The pure ambiguity of high end sound systems
The purist may rate the ultimate audio amplifier as being a straight piece of wire with gain. Possibly driving his or her Sennheiser HD800s. In South Africa this may well put you back R40 000.00. Maybe it’s time to put aside the Woo Audio, Schiit or Linear Tube and take a listen to Class D. On real speakers.
I personally like the Asus Xonar U7. I also use entry level headphones, the Senneheiser monitor cans HD380. A quick solution to what I would call very good quality. Others will disagree.
The problem with audio is never as much the amplifier as the loudspeaker. Modern speaker systems need a lot of drive. Power is now a cheap commodity with the current ranges of Class D hitting the shelves. 300W per channel Class AB is nothing new for the home either but at a price, inefficiency, weight and size. The purist may hate the idea of even testing a Class D audio system but the truth sadly is that, like tube gear, Class AB is losing ground rapidly. Not because they lack appeal but because of manufacturer focus.
Bang & Olufsen & ICEpower
There are more than a handful of manufacturers migrating to switching and digital amplifiers. One such manufacturer is Bang & Olufsen with their ICEpower range. Interestingly enough if anyone had a Bang & Olufsen setup when I was a kid, thousands of years back, was held in awe. These were the systems to be had. Yet quite often when it came to quality of reproduction critics were more ecstatic about the looks than the quality. I was one of those. Don’t get me wrong. The Danes make awesome gear, looks, build quality and sound reproduction wise but their range of Class D takes the cake. The ICEpower 700ASC is one typical example. So what are we are we on about?
The Road Ahead – kits, chips and power supplies
Class D is often bridge tied. A 30V supply would therefore be capable of driving a speaker load of 4 Ohms at 100W give or take potential circuit losses. Class D is often run off single rail supplies.
Class D is also significantly small in form factor as opposed to conventional A and AB designs because of their efficiency and therefore smaller requirement for cooling.
Class D lends itself easily to driving 2 Ohm loads. One needn’t parallel a whole bunch of BJTs or Mosfets to drive small load impedances, cheap high current Lateral or Hexfets are plentiful. Example: IRFB52N15D sells for 40.00 excl. at Mantech.
When it comes to self-construction or possibly destruction it now becomes also apparent that buying kits or ready made one can get the best of both worlds, they are very price comparative and we now also have a PCB. With low EMI (hopefully). One such board is the 250+250 into 8 Ohm class D amplifier from Parts-Express. Note the highTHD levels at rated outputs.
TPA7498 by ST Micro Electronics is another example of a very popular chip, possibly in most cases cheaper to buy the entire module than making it yourself.
Class D components are nearly always SMD and board layouts are small and precise.
The state of manufacture
Almost all manufacturers are leaning towards the Class D topology and almost certainly totally justifiable.
- Total cost of ownership for the owner – cheaper
- Weight – for those pro users – lighter (with SMPS)
- Efficency – claims of up to 94%
- Power supply – cheaper
- Specifications – on par with Class AB
- Bass response – excellent; and last but not least…
- Manufacturing and shipping costs
Distortion levels tend to be on the high side when compared to conventional designs but with the extreme high power outputs nowadays who needs to use the amplifier at full power in any event? Driving these amplifiers into clipping is a big no-no.
Disadvantages (my opinion)
Class AB or A designs seem to have a better power bandwidth at the high end of the audio spectrum. Manufacturers are quick to point out that this is not so with modern switching amplifiers.
Difficulty in repair. One cannot apply the American Shotgun approach to amplifiers of this type. Class D amplifiers are often powered from SMPS which can be sophisticated and difficult to trouble-shoot. Repairing board tracks can lead to the possibility of EMI. Active components may have a relatively short life cycle as manufacturers bring out improvements to switching chips.
Having said all of the above, modular builds are the way to go. Ensure the PSU and amplifier modules (and pre-amplifiers) are not all on one board.
Class D has a lot to offer and won’t go away.
- Designing Practical High Performance Class D Audio Amplifier – Texas Instruments (pdf)
- Design Considerations for Class D Audio Amplifiers – Texas Instruments (pdf)
- Class D Audio Amplifiers – What, Why, How – Eric Galaas, Analog Dialogue(pdf)
- How to Design a Class D Amplifier – Joey White, Hephaestus Audio (pdf)
- Yamaha Audio – The importance of stability driving 2 Ohm Loads
- The Class D Amplifier – W. Marshall Leach (From the book Introduction to Electroacoustics and Audio Amplifier Design, Second
Edition – Revised Printing, by W. Marshall Leach, Jr., published by Kendall/Hunt,
Read Next: Part Two – Practical Examples