Class D Technology

Atmasphere 11-4-2016

A drunk ran a stop sign and T-boned my car, totaling it, back in July. I had a lower back injury that was really painful (bulging disk into my sciatic nerve); its only now healing to the point that I can dress and move in a normal fashion.

Oh, wow! Sorry to hear about this, Ralph. But of course I’m glad that it wasn’t even worse, and that you are progressing toward normalcy.

We practice pragmatism, not ideology.

No wonder we usually agree about things in the discussions here. In a past thread I recall describing myself as neither a subjectivist nor an objectivist when it comes to audio, but rather as a pragmatist with a technical background.

As far as the subject matter of the thread is concerned, I have no experience with class D, and nothing in particular to offer in the way of comments.

Kijanki, best wishes to your daughter as well.

Best regards,
-- Al

Regarding the EE Times article, I took a look at some of the many patents that were issued to what was then the author’s company, JAM Technologies. It appears that most or all of their amplifier designs were intended for applications in which the amplification circuitry is provided with digital inputs, and operates exclusively in the digital domain until low pass filtering is applied to the pulse width modulated signal at the amplifier’s output.

Given that, the references in the article to jitter and to quantized output levels seem understandable, but along with the rest of the article would seem to have little if any relevance to class D amplifiers that are driven with analog inputs.

Also, JAM Technologies was described as a fabless semiconductor company (the word "fables" appearing at the end of the article is an obvious error or typo). Their designs appear to have been mainly intended for flat panel TV, PC audio, and personal media player applications, not for higher powered audiophile-oriented applications.

Some additional background I found that may be of interest: The company was founded in 1998 or 1999, and no longer exists. The author of the article was a co-founder of the company, together with another gentleman who was its chief technology officer and the inventor named in most or all of its patents. Both gentlemen left the company within a year or two after the article was published in 2005, and their subsequent professional endeavors have been unrelated to audio or consumer electronics.

Regards,
-- Al

Also what is the math relationship between bandwidth and phase shift? Where does 10X bandwidth number come from?

Hi Mapman,

If I'm not mistaken class D amplifiers typically use an output filter consisting of a series inductor and a shunt capacitor.  Together with a primarily resistive load that will form what is known as a second-order low pass filter.  "Second-order" meaning a filter that increases the amount of attenuation it provides by 12 db/octave (12 db per doubling of frequency) above the frequency at which it has rolled off by 3 db (that frequency usually being what is referred to as the bandwidth of the filter). 

The equation defining the phase shift introduced at various frequencies by a second order filter is complex, and is shown (approximately!) as equation 3 on page 2 of this reference.

To provide some perspective, however, it may be helpful to consider the much simpler case of a first order filter (6 db/octave rolloff), which is what would be formed by the combination of a series inductor and a resistive load, without the capacitor.  A first order low pass filter will shift the phase of a given frequency f by an amount equal to:

Phase shift = arctangent (f/bandwidth)

So a first order filter having a 3 db bandwidth of 200 kHz would shift a 20 kHz signal by arctan(20/200) = 5.7 degrees.

The 10x figure is a rule of thumb, as Ralph indicated, chosen to limit the phase shift introduced at frequencies of interest (e.g., at 20 kHz and lower) to amounts that are presumably inaudible.

It should also be understood that while for a pure sine wave at a single frequency any amount of phase shift will be inaudible, a musical note consists of a combination of many frequencies that are simultaneously present.  And the goal is to achieve proper alignment of the timing of all of those frequency components relative to each other.

Best regards,
-- Al
       

Kijanki 11-21-2016
One can argue that this phase delay might be beneficial since many speakers have positive phase shift at these frequencies.  I don't have data for my speakers, but I picked one of the speakers reviewed by Stereophile.  As you can see on the phase diagram (dotted line) speaker actually accelerates phase at the highest frequencies.  Phase delay caused by the bandwidth limit of my amp would improve summing of harmonics (Al, am I missing something?).  

No, I don't think you're missing anything, Kijanki, and that's an interesting point.  As is frequently the case it's possible that non-ideal behavior in one part of a system can to at least a loose approximation offset or compensate for non-ideal behavior in another part of a system, and turn out to be subjectively preferable.  And it is true that dynamic speakers in particular commonly have impedances in the top octave or two that are inductive, meaning phase angles that are positive.  On the other hand, though, there are of course many other factors besides impedance and bandwidth that contribute to a speaker's overall time domain performance.

Best regards,
-- Al
 

I have no dog in this discussion/fight/whatever, but FWIW I believe the statement Mr_m is referring to is the following:

Atmasphere 11-3-2016
Right now the technology is to the point where the amps have become practical for mid fi, but they still have a ways to go before they can challenge a good class A transistor amp or a good tube amp. There is no question that this is a technology to be watched.

FWIW I do not interpret that statement as "dismiss[ing] Class D
as nothing more than midfi......."  As I read the statement it does not by any means exclude the possibility that the best present day implementations of class D may be much better than mid-fi, while nevertheless not being as good as what well implemented class A solid state or a good tube amp can presently offer.

Personally I take no position on this, having no experience with class D.  But that is how I interpret Ralph's statement.

Regards,
-- Al