class-D amplifier

[leppy]

We need to create a Smart Audio Amplifier for Home HiFi system. What we want is an equalized volume output with power of 50W. The input will come from a CD player, and the output will enter goto a pair of speakers. In order to do this we plan on at least two stages.. 1. automatic gain control, and 2. the power amplifier.

For automatic gain control, we initially wanted to use a feedback LED,LDR system since from our understanding the intensity transfer/detection is logarithmic, thereby providing good compression of our signal (hence volume equalization). With this circuit, we planned to compress the amplitude of the signal to a known size, and then amplifying the compressed wave in the power amp. Given that the signal is compressed enough, all sources should be equalized to approximately the same volume. We hear that this might not be such a good idea, and that there is a much easier way to accomplish AGC... any ideas?

A few things to keeep in mind... the system we desire to build needs to be able to work on all frequencies. The feedback system controlling the volume of the input also needs to be fast.

As far as the power amp we hear class-E amplifiers are quite simple and very efficient. However, we haven't been able to find much information on these online...

Any help would be very much appreciated
Thank you!

 

[a31ford]

AGC, why stop there ? that is only 1/2 of the issue when dealing with "program" in a space ocuppied by humans.

Investigate what is called an ANCA, as it will do both the inital job if AGC, AND the really kewl job of keeping the volume just over the noise of the crowd !

ANCA stands for Ambient Noise Controled Amplifier.

How they work is real simple....

program source (Cd player etc) is controlled via a unity gain process amp that is feed "room noise" via a simple microphone that is configured to be "out of phase" with the program source ! Since the sum of the program and the microphone 180deg. out of phase, will be a "nul" any noise created in the room that DIDN'T come from the speakers must be "ambient" noise, therefore if the ambient noise gets to loud, the process amp turns the main amp UP ! as the room gets quiet, the mic will sense there is no ambient, and turn the volume down again.... NEAT !

Ever had the thing while watching a movie, where two people start talking in the same room ?? you can't hear the movie anymore, with an ANCA the movie's volume level would go up automaticly until they stoped talking and then slowly go back to the original volume you had set it to.

Trust Me, these things are really neat ! I have one installed on the theater system in our house in our "rec rooms" sound system, it works great if we are watching a movie, having friends over, or out and out having a big party! simply "set it" and forget it !

Oh, BTW malls & huge stores also use them, that is why no mater what is going on, you can always hear the paging system......



Regards,
Greg,

Called "Mr. Fusion" by the Locals

 

[a31ford]

Oh, ALSO, the compression thing....

In the world of "sound recording" the known limits of compression are "0 to 0" and up to roughly "5 to 1" anything over 5 and you start getting the noise floor in the program.

(lets say 80 db noise floor, and 30db program = 110db, if you are running 5 to 1 that is 20db/step, at 5 you are 80db down from program... (the noise floor in the firstplace)if you had moved to 6 you would have 20db of noise IN your 30db of program) you can get "peak ratios" that are much higher BUT those can NOT be used in the standard "A" weighted compression ratios of AGC use.

as far as input signal level being "wild" (as in tape is louder than CD. and the TV is louder than both of those) I would get a "PRE-line level" from the sources in question, that way the internal volume controls would NOT affect the levels going to your "AGC amp"and you could set them to be close, and let an ANCA do the rest.



Regards,
Greg,

Called "Mr. Fusion" by the Locals

 

[Chuckles159]

You need an AGC and a class E??? power amp of 50W per channel? Do you mean class D? How many channels? B&O ICEPower division is coming out with a 2 X 150W module with an OEM price of $14.00, It can do less power too. They will also have a 6 channel version for $38.00. The switching 120/240V power supply for 6 channels will be another $38.00.

Texas Instruments also has class D solutions in your power range for a few dollars.

For AGC, this has been solved in the analog world decades ago. There are loads of designs at

http://www.schematicsforfree.mattsoft.com

in the audio circuits section. I now use both FET and opto types depending the application. However, my next units will use the TAS3103 DSP from TI.

Also check out

http://www.diyaudio.com