We’re in the middle of our series on amplifier classes (not learning classes, but output classes) and getting on to understanding one of the very different classes of analog designs, Class D.
Along the way we’ve covered Class A, B and A/B. Today I thought we’d take a side street over to one of my favorite designs, Nelson Pass’ Aleph series. The first I knew of this was from the cover of a Hi Fi magazine showing the amp frying an egg.
I am sure Nelson wasn’t thrilled with this picture, but I also imagine he smiled as he’s got a really good sense of humor.
The point of this is heat. Here’s a Class A design that does not use two transistors in its output as I explained yesterday, solving one of the big problems we discussed.
Instead, this design uses a single transistor arrangement (there are actually multiple transistors but they all work together doing the same thing) to cover the entire signal. This type of design is even less efficient than what we described as Class A, yet has the advantage of not dividing the amplification duties between two devices (upper and lower halves of the signal).
If you’ll recall, a typical output stage of a power amplifier handles the signal in halves, the top half has one type of device and the bottom half another. These devices, in a solid state design, are very different transistors known as NPN for the plus half and PNP for the bottom half. If we were using a different type of solid state device called a MOSFET, the terminology would be slightly different: N Channel for the top half and P Channel for the bottom half.
The point is the signal is handled by different styles of devices and each style has different characteristics. When a designer puts together a pair of these device (known as a complimentary pair) he uses pre-matched types that are close to each other but not identical.
What Nelson did was use a single N Channel device to cover the entire range from plus to minus, thus never suffering from this handoff between the two device types. Many people love the sound of this design (as do I) but are a little hesitant as it produces a lot of heat for not many watts (as you can see).
You may have heard of this type of amplifier, it’s called Single Ended.