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 660O.  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

Many of you have probably heard of “Community Supported Agriculture” – some of you may even belong to one.  If so, good for you!  CSA’s are an excellent way to form a closer relationship between you and the source of your food.  CSA’s are growing at a tremendous rate as consumers slowly learn the downsides of intensive farming practices, GMO’s, etc.

CSA members receive instruction prior to helping on the farm

Along a similar vein, I would like to introduce you to the concept of ”Community Supported Amplifiers”.  Much like a traditional vegetable or fruit CSA, the members pay for the season in advance.  If the harvest is good that year, both the amplifier farmer and the members benefit.  If the harvest is poor that year, the farmer still gets the financial support he desparately needs.

A sun-ripened amplifier awaits harvesting by CSA members

Even though the amplifiers lining your hi-fi shop’s shelves look good, they are anything but good – owing to the non-sustainable large-scale practices employed.  With Community Supported Amplifiers you get the benefit of a relationship with the farmer who provides you with your amplifiers, as well as the knowledge that the amplifiers not only look good, but sound good too - due to sustainable small-scale organic practices.

An amplifier farmer holding a freshly harvested HMA-1000

Growing amplifiers is much more challenging than one might at first realize!  There are obvious considerations such as sunlight, soil pH and moisture, but if you want to grow a truly remarkable amplifier, one also grown in an environmentally sustainable manner, then you must consider the following:

• Seed quality is essential.  Use non-GMO seeds from a reputable supplier and start indoors four weeks before the growing season beings in your zone.
• A flat frequency response usually means a well-maintained trellis system.  Any uneven growth will show up in the amplifier’s frequency response plots.
• For high power class-D amplifiers it is essential to generously amend the soil with a natural iron-rich fertilizer to ward off late season ”magnetics blight”.
• High power means plenty of good silicon for power devices – don’t skimp on the silicon!  Natural sources include sand and old Dell Dimension computers.
• A cool-running amplifier requires a cool growing climate.  A nearby lake will prevent early blooming and subsequent frost damage.  Webster NY is ideal.
• A natural distortion profile means plenty of natural materials.  A well-maintained compost heap of all of your organic amplifier scraps will help with this.
• A relative of the Colorado potato beetle, the Nevada amplifier beetle, can infest amplifier fields.  Cornell researchers are looking into natural predators. 

If you do not already belong to an amplifier CSA, then please consider joining one!  Bringing the amplifier farmer and the audiophile closer together results in sonic benefits that simply cannot be realized with the intensive farming practices that typify mass-produced amplifiers.  With an amplifier CSA you will always know that you are listening to the goodness of the earth, the sun, and the farmer’s heart.

Give back to your community – support your local amplifier farmer!

P.S.  April Fool’s! 

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!

No, not an amplifier this time!  It’s my new little girl: Kilmeny Arianwen - born February 22nd, 2010.  Mother and daughter are happy and healthy!

We received the Stereomojo 2009 Products of the Year award: Power Amplifier of the Year!

From Bill Schuchard of Stereomojo:

“The Hephaestus Audio HMA-1000 is a great monoblock amplifier. It sounds fast, clean, transparent, open, and airy with powerful but neutral bass and massive headroom. It worked well with a variety of speakers and sources, takes up very little shelf space, and produces quite a bit amount of power efficiently (1,000 watts per side). Those looking for a big shiny bauble with a fancy name to show off may be perturbed by the utilitarian look and understated size and miss out on some amazing sound. The $4900 list price per pair is a tough pill to swallow from a relatively unknown company; especially for those who are frugal like myself. The spoonful of sugar that makes the pill slip down is the sound. I am very pleased to announce my new reference amplifier; the Hephaestus Audio HMA-1000 monoblocks.”

Not only is Mr. Schuchard a wonderful reviewer, but also a wonderful customer!

Interest is picking up so this evening I am spending time soldering some new amplifiers!  I couldn’t help but think, “Gee, if I outsourced assembly of the PC boards, then I could just worry about final assembly and have some extra time…”

Is this how it has started for companies in the past?  It’s so easy to fall into the trap of offloading some of the more tedious tasks to somebody else – somebody who doesn’t know or care about your customers – somebody who is only doing a good job because you are paying them to do so.  There’s not a chance I’m going to do this!  Big amplifier manufacturers may scrap more units then I will ever build, but I’m not going to change how I’m doing it.  I like to know that my customers will receive something I have created with my mind and built with my hands.

Needless to say, when you purchase an amplifier from Hephaestus Audio, you may count on the fact that I built it for you – even if I occasionally toy with the idea of doing it another way.  Maybe it was just the solder fumes going to my head? 

We received the Stereo Times Publisher’s Choice Most Wanted Components award for 2009!

From Clement Perry, publisher of the Stereo Times:

“What immediately impressed me right out of the box was the improved dimensionality and the enriched harmonic overtones. This, from a kilo-watt powerhouse no less. Don’t let the small size of these dynamos (3.5 lbs) fool you either because they really do pack a punch!”

Mr. Perry is wonderful to work with.  He gave me real-world feedback early on in the life of the HMA-1000 that was instrumental to steering it in the right direction.  I’m thrilled to receive this recognition from the Stereo Times!

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!

This is a very tricky topic.  Any simple article is never going to be enough for a novice to really be able to design a class-D amplifier.  However in this article I attempt to outline the basic requirements and considerations for designing a class-D amplifier.  It isn’t complete, but it’s a good start!

Also, in the article I attempt to draw a parallel between some of the familiar elements of a linear amplifier and the perhaps not-so-familiar elements of a class-D amplifier.  The comparison isn’t perfect, but I hope it will serve as a springboard to those not yet familiar with the intricacies of class-D amplifier design.

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!