[R-390] Class D Amplifiers

[Charles Steinmetz]

Barry wrote:

>A while back, there was some discussion on this list about outboard 
>audio amplifiers.  I don't recall whether these came up, but has 
>anyone worked with Class D amplifiers?  If so, are they troublesome 
>- specifically with respect to noise?

I have considerable experience with Class D amplifiers that put out 
serious power -- from 500w to 4kw (our first test mules were in the 
10-20w range).  Switching noise can be a real problem, although that 
can be minimized with good design.  However, even if you solve the 
switching noise problem, we still don't have fast enough switches to 
run the chopper as fast as you'd like, so the cutoff of the 
reconstruction filter is too close to the audio band and its poor 
group delay sounds awful compared to a linear amp.

>Just looking at the one below and thinking that, at the price, it 
>would make a decent amp for the R390.

Looking at the picture, the amp does not appear to have a 
reconstruction filter at all.  No surprise -- the PAM datasheet does 
not discuss reconstruction filters, and the application circuit 
(which I believe is copied by all of the board-level manufacturers) 
does not have them.

The PAM datasheet does note that the amp fails to meet FCC Part 15 
regulations with respect to noise output without additional filtering.

So, I think you can assume that this thing creates lots of switching 
garbage and you would rather not have one within a few hundred yards 
of your radio or antenna.

Best regards,

Charles