Hephaestus Audio

 

Don’t let the title scare you off: “equivalent input noise” is just another way of measuring how noisy an amplifier is.  The nice feature in this case is that it takes the gain out of the equation, so it doesn’t matter if the gain of the amplifier is 20dB, 26dB, 32dB, or 38dB – you can still compare apples to apples.

Perusing the datasheet of a class-D amplifier from one of the most highly regarded class-D manufacturers in the world (no, not me…yet), I came across the following noise specification:

5nV/sqrt(Hz) equivalent input noise

Well, is this quiet, noisy or what?  As it turns out this is extremely quiet, as quiet as a 1.5kΩ resistor just sitting there on its own in fact!  What is even more amazing is that this amplifier features an input impedance of 100kΩ!  In other words right at the input of the amplifier is something that should be generating about 41nV/sqrt(Hz) of noise, yet the amplifier only features 5nV/sqrt(Hz) of noise!?

How is this possible you may ask?  Well, it simply isn’t.  To the best of my knowledge people have not yet determined how to cancel out random noise.  Reading the datasheet a bit more it seems that the amplifier features a “minimal path voltage mode” input with an impedance of 660Ω.  That must be it.  It must be this “minimal path voltage mode” input that is used for purposes of the noise measurement, but is this the input typically used by customers?

This does strike me as a bit of obfuscation.  The other parameters of the amplifier are very good, so why give your customers this useless noise data?  How about simply providing the dynamic range or signal to noise ratio for the 100kΩ input – the one the customer will most likely use, or at the very least provide understandable noise data for all of the inputs?

For those interested, here’s how you calculate the noise of a resistor:

k = Boltzmann’s constant
T = temperature in Kelvins
R = resistance of the resistor

 

Audio components are often touted by manufacturers as “one size fits all”, but most audiophiles know this simply isn’t true.  What works for you in your system may not work for me in my system.  This begs the question: What exactly makes a piece of gear work for you?

What it all comes down to is exactly what you are looking for in your musical reproduction.  Extreme accuracy?  Recreating the ambiance?  The spirit of the original performance?  Relaxation?  This list could go on indefinitely and may vary day-to-day for many listeners.  However, chances are though that there are one or two things that are absolutely crucial for you as a listener to appreciate a given audio system.

It helps to start with a concrete example, so here are a few things that may be crucial to you as a listener and the type of loudspeaker that may fit the bill.  This exercise can be done with any audio gear, but loudspeakers seem like the most intuitively easy to grasp for purposes of illustration.

“Accurate and stable imaging is most critical to me.”

Full range drivers or possibly coaxial/coincident drivers may be the best choice.  This allows for the best integration of the source signal, often at the expense of frequency response and dynamic range.

“Maximizing the dynamic range is most critical to me.”

A large format multi-way system or possibly a horn loaded system may be the best choice.  This allows for the most dynamic range over the audio bandwidth, often at the expense of poor imaging and coloration.

“Recreating the performance space is most critical to me.”

Minimizing early reflections/stored energy is essential to avoid masking spatial cues.  An open baffle system may be the best choice, although the price is poor low frequency efficiency and the need for large woofers.

 

As for myself, I like my music to sound, well, musical!  For me this means using drivers without nasty breakup modes – even if the modes fall outside of the bandwidth I intend to use the driver in.  Unfortunately, this puts most drivers with with rigid cones out of the running: Kevlar, carbon fiber, aerogel, etc.  Some metals might be okay, such as the highly regarded Jordan JX92S or an aluminum ribbon tweeter.

Frequency response aside, I think drivers should sound good on their own without any correction networks, etc.  Here’s a little secret about driver breakup modes: They represent a lack of control on the part of the driver, therefore it takes very little to excite these modes – even if notch filters and steep crossover filters are used!  You can’t control the uncontrollable, you can only try to minimize excitation of these modes.

A driver is simply not going to be more “accurate” if your brain has to sporadically allocate resources to “hear around” nasty twin breakup peaks at 3kHz and 5kHz.  My opinion is to do yourself and your brain a favor and use musical drivers!

