly rendered transients.
@cloudsessions1 That is because in most amps employing feedback, its poorly applied- so my surmise is you've yet to hear one where the feedback was really right. Norman Crowhurst pointed this problem out 65 years ago, describing how the feedback node (the point in the amplifier where the feedback signal is mixed with the incoming signal) isn't linear; therefore the feedback signal gets distorted before it can do its job, thus creating higher ordered harmonics that have given feedback a bad rap. Its not the fault of feedback so much as poor application. Amazingly, little has been done in the last 6 decades to fix this; IMO mostly out of ignorance and a lack of will to do so.
I've heard amps with very high feedback that sound utterly relaxed in the mids and highs and portray depth with ease.
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These amps are still prototypes and being tuned as we speak. So what Cloud wrote was in reference to the first prototype a year ago. Spatial Audio Lab had that amp for maybe 6 months. The mono amps are probably 25-30% better at least. @whitestix has a pair, and Lynn and I are still tuning them and have improved them since that pair went out. What will be shown at the Pacific Audio Fest in Seattle will be very close to the final circuit, but there will probably still be a bit of tweaking after that. I would expect production and sales to be Q4 of this year and maybe Q1 of next. We have to see how it goes. Again, I hope lots of people can hear them in Seattle and give an honest opinion. They will come to market, I promise, but we want them to sound as good as they can first.
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When I was having a Bespoke Valve Input/Output Phonostage Built for me.
The designer builder was running two builds parallel, with the only differences being one had more RCA Inputs and my one had a Single RCA Input.
The Designer Builder invited me to listen once more to the design, but I could not get my head around it, there was still too much perception of weight, which was a a assessment put forward during the demo's done prior to what was supposed to be the final demo' prior to completing.
I told the designer/builder, it was time to introduce Foil Resistors and Alternative Cap's. After some toing and froing, we got there in principle. Z Foils and a selection of Copper Foil Cap's was bought in.
The impact these had on the Sonic was from my assessment something very very special, even the designer /builder was on board and had their eye on this as a Upgrade Package on the Phon's if they were to be produced in increased numbers My later investigations which resulted in the addition of Vintage Tubes selected as a outcome of Tube Rolling experiences, was the addition that is the Cherry Garnish on Top.
The Tuning/Tweaking has the capacity to produce a sonic that is very different in the attraction/likeability generated to a earlier version.
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@pindac Exactly my friend, exactly. Once the power supply is right, all the operating points are right and there is some serious listening, then the tuning begins. We are almost there...
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@atmasphere very interesting Ralph. What are some amps you’ve heard with high feedback that you’ve enjoyed? I’ll have to check them out. I’ve seen the measurements for the Purifi and its distortion is incredibly low, to the point it’s at the limits of what the AP analyzer can measure. It does have raising distortion in the treble but if I had to surmise, it is most likely low order as I find the purifi a touch sweet in the treble. I do run it without an input buffer as my preamp is up to the task of driving the Purifi module directly.
I have yet to hear the new Gan stuff that is specifically designed for audio. I know yourself and AGD specifically designed yours for audio applications unlike many of the other brands. Are you guys going to be at the pacific audio fest? I’d love to come hear your Class D amps and hear how it compares to the 300b statement monos.
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@cloudsessions1
Are you guys going to be at the pacific audio fest? I’d love to come hear your Class D amps and hear how it compares to the 300b statement monos
That would be terrific!! Even if not heard in the same system. Still worthwhile to hear the GAN class D amplifiers and how they compare with the new 300b push-pull amplifiers. My sense is that Don and Lynn’s amplifier is something quite special.
Charles
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Pindac has described the stage Don and I are at now: tuning the subjective balance ... there are a couple of nodes in the circuit where parts selection is quite audible, and we’re fine-tuning that.
I wrote an email to Don a couple of days ago that this topology is uniquely susceptible to parts coloration at the critical nodes. You get the same parts sensitivity in non-feedback SET amplifiers, but the much lower distortion of this circuit, compared to SET, exposes parts coloration more vividly. Fortunately, the right parts are available and are not super-exotic.
The circuit is inherently transparent, so there is almost nothing we can do to take that away, nor would we want to. But subjective tonal balance can be adjusted at the critical nodes. Surprisingly, the tuning has no effect on measurements, since topology, operating points, gain structure, tube loading, and bandwidth all remain the same.
Cloud Sessions asks a good question. Why does it sound this way? The best I can give is:
1) The circuit avoids both local and overall loop feedback, so there are no issues with hard clipping (transient overshoots in the FB network), stability margin (running out of gain and/or phase margin at high frequencies), or sensitivity to load reactance (which decreases phase margin and increases settling time after a large transient).
