300b lovers

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.

@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 :-)

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.

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

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.