but are they really designed that way, or do they produce two switched
wave forms to produce an (approximate) sine wave? I'm skeptical that
for audio reproduction they really create a single variable DC signal instead
of two DC signals that become an AC signal. Someone with an oscilloscope
should check.

 

A Class D approximates an analog signal by defaulting to a neutral/zero position. The subwoofer isn't driven to the null position, it simply is driven from the null position and allowed to return to the suspension's natural resting point.

In other words, a Class D amplifier only pushes. The troughs in the analog signal are created by a lack of push

 

you may be right, but I haven't found a single technical article online that
describes their operation that way, and it's counterintuitive, as a speaker
so driven would need to have twice the excursion to produce the same
volume as one driven by AC, which sounds like a recipe for disaster.

 

oth, Claus is an EE. which made your post kind of funny, since it immediately followed his.

[QUOTE=Claus,another thread on Dec 13 2009, 03:35 AM]I am an EE that is why I did not rec to go that route.

 

and if you look at all those diagrams, etc, they all demonstrate a
sinusoidal AC output.

 

For what it's worth to the discussion, I'm not an EE...but I went through 6 months of electronics repair training in the AF (and spent another 5 1/2 years working on amplifiers/radios/etc). For the past 3 years I've worked in the Semi-conductor industry as a field service engineer troubleshooting/repairing electronics (amongst other areas such as hydraulics, electro-mechanical, robotics, pneumatics, vacuum, and so on). I've been doing car installs for the past 10+ years


The switching action of the Class D is what makes it less than desirable for mids/highs. Referring back (somewhat) to the previous post on crossovers, remember what an octave is. Now, the switching frequency of a Class D amp might be 100Khz. Even with a high slope, there will be harmonics at 50Khz, 25Khz, 12.5Khz, etc. Granted, those aren't going to be very loud...but how loud does that annoying whine from your alternator have to be to bug the piss out of you?

When we cross that 25Khz line (2nd harmonic), we get into the aural range.

When we use Class D for subwoofers, we start at 100Hz and down. So that switching freq harmonic goes (approx) 50K, 25K, 12.5K, 6.25K, 3K, 1.5K, 750Hz, 375Hz, 180Hz, 90Hz - so we're at the 10th harmonic before we hit the frequencies that are being reproduced by the speaker.

Even with a lower switching frequency (say 60Khz), we still find ourselves close to the 10th harmonic before it's in the range we allow to reach the sub (30K, 15K, 7.5K, 3.75K, 1.80K, 900Hz, 450Hz, 225Hz, 112Hz, 60Hz).

And it's not just the volume drop-off difference...it's the number of harmonic frequencies played. Take the first instance. A woofer plays between 100Hz and 2.5Khz - with a 100Khz switching freq, there are 4-5 harmonic frequencies. The tweeter on that switching freq is seeing 5-6 harmonic frequencies (depending on xover being at 2.5Khz or 3Khz). The subwoofer sees 2 (90Hz and 45Hz) very low order (10th and 11th respectively).

The same holds true for lower switching freqs (60Khz).

 

Right, because you have power, then you don't, then you have power, then you don't...

A/B amplifiers are powered both ways, the Class D is powered only one way (the push). The suspension on the subwoofer is what brings it back to zero, not the current. This is why infinite baffle subwoofers can't handle as much power (and, in addition, why enclosure volume is so important - the vacuum created when the cone is extruded helps the suspension)

 

no, that diagarm is showing an output voltage which is varying from positive
to negative, not positive (or negative) to zero. As I understand it, when the
input voltage is positive, the amp generates a positive PWM signal, and
when the input signal goes negative, a negative signal is generated. The
signal then passes through filters which restore the signal to sinusoidal AC.

 

You guys lost me a page ago.