Hephaestus Audio

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!