REAL ADVICE ABOUT PICKING AUDIO POWER AMPLIFIERS

I'm always weary of reviews from magazines. There are hardly any bad ones in fear of "burning bridges", getting sued, or losing a potential advertising client. No magazine in their right mind will admit to this. In fact they will deny it tooth and nail. Same deal for "product rewards"; many are bought and paid for indirectly.

Reviews that simply show measurements and no opinion whatsoever can usually be trusted, but are hard to find. Measurements can't tell you everything, but can tell you a lot. Manufacturers rarely falsify or embellish measurements due to fear of getting caught. However, there are some tricks that can make measurements look better, such as rating power at 10% THD, but this is also very rare. The most trustworthy source of how something sounds (beyond measurements) is actual customers. However, people have a tendency to like whatever they bought, so watch out for reviews that have no comparisons to anything else.

Regarding well recognized brand names, don't believe that just because a brand is well known and has a long history that they only make spectacular products. I had a WELL KNOWN (maybe the MOST well known amp brand of all) amp and was explaining my total disappointment with it to a friend. He said something like "I can't believe you bought one, everybody knows that amp sucks. It sounds just awful and was a complete failure for them". He also said that despite the bad sound of this design, they sold a TON of them.

One important figure is signal to noise ratio. Anything less than 100dB with respect to rated power should be avoided like the plague. There's no reason in this day and age that you can't design just about any type of amplifier at least that good. Noise is the enemy of good sound! If there is an SNR measurement at 1W, look for at least 85dB. This is A-weighted. Unweighted SNR is not as meaningful of a spec since the human ear has frequency dependent sensitivity curve. The "ear near the tweeter" test is actually quite useful if you have a single tweeter that's no more than 2" long in any direction. Listen not only to how much noise there is but also the character of the noise. Quality amps are dead quiet or close at idle.

Look for amps with THD+N below 0.01% for a good portion of the power curve and at all frequencies. This is especially important at low levels, so look for THD+N well under 0.1% at 1KHz, 100mW. Unfortunately, many manufacturers don't provide this spec, and if they do, it's probably on a graph (THD+N versus power), not in text, and sometimes you need to interpolate. Look for amps with direct coupled output. Output transformers are fine for distributed audio in your local supermarket sound system. They saturate at low frequencies.

Another one... Damping factor under 50 is a red flag. If damping factor or output impedance (reciprocal of this divided by the load impedance is damping factor) is not mentioned, it's probably poor. Also, as damping factor goes higher and higher it means less and less. I doubt any human being can hear the difference between an amp with a damping factor of 300 and one with a damping factor of 450 that's otherwise the same.

Regarding the "sound" as perceived by any reviewer comparing two or more amps, if the comparisons aren't double blind, they aren't super valuable. The mind simply can't remove bias reliably otherwise. Single blind is much more meaningful than non-blind. You might also be surprised how many so-called "golden ears" fail double blind listening tests (essentially can't tell the difference) when the amps are similar! It's funny to hear the excuses when you call them on it. This is especially amusing with aftermarket modifications. My theory is "buy something that was designed right in the first place".

Anyway... Regarding output power, watch out for manufacturers of anemic amplifiers that tell you their 30W amp is great for driving your 10-driver per channel tower speaker system. You need adequate power to drive your speakers through peaks unless you never "turn it up". Big speakers need big power to play loud and clean at the same time. I recommend a few hundred watts minimum per channel unless you're driving a pair of bookshelf speakers in a small room. Also watch for amps with lots of power but bad specs otherwise. Beware of amps with a spec called "instantaneous peak power" of something similar. This is technically twice RMS output power at rated output right before clipping. If you see this rating and it's NOT twice rated power, there's something fishy going on. This spec is used deceptively so beware of its use either way.

Regarding inputs, when possible, insist on balanced inputs or both unbalanced and balanced. Balanced audio is inherently better due to built-in noise cancellation, and if you can afford a good preamp with balanced outputs, use one for your primary audio system.

Here's something I wrote regarding pro amp measurements more than 3 years ago:
http://www.livesoundint.com/archives/2004/dec/power.pdf
Yes, I worked in pro sound for several years, but learned a lot. I know lots of high-end people think anything pro audio is junk. They are just wrong.