Ayre Acoustics QX-5 Twenty – The Digital Hub

[AmusedToD]

Looking forward for @austinpop to compare the SOtM clocking ability through the “trifecta” driven by the external Cybershaft master clock to the single built in superior clock as the one in the QX-5. 

 

I have read somewhere that it’s possible to order a stripped down QX-5 without some of the modules (for example without the USB module) and have a lower overall price of the unit, that should be interesting.

 

And finally, a question for @Charles Hansen, which type of clock will be used in the upcoming QX-8 and what kind of performance should we expect compared to the Morion one in the QX-5?

[AmusedToD]

20 hours ago, Charles Hansen said:

 

Hello Amused,

 

There is a lot of hype going around about clocks these days, and unfortunately it is all almost all just marketing hype. All of this talk about "femto clocks" and so forth is just that - talk. There are a lot of things that affect clock performance, but the question is "Which clock parameters correlate with audio quality?".

 

It turns out that a single jitter number is completely useless, and actually almost always misleading. When a "jitter spec" is given, it is the sum of the phase noise over a particular frequency range offset from the carrier (operating) frequency of the oscillator. For all telecommunications work, this range is specified as 12kHz to 20MHz offset. Please refer to:

http://community.silabs.com/t5/Timing-Knowledge-Base/The-12-kHz-to-20-kHz-Jitter-Bandwidth/ta-p/114177

for information as to how this standard came about.

 

It turns out that this number is almost meaningless for audio performance. Instead it is close-in phase noise - no more than 20kHz (the audio band) offset from the carrier. Low levels of lose-in phase noise are very difficult to achieve. Factors that affect this include the crystal, the oscillator circuit, and the power supply to the oscillator itself. For crystals there are two types used for audio. By far the most common is called an AT strip-cut. Depending on the care in manufacture, these can range from fair to excellent in performance. The oscillator circuit itself can also affect phase noise dramatically, but depending on the circuit, the effects may either be in the important close-in range or in the unimportant (for audio) large offset from the carrier. Finally there is the power supply to the oscillator circuit. Most oscillator circuits have very low power supply rejection ratio (PSRR). This means that the power supply is just as important as the oscillator circuit in achieving low levels of close-in phase noise.

 

Measuring phase noise is extremely difficult. Most crystal and oscillator manufacturers use an HP (Agilent) phase noise analyzer, but it costs $100,000 brand new. If you are lucky you can find a used one for half of that price. One problem is that there is so much variation from crystal-to-crystal that in phase noise performance. An even bigger problem is that crystals vary in phase noise performance over time. Getting a good measurement on an oscillator can literally take a week or more as (apparently) surface contaminants on the quartz surface "burn-off" (evaporate) as the oscillator is used. There are two things that help here. One is to only by from high-quality sources with outstanding quality control, and the other is to keep the oscillator running all the time. One of the big things that causes a sound improvement in digital products as they "warm up" is the fact that the oscillator is settling down and the phase noise is lowering.

 

Probably the most important thing is the power supply for the oscillator. All of the IC-based voltage regulators have quite a lot of low-frequency noise that directly translates into close-in phase noise - no matter how good the oscillator circuit and crystal used, as all oscillator circuits have poor rejection from power supply noise. This is one area where Ayre has a huge advantage. We build our own discrete regulators, not only for the audio circuitry, but also for everything that is in the clock path from the oscillator to the DAC chip itself. Our discrete designs typically have 10x lower noise at low frequencies than even the best IC-based voltage regulators.

 

When it comes to the crystal itself, there is only one more thing that will help lower the close-in phase noise. That is to use an SC-cut crystal. This can provide a 10dB improvement in close-in phase noise compared to even an excellent AT strip-cut crystal. One problem is they are much more difficult to use as they always have spurious resonances close to the main carrier. If the oscillator circuit is not designed properly, the circuit will "hop" from the desired frequency to the spurious frequency - often many times per minute (or even second), which translates into massive amounts of low-frequency phase noise. The other problem is cost. Not only does an SC-cut crystal cost over 100x as much as an AT-strip cut, but they also generally require an "oven" - a small, sealed metal box with a thermostatically controlled heater to hold the crystal at a constant temperature. That adds another 5x to the price. This is what is usually found in the $5000 (internal option) to $100,000 and even $20,000 external clocks.

 

In the QX-5 Ayre worked with a crystal company to develop an SC-cut crystal that has the same low phase noise, but does not require the oven. This (barely) allowed us to get the very best possible performance available in a real-world priced unit. The less expensive Ayre products use an AT-strip cut crystal, but from specially selected manufacturers that can control the quality of the crystals and provide consistent performance. The biggest difference however is the ultra-low noise power supply regulators that we use. This allows the less expensive Ayre products to exceed the performance of what many other manufacturers call "femto clocks" (because the jitter from 12kHz to 20MHz offset (which is meaningless for audio applications) is less than 1 picosecond and so would normally be measured in tens or hundreds of femtoseconds.