Signal to Noise Ratio (SNR), or Dynamic Range, is a measure of the maximum resolution of a piece of audio gear.  The larger the number, the greater the possible range from very, very quiet sounds to very, very loud sounds.  It is easy to calculate, for example:

Noise level with no input signal = 70uVrms
Signal level at full output = 1000Wrms into 4Ω → 63.25Vrms
SNR = 20 log (63.25Vrms / 70uVrms) = 119dB

Pretty simple, huh?

Now sometimes the Signal to Noise Ratio may mention that it is “A-Weighted“.  This is an attempt to adjust the value based (very roughly) on the way we hear – i.e. the “Fletcher-Munson” curves.  The end result is a higher Signal to Noise Ratio than if no weighting were used.  Is this simply cheating to improve the on-paper specifications?  Probably, but at least it has some basis in psychoacoustics.

Speaking of cheating, I came across an amplifier with the following specifications:

Noise level with no input signal = 90uVrms (A-Weighted)
Signal level at full output = 1000Wrms into 4Ω → 63.25Vrms
SNR (according to specifications) = 120dB (A-Weighted)
SNR (according to calculations) = 20 log (63.25Vrms / 90uVrms) = 117dB (A-Weighted)

What’s going on here?  There’s a 3dB discrepancy between the spec sheet SNR and the calculated SNR!  The trick in this case is that the manufacturer is using the peak output power (2000Wpeak) of the amplifier instead of the RMS output power (1000Wrms).  It is then compared against the RMS noise level – i.e. it’s not comparing apples to apples!  Here’s the calculation:

Noise level with no input signal = 90uVrms (A-Weighted)
Signal level at full output = 1000Wrms into 4Ω → 2000Wpeak into 4Ω → 89.44Vpeak
SNR = 20 log (89.44Vpeak / 70uVrms) = 120dB (A-Weighted)

There it is now!  So if you are simply “creative” with you calculations, you too can add 3dB to your amplifier’s SNR.  Is this cheating?  Yes, I believe it is.  However, you are now armed to check if the SNR numbers add up for a prospective piece of audio gear, to help weed out the cheaters.  Good luck!

In order to evaluate two components in an AB test, it is essential that they are carefully level matched.  If the gain differs by only one dB, the louder unit will almost always be perceived as sounding better.  Unfortunately, this phenomena has been exploited by unscrupulous hi-fi dealers to promote overpriced components, as I have personally witnessed:

My friends and I once went to a local hi-fi shop to check out the goodies.  There were some very lovely components we had the opportunity to see and hear, but there was also a one meter length of heavy power cord retailing for about $2,000 (this was over 10 years ago).  The salesman conducting the demos (err…Jim) saved this gem for last – here’s the process he used (I watched very carefully):

• Carefully set volume knob to specific level
• Play track from reference CD to completion
• Turn down volume knob to zero
• Insert special power cord between power strip (!) and amplifier
• Carefully set volume knob to specific level
• Play track from reference CD to completion
• Turn down volume knob to zero

Sure enough the second time the track was played it sounded significantly better!  It would have been truly amazing if I had not been paying really close attention: When Salesman Jim carefully adjusted the volume knob the second time – the time with the special power cord in circuit - he turned it up just a little bit higher!  It wasn’t enough to notice it was louder, but it was enough for all of us to think it sounded much better.

My friends had unfortunately not noticed this scoundrel’s trick.  I’m sure I only did because I knew to look for it.  I quizzed Salesman Jim on exactly what made this power cord work.  At first he tried to dodge the question, but then it went something like this:

Salesman Jim: “Okay, okay it’s a ‘PFM Circuit’.”
Me: “A ‘PFM Circuit’?  I’ve never heard of that – what’s it mean?”
Salesman Jim: [laughing] “Pure Fricking Magic.” [note: slightly edited]
Me: “Okay, ha ha, seriously what is it?”
Salesman Jim: [growing annoyance with this punk] “Hey Bob, what’s the technology of this [brand] power cord?”
Salesman Bob: “Uh…geometry and metallurgy.”
Me: “Geometry and metallurgy?”
Salesman Bob: “Yup, that’s it.”
Me: [sigh] “Okay, well thank you guys for your time.”