2) There are no differential stages to current-limit when one tube saturates or clips, taking the other tube along with it ... instead, the paired PP-mode tubes are functionally in parallel, helping each other out when the opposite-phase tube saturates or clips. Avoiding series-mode operation has a big effect on subjective dynamics. No SRPP’s, no split-load inverters, no long-tail pairs, no cathode followers.
3) Last but not least, each stage is individually optimized as much as possible for intrinsic linearity over the audio band. This is a matter of optimizing loads and minimizing the effect of Miller capacitance on the preceding stage.
Interestingly, Class D amps are free of Class AB transition artifacts, so there’s an entire class of coloration that just isn’t there. The big issue for Class D is nanosecond precision of timing for the pulse-width modulation (Class D is pulse-width-modulation, akin to FM, and not PCM), and insensitivity to reactive loads affecting the PWM modulator.
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What are some amps you’ve heard with high feedback that you’ve enjoyed? I’ll have to check them out. I’ve seen the measurements for the Purifi and its distortion is incredibly low, to the point it’s at the limits of what the AP analyzer can measure. It does have raising distortion in the treble but if I had to surmise, it is most likely low order as I find the purifi a touch sweet in the treble. I do run it without an input buffer as my preamp is up to the task of driving the Purifi module directly.
I have yet to hear the new Gan stuff that is specifically designed for audio. I know yourself and AGD specifically designed yours for audio applications unlike many of the other brands. Are you guys going to be at the pacific audio fest? I’d love to come hear your Class D amps and hear how it compares to the 300b statement monos.
@cloudsessions1
Our amp of course, which you have to imagine we’ve compared extensively to our class A triode zero feedback OTLs.
The AGD Audion (although I don’t know how much feedback is employed in this design)
Orchard Audio
Digital Amplifier Company (unfortunately the designer passed away last year)
We won’t be at the Pacific Audio Show- I have a prior booking.
@lynn_olson
Interestingly, Class D amps are free of Class AB transition artifacts, so there’s an entire class of coloration that just isn’t there. The big issue for Class D is nanosecond precision of timing for the pulse-width modulation (Class D is pulse-width-modulation, akin to FM, and not PCM), and insensitivity to reactive loads affecting the PWM modulator.
Yes, crossover distortion artifacts are impossible for any class D using an output filter. Timing issues (which cause an unpleasant hiss) are solved by a self-oscillating topology. This is done by exceeding the phase margin of the amp and the oscillation is used as the switching frequency. This makes for a very high stability amplifier and very large amounts of feedback. Since the feedback is not distorted prior to doing its job, higher ordered harmonic generation is avoided. Essentially the feedback simply reduces the existing distortion (in our circuit this tends to be lower ordered harmonics). Distortion vs frequency winds up being a flat line across the audio band. So it sounds very much like a zero feedback triode amplifier, but more ’focused’ owing to lower distortion, which otherwise obscures detail. You should try it- you make excellent amplifiers and know how they are supposed to sound, so you are in a good position to see how this technology advances the art.
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Amplifiers using these tubes run very hot.
Very undesirable given the need for separate air conditioning which affects the sound Quality. super inconvenient situation
It's like running a portable heater in the room continuously which is horrifying.
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I would think the majority of people who buy class A amps of any sort are quite well aware of how hot they run and are more than happy to deal with it for the sonic benefits. As far as separate AC, that notion is absurd
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@donsachs
+1.
Exceedingly absurd and silly.
Charles
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I live in the desert where it reaches over 115 degrees and run my KT88 tube amp nightly and did the same thing when I had my coke bottle 845 and 300b tube integrated amplifier. No issues with heat in my room 😁
Wig
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If a cool room is important, you want Class D (90% efficient or better) and speakers that are more than 1% efficient (a true Theile/Small 92 dB/meter/watt), as well as switching (not linear) supplies for all the other audio components. That’ll make no more heat than a transistor radio.
Class A vacuum tube is the exact opposite. Constant power draw regardless of signal, from silence to clipping, plus heater current of a few watts per tube, and a few watts of excess heat from the linear regulators. Similar to a 1963 all-tube color TV. My Panasonic 58" plasma HDTV consumes 500 watts, a bit more than a pair of Statements.
To put that in perspective, the same as four or five 100-watt conventional light bulbs, or the heat emitted by two people at rest. A light to moderate additional load on the A/C system, less than 1/10 of its capacity. Of course, if your A/C is running more than 50% to 70% of the time (a 50~70% duty cycle), probably not a good time of day to use the oven or turn on vacuum tubes.
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Stereophile published an article on the Westinghouse 300B, which, unlike cheap modern 300B triodes, had a triple coated cathode which lasted longer and was the secret to the better sound. If you can tolerate less power the NOS 45 globe tubes were made the same way and many people find them richer sounding than the cheap modern 300B.