 

Don't be fooled though. A "femto second" oscillator can have far higher amounts of phase noise at low frequencies - within 1kHz or less offset from the carrier. Ayre has found that even very low offsets - down to 10Hz, 1Hz, and even 0.1Hz can have large effects on the sound quality of digital playback. The next step is delivering that low phase noise signal to the DAC chip itself, where it really counts. Ayre uses impedance matched traces and isolation stages help out here. Plus there is internal logic in the DAC chip itself that needs ultra-low noise phase chip before it reaches the actual conversion stage. It then becomes just as important that power that supplies the clock pins also have extremely low noise.

 

As you can see, there is far, far more to having a converter free from jitter problems than just paying $10 or $20 more for one of the Crystek oscillators that are advertised as being "femto clocks". Those are simply pre-canned oscillators employing AT strip-cut crystals that have a reasonably quiet power supply - for an IC-based internal power supply regulator. Ayre has found suppliers of crystals with equal or better quality but combine that with our custom ultra-low noise discrete power supplies to achieve even better performance. To achieve the ultimate requires using an extremely expensive SC-cut crystal, so that must be reserved for our more expensive products. But even there by developing a way to eliminate the need for the oven, Ayre can afford to use that very highest level of performance clock without having to spend multi-kilobucks for just a clock or a complete DAC.

 

Hope this helps,

Charles Hansen

Charles Hansen

 

 

 

Hi Charles,

 

Thank you for the time and effort in replying to my question in great detail, much appreciated!

 

So basically the QX-8 will have the AT-cut crystal which is not sourced from Morion, right?

 

 Is the clock quality and performance the only difference to expect between the QX-5 and QX-8, or are there other differences (in the PSU section perhaps)? Any hint regarding the price of the QX-8?

[AmusedToD]

On 26.9.2017. at 3:07 AM, Charles Hansen said:

 

Hello Amused,

 

An SC-cut crystal is extremely difficult to manufacture. It is called a "double-rotated" cut, as a quartz crystal has a growth structure along a specific axis (the length) and then the arrangement of atoms grows at specific angles to this axis. Most crystal cuts (such as AT) simply cut at one specific angle to the quartz crystal (which you have no doubt seen before - they look like hexagonal rods). Just to purchase a bare SC-cut crystal will cost between $100 and $300. This compares to a complete AT strip-cut oscillator, which will typically cost between $0.50 and $1.50.

 

The Crystek parts that are used in many "high-performance" audio applications (and called "femto-clocks" by at least one manufacturer) are AT strip-cut and cost around $20. We have tested them and while they are better than most of the mass-market AT strip-cut, they are no better than certain specific brands that we have found to offer equally high performance at a much lower cost. Making crystals is part science and part art. There are many, many variables that can affect the performance of a crystal oscillator, and 99.9% of the the measurable factors only apply to the telecommunications market (see the above post with the linked reference to SONET) and has no bearing on how the oscillator will perform for audio applications. It is unclear what Crystek is doing to achieve their higher-than-average level of performance. It may just be that the parts include a low-noise power supply regulator inside the can, but that advantage is negated in the way that Ayre uses oscillators as our power supplies are even lower noise than any IC-based regulator. (By the way, when looking at manufacturer-supplied performance graphs of of oscillators, they will always use the best possible power supplies to achieve the best possible performance curves. Those power supplies may be special $20,000 supplies, but they don't tell you about that...)

 

Also be aware that there are OCXOs (Oven-Controlled X-tal Oscillators) that use AT strip-cut crystals, so the presence of a crystal oven is no guarantee that the unit uses an SC-cut crystal - nor how good the oscillator circuit is, nor how good the power supply circuit is. Trying to pin down the sonic performance of any product simply by running down a checklist of design features is about as useful as trying to pin it down on the basis of specifications - both of them simply don't work.

 

In the end it comes down to the manufacturer to make intelligent choices. Ayre can achieve much better performance from an AT strip-cut than others can when using SC-cut crystals by knowing which manufacturers provide consistently high quality crystals, using them in oscillator circuits that keep the close-in phase noise down, and (perhaps most importantly) using discrete power supplies that have much lower levels of low-frequency noise than any IC-based regulator available.

 

These are the kinds of things that allows Ayre to achieve higher levels of performance at any given price point than most other manufacturers. A good example is AustinPop. He has Ayre's `$2,000 DAC and wants to replace it with something that has more features than he currently has. To achieve a comparable (or better) level of sonic performance along with those extra features, he is looking at spending 3x to 4x the price.

 

As far as future products, I have learned it is best not to comment on them before they are actually shipping. Clearly there will be other differences besides just the change of the crystal used for the master audio clock, as otherwise there would be very little delta in the overall price. But just as the Pono Player was able to offer "much of the performance of the QB-9 DSD" (I believe those were John Atkinson's words in his Stereophile review of the Pono Player) for $399, I'm sure that people will be happy with the performance of the 8 series products. (We wouldn't bother to make them if we felt otherwise!)

 

Hope this helps,

Charles Hansen

 

Thank you very much, Charles. Your contribution here is priceless.

 

As @austinpop has just picked up the QX-5 for a listening test (and hopefully a direct comparison with his SOtM reclocking “trifecta”), I think this and the other big thread are about to become much more interesting now after he provides us with his inputs and comments. 

[AmusedToD]

Such a sad day, Charlie was kind enough to respond to every question on this forum very patiently and with great passion. This is a huge loss for the audio community. R.I.P.!