So there you have it folks - the secrets to any mysterious audio gizmo: PFM Circuitry and Geometry/Metallurgy – what will they think of next!

 

Please forgive the bitter tone that follows in this post, but recently I came across an “audiophile” device that I feel may damage the already precarious position of the audiophile industry.  First however a little necessary physics background…

A “Blackbody” is an object that does not reflect electromagnetic energy incident upon it, or allow any to pass through it.  Any emission from it is entirely thermal in nature with no characteristic emission/absorption lines from any element.  This is referred to as “Blackbody Radiation”.  Classical theory describes this spectrum with the Rayleigh-Jeans Law:

is the power per unit area
is the speed of light
is Boltzmann’s constant
is the temperature in Kelvin
is the wavelength

This is in agreement with observation at lower frequencies, however the problem with this result is that at higher frequencies (i.e. as ) the power approaches infinity!  This is certainly not in agreement with observation and has been referred to as the “ultraviolet catastrophe”.

A physicist by the name of Max Plank made an amazing assumption that was to prove instrumental not only for resolving the paradox of the Rayleigh-Jeans law, but for all of quantum mechanics.  The assumption is that radiation can only only assume discrete energy values:

is the energy level
is Planck’s constant
is the frequency.

Therefore the energy between two adjacent energy states is given by:

The power per unit area spectrum that results from this important assumption agrees with observation and is given by the following expression:

Now I return to the bitter part of my post.  As I make clear in my Engineering Perspective article, I am not a big fan of overpriced audio interconnects.  I am not saying there isn’t a subjective improvement, but that it may not provide the greatest benefit to cost ratio for the audiophile.

However, at least these overpriced interconnects are functional.  At worst, they get the signal from point A to point B.  Even something as esoteric as a special wooden volume knob that improves your sound at least allows you to turn up the volume!  But a product that claims to dramatically improve your system by acting as an electromagnetic blackbody for your equipment is a complete waste.

There are people in this world with incredible perception that can detect “impossible” subtleties.  The purpose of the audiophile industry is to cater to these individuals – those for whom minute differences matter.  Please try to give these gifted individuals the audio experience they deserve and not just another piece of junk backed up with horrible pseudoscience.

There is much debate about optimum grounding strategies in audio electronics.  This article explores a ”Star Ground” versus a “Ground Plane”.

It may come as a surprise that, despite the common use of the star ground approach in audio electronics, the ground plane is superior.

Also included in the article is a list of helpful advice for a successful layout, particularly for high-power switching audio designs.

Enjoy!

When I stop and think – imagine - the most fantastic images come to mind.  I know I’m getting the smallest sliver of a glimpse inside of my subconscious and my conscious mind is futilely attempting to make sense of it in the terms it understands: vision, hearing, etc.  This process is so amazing that I’ve never been able to convey it very well, although I’ve no doubt that other designers have a similar experience.  Most likely even more so.

That’s the first step.  Thinking.  Imagining.  However, for others to benefit from this process it is necessary next to do something.  A former professor of mine, Carl Pavarini, once said “Ideas are worth their weight in gold.”  After the class full of engineers enthusiastically agreed, Dr. Pavarini then posed the question “How much does an idea weigh?”  Ouch.  It’s a tough pill to swallow, but it’s painfully true.  You must do something - whether your medium is paint, words, code, or solder.

I’m not sure what’s more incredible: the fact that I have the great joy of experiencing these creative visions or the fact that so many others have as well!  The following quotes provide terse insight to the inner workings of great minds.  These are the thoughts of the giants on whose shoulders we have all stood.  The best innovators distinguish themselves by not fearing to stand at such a great height.