Around 2005 there was a Japanese 833A SET amplifier driven by 300B transformer coupled to the grid of the 833A and rumor had it the 833A was run on only 1000 Volts with a positive grid bias to force it to draw more plate current. It sold for $350,000, but the parts and labor could scarcely cost a percent or two to manufacture it. You would not need high sensitivity speakers for it, you can use planar magnetic speakers if you want. Lundahl has probably the best coupling transformer there is for coupling a globe 45 to the 833A grid which can run on zero or ground bias for eliminating a bias supply capacitor from the signal path and this is more than enough power for driving less sensitive speakers. Either a globe 45 or a WE 300B will work for this. Hammond makes a more robust air-gapped output transformer rated at higher DC current than smaller more expensive big-name brands and it is perfect for the 833A.
This is relatively easy to build yourself for less than $2000 retail in parts if you know how to be careful with the 1000 Volt power supplies it needs.
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45 is indeed very very nice sounding.
Cost has gone up quite a bit though. $2k will be barely enough these days for a high quality 45SET DIY project, assuming no fancy enclosures and not using big name irons like ISO or Hashimoto.
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Not going anywhere near an 833 or those Eimac transmitter tubes. No thank you. Those things light up a room with a dazzling white light and get serious, room-heater hot. They are designed for transmitter use, with forced-air cooling that ducts to the outside.
And if somebody chooses speakers that are 0.2% efficient, well, what did you expect? Class D or Class AB for you, as well as a dedicated AC power outlet that goes straight to the circuit-breaker box.
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@wig Where do you live? I live in Tucson so I'm guessing Phoenix area?
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@dz13
Just outside of Las Cruces.
Wig
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@drbarney1
I agree with Lynn in that I won’t work with power supplies over 550-600V. The teflon mil spec wire is all rated at 600-630V so you would have to source 1200V wire for headroom. All normal parts are rated at 630V so you would have to source special caps, resistors, etc... You don’t mess around with 1000V or you will have a fire. Also the transformer set in these mono amps including the custom interstage transformers and choke would eat up most of your budget. I like some Lundahl iron, but I found that their IT started ringing above 15KHz in my circuit, so you would most likely have to get custom iron to do your project as I did. So yes, you could certainly build the project you discuss, but it isn’t simple, there are lots of issues and sourcing of custom parts rated for 1kV and if you use Edcor or Hammond iron you or something else of that ilk you will get medium grade amp, not one that has incredible detail and sounds alive. So if you build that amp with $2000 worth of parts you may be able to do it, but if you spend a lot more you will get a much better amplifier. You will still have a fire risk if you don’t make sure every single part in there that sees high voltage is running at no more that 70-75% of its rating. Easy to do at 500V, but a lot harder at 1000V.
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I use Tubes that also run quite hot, the Amp's these tunes are utilised in are positioned in free space away from other Structures within the room.
I have in the past owned a Portable Air Conditioner Unit, that produced Icy Cold Air, this has been reserved for use when the External Temp's are getting up to the +25 Centigrade to help with sleeping. When using the room the system is set up in during these periods of high Temp's, I have never felt there was a need to run the Portable AC Unit during a listening session.
If increasing Room Temp's are a concern, as a result of a Tube Amp' being used, and a Cooling Device is at hand, cooling the Room in advance of the listening session is certainly going to cool the room and not impact on the Sound being produced.
My cautionary advice to an individual interested in using Tube Devices is no different to the advisories I was given many years past, which is to make sure there is always 'eyes on' the devices in use for the bulk of the time they are Powered On, and have a revisit shortly after being Powered Off. After approx' 30 Years of using Tube Devices, I have not had a concerning experience, only the oddity of a electrical glitch, a spent Valve or a blown fuse.
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Very good article about how the brain distinguishes between reality and imagination. Since audiophiles are endlessly arguing about perception and reality, this is a useful read:
Quanta Magazine
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Hi @donsachs @lynn_olson ,
I have 300B SET with 6f6 driver in triode mode. I did 2nd harmonic distortion calculation using 3 point method and 6f6 plate characteristic graph. So with RC coupling that I have right now I get more than 2% distortions with voltage swing 140v pick to pick and when I go to interstage transformer coupling I can get around 0.7% distortions with 160v pick to pick. The difference is huge!
I have a couple of question about using transformer. Do you use load a resistor after the interstage transformer? What kind resistors do you use? I'm going to use Hashimoto A-305 transformers. As I understand Hashimoto recommends 100K load resistor for this transformer.
Regards,
Alex.
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@alexberger
Hi
I am sorry, but I am not giving anything about the design of the amp away. I had ITs custom wound for the project. My amp is PP, but if you need a load resistor with your circuit then I would try two kinds. A good quality wirewound of sufficient rating, and a good old Kiwame 2W or 5W depending on what you need. I usually use wirewounds, and not cheap ones, but Ohmite or Vishay or something else of quality. Which you prefer will depend on the circuit, your ear, and the Hashimoto iron. The resistors are cheap so try both kinds and see what you think.
