PS AUDIO: Class F amplification

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Paul McGowan writes: I think one of the very basic problems with people’s opinions on class D amps is the hierarchy of labeling – I mean, as a child if you get an “A” in school you’re pretty happy; get a “D” or an “F” and you’ll probably be having a conversation with your parents.  So naturally class A seems better than class B or C or D just from the sound of it.  ’Tis a shame we use such a system to identify the type of amplifier topology.

Yesterday I began to describe the importance of the input stage on a Class D amplifier.  Imagine taking the worst sounding cheap receiver or preamp you can think of and placing it in front of the best sounding power amp you can think of.  What do you think the results would be?  Would the great sounding power amplifier make the poor sounding receiver or preamp feeding it better?  Or would the better sounding power amplifier merely magnify everything wrong with the device feeding it?  I would go with the latter as it’s been my experience that garbage in equals garbage out – just more of it.

Such is the importance of the input stage because this is the point that we can make some of the most significant improvements and important sonic decisions for an amplifier and the way it is going to sound.  Place a tube or a FET in the input stage of a well designed class D output amp and the entire amplifier will sound very much like a tube amplifier.  Remember the old MOSCODE hybrid amplifiers from “Dr. Gizmo”?  These were fine sounding amplifiers with a tube front end and a solid state output – exemplifying perfectly my point about the importance of the input stage.

I cannot emphasize just how important this stage is.  In our upcoming PerfectWave Power Amplifier we went through a number of variations and found significant benefit in building an all FET stage with parameters very much like a tube – in fact, this amplifier sounds, for the life of me, more like a powerful tube amplifier than anything I have ever heard.  Why?  Because of the input stage.

Here’s a great rule of thumb for you: if you do not honor the music at the input to a device, everything you do that follows is only adding insult to injury.  Really.  You must get the music to the output stage without any damage whatsoever, and then you’ll have a chance at building a musical amplifier.