 

“I do not think there is any thrill that can go through the human heart like that felt by the inventor as he sees some creation of the brain unfolding to success… such emotions make a man forget food, sleep, friends, love, everything.” — Nikola Tesla

“But I think Steve’s [Jobs] main contribution besides just the pure leadership is his passion for excellence. He’s a perfectionist. Good enough isn’t good enough. And also his creative spirit. You know he really, really wants to do something great.” — Andy Hertzfeld

“People take the longest possible paths, digress to numerous dead ends, and make all kinds of mistakes. Then historians come along and write summaries of this messy, nonlinear process and make it appear like a simple, straight line.” — Dean Kamen

“An essential aspect of creativity is not being afraid to fail.” — Edwin Land

“One of the unfortunate things about our education system is that we do not teach students how to avail themselves of their subconscious capabilities.” — Bill Lear

“One always has to remember these days where the garbage pail is, because it’s so easy to make sounds, and to put sounds together into something that appears to be music, but it’s just as hard as it always was to make good music.” — Robert Moog

“With all the knowledge and skill acquired in thousands of flights in the last ten years, I would hardly think today of making my first flight on a strange machine in a twenty-seven mile wind, even if I knew that the machine had already been flown and was safe.” — Orville Wright

“Knowing is not understanding. There is a great difference between knowing and understanding: you can know a lot about something and not really understand it.” — Charles Kettering

“Opportunity is missed by most people because it is dressed in overalls and looks like work.” — Thomas Edison

“The real secret of success is enthusiasm.” — Walter Chrysler

“What is it that I like so much about the house you’re building for me, Howard?”

“A house can have integrity, just as a person,” said Roark, “and just as seldom.”

“In what way?”

“Well, look at it.  Every piece of it is there because the house needs it – and for no other reason.  You see it here from the inside.  The rooms in which you live made the shape.  The relation of masses was determined by the space within.  The ornament was determined by the method of construction, an emphasis on the principle that makes it stand.  You can see each stress, each support that meets it.  Your own eyes go through a structural process when you look at the house, you can follow each step, you see it rise, you know what made it and why it stands.  But you’ve seen buildings with columns that support nothing, with purposeless cornices, with pilasters, mouldings, false arches, false windows.  You’ve seen buildings that look as if they contained a single large hall, they have solid columns and single, solid windows six floors high.  But you enter and find six stories inside.  Or buildings that contain a single hall, but with the facade cut up into floor lines, band courses, tiers of windows.  Do you understand the difference?  Your house is made by its own needs.  Those others are made by the need to impress.  The determining motive of your house is in the house.  The determining motive of the other is in the audience.”

 

This is the way Howard Roark, the main character of Ayn Rand’s The Fountainhead, describes the house he has designed for friend and supporter Austen Heller.  This novel is rare in its accurate illustration of the way many creative types feel about design – whether architecture, art, or amplifiers.  Non-functional ornamentation can provide transient beauty, but lasting beauty comes only from functionality.  Some of the most beautiful designs in the world are beautiful for two simple reasons:

1) The design serves a purpose better than other designs
2) The embodiment of the design is a function of the purpose

The human body is the best example I am aware of.

Anybody ever notice how “amplifier kits” sound the way that “beer kits” taste?

I recently took up homebrewing and, not knowing any better, I smiled as the nice LHBS man filled up my arms with “beer kit” paraphernalia.  I read the instructions carefully, bottled it up happily, and waited patiently for EIGHT weeks as it aged to “perfection”.  For any of you who have tried a beer kit, you are probably aware that “perfection” is a term to be applied loosely…

After dumping out bottle after bottle of icky beer, I was motivated to try making some beer from scratch.  This took lots of research that involves terms particular to beer making such as “degrees Plato”, “brix”, “original gravity”, “final gravity”, “wort” (pronounced “wert”), etc.  Make no mistake that I still made plenty of errors as I learned the ropes of making beer from scratch, but with a little tenacity I finally made something pretty good.  Please see the recipe below.

