Why no AGC in amplifiers?

[reedal]

So, to start off, I'm posting this in the D'Amore section because I hope it will get the attention of Tony D who I'm quite sure knows more about amplifier circuits than 90% of the people on this forum. I know enough about electronic circuitry to do my job in the navy, which was a lot of different courses crammed into 9 months. I'm no pro, but I can get by.

Second, for those who don't know what AGC is, it stands for automatic gain control. It is used in a lot of communication type radios and systems. AGC takes whatever input voltage comes in, low or high, and adjusts the output to be the same consistently. It is by no means a new technology in the electronic world considering the age of some of the things it is used in. For all I know it's used in our everyday radios/stereos/head units (I really don't know).

As for the question, why aren't we using automatic gain control in our amplifiers? Why is that not incorporated into the design of the circuit boards? With my recent testing of my preouts from the pioneer 80prs (there's a thread about it) I discovered for myself that the AC voltage your head unit outputs (via the RCA preouts), varies with the frequency you are playing in order to set your amplifier gains. So why is there no AGC in these amplifiers to constantly match that preout voltage coming in? It would create a more consistent output power to our speakers, which is what we all want anyways correct? Granted, you would still have varying output power due to impedance rise, but I believe that it would be a higher output if this AGC were incorporated into the design of the audio amplifiers we use everyday.

This question hit me like a brick wall this morning, and I've been itching to ask it all day long.

Is there a reason it is not used for this application? If so, why?

Is my thought process wrong?

Or maybe my test of the RCA outputs varying AC voltage was inconclusive due to my use of a cheap $30 DMM from Home Depot? I don't believe it is, but if someone can prove that wrong, please do.

Please, discuss, ask questions, whatever. There's a lot of people on here smarter than I, and this is definitely discussion worthy in my mind, so any input is welcome, and hopefully the great Tony D himself, and hopefully Tony C too, will grace us with their words of wisdom in this area, being designers of some amazing circuitry themselves.

Thanks!

[Jessica]

i can imagine an amplifier racing around adjust gain to match the signal and all your music will lose all of its dynamics. Basically you will be play the same volume all the time.

[reedal]

Right you are. I didn't even think about that. Well, that ends that. Damn. It didn't even come to mind that the voltage changes with volume adjustment on the head unit too. Welllllll

/thread

If any mod wants to lock this up to prevent further discussion of a pointless topic now that the variable I forgot has been brought to attention, that would be great. Good shit, duck.

[reedal]

Bye-bye EQ, volume knob, etc.

Damn. And there I was thinking I was onto something. There's always a forgotten variable.

[Jessica]

I think what you are after is more like automatic gain limit, keeps you from pushing the amp into clipping. That would be ok, except people would hate it because far too many kiddies only get loud by cranking the gain like its a volume knob. Also, some people who compete push into clipping to gain a tenth or so when needed.

The other factor is that sinewaves are busy busy busy, look at music and not just a clean test tone and imagine trying to determine if any of the mumbo jumbo is clipped or not. Hell some music has sines that look like clipping but arent.

I think the cost to build a logic gate that could filter all the out would make an amp prohibitively expensive for mass market, especially when they are tools like o-scopes and dd-1's that make setting gain correctly pretty easy

[reedal]

A gain limiter would be interesting. But I guess you're right. The man hours to design it and then the materials to produce it would be costly compared to an already working product. Because the more people clip, the more damage they do, the faster it dies, then they buy another, putting more money into the company. Although, I will say I clipped the ever-loving fuck out of my kicker cx1200.1 before I understood how to set gains (3/4ths of the way up ftmfw right) and that thing is a tank, even at .5 ohm when only rated to 2ohm. And it still works perfectly to this day. It's definitely a stout amp. Maybe there's a reason why they're products are priced so high compared to other brands that offer more power for less money. Reliable brands that is.