[R-390] class d amplifiers

[Bob Camp]

Hi

Not to bash the R-390, but it’s not as RF tight as it might be, at least not after you pull off the top and bottom covers etc. To really get the best out of it, you don’t want a bunch of local RF right at the radio. I can fairly easily set up a signal generator and hear it on the 390 without any need to attach it to the antenna input. 

Non-switching MOSFET based amps are pretty cheap. I’d stick with one of them. By the time you properly filter / shield / suppress the class D amp (if you can at all), you will have spent more money than you might have saved. 

A 390 starts listening (very well) not to far above the frequencies these gizmos switch at. You don’t have to get to a very high harmonic to have trouble. They very much need to be “square” out to the 5th or 7th harmonic to keep any sort of efficiency at all. That’s at say 40 or 80 V p-p. You need to get that down by 120 db or more ….

Bob

On Feb 8, 2014, at 10:39 AM, John Wendler <wendlerjrv at gmail.com> wrote:

> I worked with a ti class d amp about 4 years ago for a UHF radio product.
> 
> I cannot speak to the HF noise generated by the chip you identify, but I
> would certainly put any class d amp in a good shield with feed through
> filters as a precaution.  Just because it meets FCC does not mean it won't
> cause you heartburn in proximity to your antenna.
> 
> Several of the reviews mention white noise when the volume is down - I
> might call apps engineers at several different class d manufacturers to see
> if that is characteristic of the class d or whether their product is
> better.  Analog Devices and TI come to mind.  You would need to buy one of
> their eval boards if you are unable to work with SMT - many of those chips
> heatsink through a ground paddle in the bottom of the chip.
> 
> The biggest problem I had was trying to measure the output with an
> oscilloscope.  The output on my chip was full differential; hooking the
> ground lead to the chip blew the chip out.  This chip is similar.
> 
> You have to use a differential probe or two probes with your scope channels
> in a differencing configuration.  You cannot use a speaker where one
> terminal is grounded.
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