## Archive for April, 2010

### Equivalent Input Noise

Tuesday, April 6th, 2010

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:

$v_{n}=(4kTR)^{1/2}$

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

### Community Supported Amplifiers

Thursday, April 1st, 2010

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.

P.S.  April Fool’s!  🙂