This whole experience bears an uncanny resemblance to “amplifier kits”.  This is where you take somebody’s amplifier controller and simply add the right ingredients – output stage, output filter, and power supply are typical - to build a complete amplifier.  It all sounds so simple, like the beer kit, and great results are promised, but these amplifier kits never quite manage to live up to it…

After throwing out the amplifier kit (i.e. “dumping the bad beer”) some are motivated to do research to figure out what is needed to make an amplifier from scratch.  There are many terms particular to amplifier design such as “dead time”, “slew rate”, “THD”, “EMI”, etc.  There are plenty of errors still in store for the would-be amplifier designer, but again with some tenacity it is possible to develop an amplifier that will shame any amplifier kit out there.

“Dry Ice Ale” Recipe

• 6 lbs Briess 2-row malted barley
• 2 oz Glacier hop pellets (added during boil)
• 2 oz Cascade hop pellets (added to secondary)
• 11g Nottingham ale yeast, proofed
• Bring 5 gal water to 160F (fill pot with water night before)
• Add grain and steep 30 min stirring occasionally
• Remove grain and bring to boil (this is a no-sparge process)
• Boil 30 min while adding 2 oz Glacier hops continuously
• Put in ice water bath and stir gently for 20 min
• Pitch proofed yeast and pour into primary
• Fill airlock with water and let ferment 1 week
• Transfer to secondary, add 2 oz Cascade hops
• Fill airlock with water and let sit 2 weeks
• Transfer to bottling bucket and add 1/3 cup sugar
• Fill bottles and let carbonate 1 week
• Put in cellar and let rest 4 weeks
• Serve at cellar temperature and enjoy!

“We aren’t making the best products just because some customer suggested them to us, or even assured us of big orders, but because we have a passion to bring some art, in which we have a large personal investment, to the pinnacle of perfection.” – Barrie Gilbert 

This quote comes from Ch. 18, ”It Starts with Tomorrow”, of The Art and Science of Analog Circuit Design.  If you love analog circuits, then this book should be placed right next to your copy of The Art of Electronics, if it isn’t there already.  Please refer to this IEEE article, “The Gears of Genius“, for more on this amazing designer.

There is a section of this chapter that deserves a brief summary – a bit of pseudohistory entitled ”History through Dark Glasses”:

 

“…the first step to a successful product is thorough market research,” reads a young Thomas Edison in his new copy of “Yours is the Market: How to Find Out What People Really Need and Thereby Become Rich and Famous” by Harvard professor Yucan Sellum.  It is essential, says professor Sellum, to listen to the “Voice of the Customer” (VOC).  Young Mr. Edison now goes door-to-door to conduct the needed market research asking if his potential customers see any possible improvements in the way they light their homes.

Door 99

“First, if you can invent a stronger, brighter gas mantle, people will beat a path to your door.  Those durned things are always breaking!  And second, if you can invent a way that causes leaking gas pipes to be self-healing, you’ll quickly find yourself off these streets.”

Door 100

“Now, if you find a way to make our oil-lamps burn twice as bright and and twice as long from one filling, that would be something you could sell.”

Edison returns to his lab, dejected.  He has adhered to professor Sellum’s advice that he must conduct his marketing research “in such a way that…one only elicits those facts which the customer freely wishes to impart to the researcher.”  “Too bad nobody ever asked me if I had any ideas of my own,” sighs Edison as he gazes at his new tungsten lamp powered by a generator spinning in the basement…he then writes:

“Trip Report, 18th November, 1878.  Spent all day doing a VOC in Menlo.  Spoke with 100 people re lighting improvements; got good info. from 83….Action Item: Write Product Development Proposal re Improvements to Gas Mantles and Oil-Lamp Wicks.  Do before Monday exec. council mtng.  Call KJ to consider weaknesses in present methods of mnfng mantles.”

 

After your masterpiece is created you must listen carefully to your customers and adapt quickly.  However, for that first critical, tenuous, and beautiful spark, you must listen only to the inner dynamo that drives you to create that which has never before existed.