There are many types of "gain clones" now. In general it has come to mean a small and easy to build amplifier built from an op-amp (sometimes discrete power audio transisters, but usually not...also remember that op-amp is a general name for a type of amplifier that is often built from many transisters, MOSFETs, and such inside it...as in a complete package that behaves in a certain way). Since there are many upon many types of op-amps, you can get many different quality and power ratings. Since all op-amps are wired together about the same, one design can generally be used for another.
I have 3 or 4 different op-amps I purchased last year I'm trying to get working correctly. I think with that latest schematic I mentioned I now have the missing pieces that nobody ever talks about if there is trouble. I have the new schematic drawn out and on the dump site.
The op-amp's I mentioned to you in the link are the higher powered and mostly better versions of the many types available. Each has a purpose and people war constantly over which is better (and sub-war over which power supplies and designs are better). My few I have are lower powered and mainly designed to power computer speakers, car radio's, and TV's.
You compare quality by comparing the various frequency, rise time, and distortion specs between different op-amps. The nice thing about building your own is that you can increase quality (slightly) by purchasing better and some bigger parts than what you'd ever get in crappy consumer equipment. In reality, most designs are about the same quality wise...provided that the design is decent to begin with. When people war over which is better, that is highly subjective. Speakers, mounting cabinets, room noise and dynamics, and even if the person has unknown hearing damage are huge variables that are difficult to quantify. That's part of the reason I'm so interested in doing my calibrated frequency sweeps and tests. It's hard to argue the science of those.
With all that being said, don't ever buy $10,000 components. The "audiophiles" (as defined by wikipedia.org) who preach that crap are morons. I have to be careful here because I fall into peripheral audiophile category in that I like to build and tweak, but I have no belief whatsoever that a vinyl record player + vacuum tube amplifier is better than a CD player + modern amplifier (given both are well designed to being with). If you ever want to spend that much money, give it to me, I'll give you back a $100 amp and pocket the rest. :)
That's not to say your completed home theater system will never be more than $10k, just that your individual components shouldn't.
The biggest thing that makes the hugest difference in sound in a home theater setup is the speakers (then followed by the playback source, then followed by the amplifier and EQ equipment...all of which should be low noise and low distortion). No matter how good the sound source, crappy speakers = crappy sound. In the home theater setup, a good set of speakers will cost some real money. I really want to try and build my own so I can mix and match better speakers than what's available for the general consumer (general consumer = expensive crap). I think part of the reason the living room speakers sound so much better than anything else I have (both large and small) is the tweeters/horns. Clean high frequency range makes for clarity. Clean high frequency is something I just don't see in crappy consumer speakers. A good tweeter/horn isn't that expensive, so I have trouble understanding consumer grade speakers.
Putting amps and speakers together, I want to do what is commonly done in the professional world of bi-amping or even tri-amping. This means there is a separate amplifier for the low frequency woofers and a separate amp for the higher frequencies (tri-amp is a third amp for the tweeters). Sub woofers also have a separate amp but are in a separate speaker box. Here I'm talking about a single speaker box with 1 to 3 screw terminal blocks on the back, each capable of using a separate amplifier. My big PA speakers have the terminals for bi-amping on the back.
Why is all this done? When a coil of wire is moved around a big magnet, it acts like a little generator and creates current. This happens to the speaker on rebound. Sending current back into the wires provides unwanted energy that moves the other speakers. If the other speakers move when they aren't supposed to, that is distortion and bad.
Low frequencies also require a lot more power to move than higher frequencies. In a balanced system, a low frequency woofer may need 50watts of power while the high frequency horn in the same box may only need 5watts. A good engineer will split the loads and put a big amp on the woofers and a smaller (and usually much clearer) amp on the high frequencies.
This way power is evenly distributed for the best of both worlds and the back EMF problem caused by the big speaker doesn't bleed into the other speakers.
Enter the op-amp into the equation. These are cheap (about $10 + whatever for parts and power supplies) and relatively simple to build. Given that, I can potentially have one op-amp per speaker driver for both matched efficiency and clarity. I'd also probably need a pair of small signal level op-amps to act as pre-amplifiers/buffers to feed the power op-amps so that the signal doesn't get sucked too low.
THIS RIGHT HERE is where you get the $10k amp quality for a fraction of the price. And in reality, the op-amp setup would probably sound far better if you used decent parts. Remember the "prosumer" wannabe class is all hype and 1000% profit with very little delivered on that in actuality. This is partly where the build it yourself audiofiles preach their religion...and in that aspect, many are correct.
Before the next paragraph: Amplifiers come in different class ratings. Generally there are A, AB, B, and D. Class A amps are power hungry monsters that are designed for supreme clarity. I have a Class A Reference Amp in the rack in the storage room. It is heavy, very low wattage out, eats lots of power, but has an extremely clean and noiseless operation (which is why it is called a reference amp). A Class B Amplifier is a much lower power amp but suffers a lot with high frequencies (crossover distortion). Unfortunately these are often used in TV's, computer speakers, and cheap car stereos. A Class AB amplifier tries to merge the best of Class A and B. It wastes less power than Class A but more than Class B. It isn't as clear as Class A but is far better than Class B. Class D Amplifiers are somewhat recent and act more like a variable switching power supply than anything else. This means they are very power efficient. Their weakness is in high frequency clarity, though. Class D is gaining popularity (and getting its weaknesses solved) but still has a way to go before full and widespread acceptance.
Now merging Class info with everything above, this is what I want to do...
I have a Class B op-amp chip I want to use with a small bookshelf speaker I rebuilt into a mini sub-woofer. Since Class B sucks for high frequency and sub woofers aren't supposed to get high frequencies, this should be a good match. Class B also wastes less power and sub woofers suck power like there is no tomorrow.
If this works out ok, I'll also try hooking it up to a regular woofer to see how it handles. Once again, the results should be good...at least on paper.
Moving up one, the midrange will be on a Class AB Amp with slightly lesser wattage needed to drive the same sound levels out. AB has better frequency response but wastes more power than Class B. Overall this is reasonably balanced.
Moving up to the tweeters, this will be on a low wattage Class A amp. Since tweeters don't suck watts and Class A isn't power efficient, this should be reasonably balanced. Since tweeters do need clarity, Class A should be the best that can provide that...provided I can find a Class A op-amp. There are "hacks" to bring an op-amp into pseudo Class A mode.
Once again, since all these speaker drivers have their own amplifiers, back EMF won't be a problem.
Also looking at this type of design, the power hungry low frequencies won't try to suck power away from the power efficient higher frequencies, thus leading to an overall more consistently balanced audio output no matter what is being played.
Oddly the active pre-amp/crossover circuits will probably be more complex than the power amplifiers driving the speakers.