About a simple class D amp

Meh, it's just two cathode followers in series. What gets me is, I've seen at least one 10kW shaker table amp (the manufacturer escapes me, but shaker tables is basically all they do...) which was built with about 50 x stud-type NPNs (rated about 200V 2A each) per water-cooled rail. A three-phase transformer sitting in the bottom provided isolated power to the two halves. It ran class AB or so. The whole thing (including signal generation) occupied three full racks.

Tim

Much better to completely ignore the 555 altogether. The 555 isn't bad in and of itself, it's a fine switching device and oscillator building block. However, using pin 5 to control it is essentially stupid. You can linearize it, principally by controlling charge and discharge currents, but that's extra circuit. You can also use two, one as a clock generator and the other as a variable delay, but that's still more circuitry. You might as well start over with another general-purpose 8-pin chip, like so:

Scrunched together on a bit of perfboard, this circuit hardly occupies a square inch, it's quite easy to build. Couple the logic-level signal into whatever output stage you want.

Or for that matter, if you're adventerous, you can use the LM311's beefier output stage directly for maybe 1W total output. ;-)

Tim

Another idea:

in ---!!---/\/\--+------/\/\-----+---))))---!!--- Out ! ! ! !\ HC14 ! +---! >O-------- ! !/ --- --- ! GND

With HC14, you have six gates anyway. I believe there used to be a standard design of a reasonable class D amp using one 4069 and two FETs.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

Many years back I saw one that used CD4049s. The whole circuit used the 4049 as all of the gain and switching elements. Someone did it as a joke in the first place.

Many years ago I made my own class D power amplifier. It sounded very good. The whole thing was discrete. The sawtooth was very linear and the comparators were fairly fast. This made the open loop linearity fairly good. The feedback cleaned it up the rest of the way.

Class D amps is one of the things that I design as the business.

There is big problem with the open loop linearity of the class D: the effective time of switching depends on the operating point. There is a knee in the transfer curve between the continuos current and discontinuous current conduction modes. Due to that effect, the attainable open loop THD is at the order of 0.5...1%. The feedback of the 1-st order has to be shallow, and the feedback of the higher order distorts the saw. Higher clock rate is a cure for many sins, for the cost of EMI and power. It takes a lot of optimization to get the quality audio from the class D amp while maintaining the good power efficiency.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

you'll need stronger gate-drive on the mosfets also need a transformer that can handle the harmonics without overheating.

ISTR Ling and Umholz-Dickie from way back, there were a couple of others as well.

Dickie, that's it!

Tim