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There is a simple and inexpensive means of dealing with heat due to amplification. Ducts in the ceiling above the amplifier(s) can be added, along with dryer style hose the connects via a squirrel fan to the outside. This can be installed for a few hundred dollars (mostly labor), is quieter and far less power draw than air conditioning.
Do you use load a resistor after the interstage transformer? What kind resistors do you use?
@alexberger If any audio transformer is not loaded it can 'ring' (which is to say it will generate harmonic distortion, which can be quite profound). The correct loading will cause a state of 'critical damping' in the transformer, where when a square wave is put thru the device, it will have little or no 'overshoot'. Since the grid of a tube tends to be very high impedance (other than its capacitance) some form of resistive loading is a good idea to explore!
Since you sound like you are up to something with your own design, I recommend an empirical approach, which might be to drive the transformer with a square wave and have it drive in turn a power tube which is operating normally. A potentiometer and oscilloscope's probe across the output of the transformer would then allow you to vary the potentiometer and observe the result on the square wave. In this manner you can exactly dial in the correct loading value.
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I agree with Atmasphere. Some things cannot be accurately modeled. Put it on a bench, attach a dummy load, light it up, and measure square waves. Tune for nice-looking leading (and trailing) edges. Measure both low-level (below 1 watt) and also just below clipping.
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Any indication on target preamp pricing?
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I just have the first production one running in now in prep for Seattle show. It will be at least $5000 as it is cost and labour intensive and all very high end parts.
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Although a glance at a schematic might lead you to think it is simpler than a classical (Golden Age) PP KT88, the parts are more expensive. Way more expensive, just as 300B’s (as a group) are more expensive than KT88’s (as a group).
Another factor is sensitivity to parts quality. An amp with 20 dB of feedback (which is nearly all Golden Age amps) tends to wash out differences in the sonics of different parts. This is exactly what feedback is meant to do ... 20 dB of feedback is a 10:1 reduction of all sources of coloration. A zero-feedback amp, by contrast, reveals the sonics of every single part, particularly at critical nodes in the circuit. This raises costs compared to the PP KT88 equivalent.
Depending how you feel about the sonics of solid-state and feedback, you can travel a continuum between modern Class D, with sophisticated and complex feedback, to Class AB transistor or MOSFET with 20 to 40 dB of feedback, to Class AB push-pull pentode with 20 dB, to Class A with zero feedback. Each type sounds different and has different distortion spectra.
Comparisons between modern Class D and all-triode Class A are not absurd, despite radically different technologies. Class D and Class A both skip over the many difficulties with Class AB device switching, whether bipolar transistor, MOSFET, or pentode (each device type has different artifacts associated with the AB transition). The difference is Class D switches at 100 kHz or higher, with pulse width translating to signal level, while Class A is non-switching and like a preamp, fully analog from start to finish.
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@lynn_olson
Another factor is sensitivity to parts quality. An amp with 20 dB of feedback (which is nearly all Golden Age amps) tends to wash out differences in the sonics of different parts. This is exactly what feedback is meant to do ... 20 dB of feedback is a 10:1 reduction of all sources of coloration. A zero-feedback amp, by contrast, reveals the sonics of every single part, particularly at critical nodes in the circuit. This raises costs compared to the PP KT88 equivalent.
I do not have the resources to confirm this via test bench measurements as you do. I can only rely upon listening experiences with my own audio system. When I placed my 300bSET zero NFB amplifier in my system the first time (2009) the immediate difference/improvement was the sense of naturalness. Very stark and unequivocal. In my system realism and authenticity took a step (Or two) forward.
Charles
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The difference is Class D switches at 100 kHz or higher, with pulse width translating to signal level, while Class A is non-switching and like a preamp, fully analog from start to finish.
@lynn_olson most quality class D amps switch at 500KHz or more. Also to be absolutely clear, class D is 'like a preamp, fully analog from start to finish.' A very different type of circuit, but analog nonetheless.
I do not have the resources to confirm this via test bench measurements as you do. I can only rely upon listening experiences with my own audio system.
@charles1dad Lynn is correct- if you are not running feedback then everything in the amplifier design right down to the component quality, wire and solder makes a difference. When you run lots of feedback you get rejection of things like that- often including sensitivity to line voltage.
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That’s why I am agreeing with what Lynn wrote. Zero NFB seems to uncover a masking effect. The subtleties and nuance of virtually everything is revealed as noted.
Charles
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Zero NFB seems to uncover a masking effect.
@charles1dad That's not quite correct. It depends on how well the feedback is executed and as I've been pointing out a lot recently, with most amps made in the last 70 years or more that's not happened.
But if its done correctly, amps using it can be amazing- no hint of dryness, nice soundstage; everything you want in an amp.
I've described what's needed often enough, no need to repeat it here.
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