Path: utzoo!mnetor!uunet!husc6!cmcl2!phri!cooper!gene
From: gene@cooper.cooper.EDU (Gene (the Spook) )
Newsgroups: sci.electronics
Subject: Re: switching amplifiers
Message-ID: <1190@cooper.cooper.EDU>
Date: 3 Feb 88 01:10:05 GMT
References: <144@Giraf.dde.uucp>
Organization: The Cooper Union (NY, NY)
Lines: 57

in article <144@Giraf.dde.uucp>, mbe@dde.uucp (Martin Berg) says:
> 
> In article <3539@megaron.arizona.edu>, wendt@arizona.edu (Alan Lee Wendt) writes:
>> Is it possible to build an amplifier just like you would build a
>> switching power supply?  Except that instead of feedback circuitry
>> to maintain level dc, change the feedback so that the output is
>> maintained at whatever the input is, times 1000.  What would such
>> a beast be good for?
 
> Some years ago, Sony made an amplifier (type identification ?) where
> the output transistors where driven in on/off mode (i.e. a switching
> output stage).
> I think they called it 'Class D' operation.

The circuit (amp) you're referring to is either a class D, E, F, G, or H
model. Most of them are similar, except with a few unique features which
were patented by the company who developed it first. In this case, I
*think* Sony did the Class G amp. This sucker acted like a variable-
output SMPS (switch mode power supply), and the switching frequency was
near or actually in the MHz range. Also, that beastie had to be fully RF
shielded to keep from killing any other RF stuff that happened to be near.

Class D amps are great for motor controller circuits; the natural
inductance of the motor acts as a low pass filter. As for speakers, I'm
not sure the coil inductance is enough to filter out high frequency
components (as if anyone could hear them  ;^) ), but these components
may just leak out through the speaker wires enough to cause trouble.

The Sony version, if I'm not mistaken, uses two power supply voltages
of each polarity, for example, V+, V++, V-, and V--. The lower-in-
magnitude supplies are for low power outputs. This keeps the P(out)/P(in)
ratio as low as possible. When extra power is needed, the higher-in-
magnitude supply kicks in through a blocking diode to provide the
needed voltage. (Forgive the wording, but I guess you know what I mean.)
A good simple reference might be _Oscillators_Simplified_ by Delton Horn.
There is a chapter which describes some of these classes.

(By the way, if anyone has a more technical reference to the different
amplifier classes, could he/she email me a listing? Thanx in advance!)

Basically, Class D's started things off. Different variations and
improvements became different classes of amplifiers. At the very least,
it sure is an interesting subject.

					Spookfully yours,
					Gene

					...!ihnp4!philabs!phri!cooper!gene


	"If you think I'll sit around as the world goes by,
	 You're thinkin' like a fool 'cause it's case of do or die.
	 Out there is a fortune waitin' to be had.
	 You think I'll let it go? You're mad!
	 You got another thing comin'!"

			- Robert John Aurthur Halford