300b lovers

True, push-pull substantially reduces distortion, but in reality it only reduces even-order distortion ... 2nd harmonic, 4th harmonic, 6th harmonic. etc.

@lynn_olson This statement is not correct. If the circuit is fully differential and balanced (in effect, PP from input to output) distortion is compounded less from stage to stage. The result is less 3rd harmonic than you would otherwise see if only the output section is PP.

In addition I should point out that the 3rd harmonic is innocuous in the same way as the 2nd as long as it is at the same level or less as seen in an SET, which as you pointed out in most cases (except for the example I just gave) it will be.

A properly functioning tape recorder makes a dominant 3rd harmonic that is actually higher than seen in most PP tube amps at full output. No-one really seems to complain about the ’sound’ of tape; rather people seem to like it quite a lot!

How about using SE as opposed to PP in the first stage and a SE to PP interstage transformer between the first and second stage?

@ffzz When you combine single ended and PP in one amplifier you get two distinct non-linearities (since neither circuit is perfectly linear). PP circuits generate what is known mathematically as a ’cubic non-linearity’, while single ended circuits generate a ’quadratic non-linearity’. The cubic or quadratic function describes a lot about how the higher ordered harmonics behave in the circuit. This is important because higher ordered harmonics can contribute to harshness and brightness, as that is the tonality assigned to them by the human ear.

When you combine both circuits in the same amp you get both non-linearities. Norman Crowhurst (one of the technical gurus that wrote about tubes and other technical/engineering topics related to audio) wrote about this 65 years ago, stating that the result is a more prominent 5th harmonic. With this understanding I’ve not found it surprising that SET owners prefer their amps over PP simply because the PP amp combined both types of circuits with the result described.

@donsachs 

Hi Don,

I have always wanted a 300B system. Two quick questions. I currently have a pair of KEF Reference 5 speakers @91db. How well will your 300b power them? And can you give me a ballpark price so I can start groveling to She Who Must be obeyed?😁

Thanks in advance.

Jim

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.

@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.

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.