nw_avphile diyAudio Member

Join Date: Mar 2003 Location: Northwest

Blind Listening Tests & Amplifiers I'll probably be outcast here, but what are everyone's thoughts about blind listening tests of amplifiers?



I've built several of my own amps (some of my own design and some from others) and spent a lot of time listening to high-end gear (which I used to sell) from the likes of Audio Research, PS-Audio and others. I firmly believed there were significant audible differences between amps as I'd heard them with my own ears.



Then I started to hear about some convincing blind tests and finally conducted my own. I was stunned at the results. I couldn't tell a $300 amp from a $3000 in the store I was working at. Neither could anyone else who worked there. It was a major blow to my audio belief system. I'd always thought Julian Hirsch (main reviewer for Stereo Review) was an idiot for arguing all high quality amps, kept within their power limits, sound the same. It turns out he was right!



Since then, I've read Douglas Self's book on amplifier design where he says the same thing--high quality amps with flat frequency response, low noise and low distortion all sound pretty much the same if kept within their power and current capabilities.



I can certainly hear the difference between a class A single ended amp (which measure very poorly) and a conventional amp in a blind test. I can even tell the digital amps I've been involved with (Tripath Class T and various Class D designs) from a class AB amp in a blind test but they also have some measurable problems. But, between high quality low distortion conventional amps that measure well, I flunk the test and so does everyone else I know.



For a recent non-believer, I used an Onkyo SR500 Dolby Digital receiver--purchased reconditioned for $200 (they're $250 - $300 new) against some well regarded separates. It's rated at 65 watts x 2 stereo per the FTC guidelines into 8 ohms. Distortion is 0.08% from 20-20k from 1 watt to 65 watts into 8 ohms. It has a "direct" bypass feature that supposedly bypasses all the digital/DSP for analog stereo signals.



The Onkyo was put up against the well regarded Bryston 4B 300 wpc power amp and a Bryston 2 channel pre-amp. They were driving a pair of expensive floor standing KEF speakers and the source was a high-end Marantz CD player. The person who owns this system is very proud of it and has spent a lot of time getting what he considers to be the best sound possible.



I had the Bryston owner pick the level he wanted to do the comparison at while listening to his system. I then used pink noise to level match the Onkyo to his system while he was out of the room. The Onkyo was running in its "Stereo Direct" analog mode.



I called him back in to listen, he sat down in the sweet spot and I replayed the same CD track he'd used to pick the levels. He immediately started complaining about how bad the Onkyo sounded. He said it sounded thin, compressed, harsh and a few other things. I smiled and turned the Onkyo off and the music kept playing. He'd said all those negative things about his own Bryston gear!



With him red faced, we proceeded to do at least an hour of listening with me swapping cables, or only pretending to, when he requested a switch. He listened to his favorite audiophile CDs. I did lots of swaps and fake swaps and during each would ask him which he thought he was listening to. In the end, his answers were roughly 50% correct which is the same as if he'd been randomly guessing. He even finally admitted, he couldn't tell which was which and WAS only guessing! I took his place and also couldn't hear any difference between the lowly receiver and his prized Bryston gear.



For the analytical among you, I've done some input/output null difference testing as well. This is where you use a wide bandwidth analog (even passive) circuit to subtract the level matched (nulled) input from the output of an amplifier driving real speakers using real music--no sine waves or resistors here. The resulting difference signal is made up of ANY distortion or deviation produced by the amplifier. Some distortion analyzers work this way.



My null tests have shown that even a modest $250 receiver can manage a -60db or better residual signal driving my relatively difficult speakers to fairly loud volumes. Higher quality amps can easily exceed a -70db null. Those are very low levels of residual distortion. Keep in mind this test reveals ANY kind of distortion--audible or not--including THD, phase distortion, IMD, TIM, slew problems, feedback issues, frequency response deviation, etc.



You could argue that you might be able to hear things that are 60db below the signal, but I'm skeptical. It's easy to hear -60db worth of noise during quiet passages of music, but it's downright tough to hear something that's actually 60db below the signal. For those of you who have a volume control calibrated in db, turn it up to a comfortable listening level and then reduce the gain 60db (if you can without shutting the sound off) and see how much is left. It's hard to hear even in a quiet room!



I'm not trying to start a flame war here, but I am suggesting those of you who think high quality amps have magical qualities like "warm", "airy", "detailed", "liquid", etc. might want to do some blind testing. Most of the differences I thought I heard evaporated once I didn't know which amp I was listening to and the others can be attributed to other factors.



It's been my experience that hardcore audiophiles will dismiss the blind tests as flawed and they refuse to participate in these tests or acknowledge the results. Most of the magazines, of course, don't do them as it would be really bad for ad sales across the board. It's hard for any manufacture to sell a $3000 amplifier when the magazine demonstrates it sounds the same as a $300 one. The same goes for salespeople at high-end dealers. The owner of our store told us to keep our mouths shut as "our test was invalid" when we made our blind discovery.



Don't get me wrong here, I'm not saying a $250 amp is all anyone needs. As you spend more money you tend to get more power, higher current capability, bigger power supplies, better construction, longer life, etc. Many of these are especially useful if you have inefficient and/or difficult to drive speakers and like it loud. But listening at the sorts of levels you'd use for critical comparisons, there's usually not enough of a difference to tell quality amps apart in a proper blind test.



I'm also not saying that some amps don't have "euphoric distortion" that some people like (i.e. single ended amps). Some amps are measurably different (i.e. have a rolled off high-end). These amps are relatively easy to pick out in a blind test. I also know the thrill of building your own amp and the extra enjoyment you get from having built it.



Finally, I've found few people that have actually participated in a proper blind amplifier test. Next time you're evaluating an amp, take a few extra minutes to level match it to a "reference" amplifier and have someone else swap cables in a way that you don't know which you're listening to. If you believe amps have their own "sound", you might be VERY surprised at the results!



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