Any amps that "double-down" into 4 ohms in a magazine/lab tests? (1 Viewer)

[Ricky T]

....and more marketing BS behind "doubling-down" capabilities.

 

[Dennis B]

There's a very interesting article on Dec. 2001 Hi-Fi News, called "The Weighting Game". The author claims distortion (THD) is a very good spec to rate amps, provided a weighting scheme like n^2/4 (suggested by Shorter and Wigan back in 1950) is used, instead of the current unweighted measurements. In the above formula, n is the harmonic number. This also accounts for intermod distortion, which is much more important than harmonic distortion for complex signals like music and speech. It also correlates perfectly with subjective tests taken to classify amp distortion, which I found very impressive.

He ends stating that Shorter's scheme may not be used simply beacause in 1950 there was no practical way to apply it or because of audio industry indolence.

Recommended reading.

 

[jeff lam]

Many of the top car audio amps are rated to double power into a 2 ohm load from a 4 ohm load. Usually the THD rises when you drop the impedance. It's not really that important of a spec to be able to double the power into a specific load. Just get the amp with the proper power specs for your speakers. Who cares about doubling power if you won't use it this way?

 

[Peter Johnson]

There's a very interesting article on Dec. 2001 Hi-Fi News, called "The Weighting Game". The author claims distortion (THD) is a very good spec to rate amps, provided a weighting scheme like n^2/4 (suggested by Shorter and Wigan back in 1950) is used, instead of the current unweighted measurements. In the above formula, n is the harmonic number.

Having said that, THD is easy to reduce. Use a bit of global negative feed back, and all of a sudden youve got 0.0000001% THD at 100W.

The problem is global negative feedback, which is used in most amp designs is almost universally accepted to sound like crap.

 

[chung]

Global feedback, implemented properly, sounds great. When not implemented properly, sounds not so great.
I don't think it is universally accepted at all that global feedback sounds like crap.

 

[Saurav]

Hmm... I think it seems to be accepted that if you can avoid global NFB by carefully designing the rest of the system, that is the preferred way to go. Of course, that can easily be taken to the other extreme, where the design makes other compromises just so the marketing literature can proclaim "no global negative feedback!". As in most things in life, balance is usually the key. Also, a lot depends on what you're trying to do. Designing a DIY home-brewn amp to drive a specific pair of speakers is totally different from designing a commercial product which would be expected to work with a wide range of speaker loads.
Spoken with all my vast experience of amplifier design, which is precisely zero

 

[chung]

Suarav:
If you want to read about power amp design, check out this paper:
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The key to great solid-state amp design is to minimize distortion of every stage before global feedback is applied. Any paying attention to every detail helps, too.

 

[Saurav]

I wish you'd start spelling my name right That's a really neat paper. Who is Douglas Self? Interesting how most of his references are his own papers... of course, I wouldn't recognize the name of a top designer even if I saw it in there. Have you listened to any amps designed/built by him? Is his 8 category distortion breakup a generally accepted idea in the industry?

The key to great solid-state amp design is to minimize distortion of every stage before global feedback is applied.

I sometimes wonder if guitar/keyboard amp designers have more fun, because there you're actually trying to work with the distortion, instead of always trying to stomp it out.

 

[chung]

Saurav:
Sorry about mispelling your name.
I like Doug Self's approach to power amps. It is very straight-forward, and to me at least, shows that he has extensive experience working with this type of circuits and he understands them well. A lot of what he talked about on that page are fairly commonly understood among circuit designers, but it is nice to see him summarize those concepts neatly.
He said that over 20,000 of those amps have been built, so I assume that the amp is available commercially, although I don't know what is the brand name. Perhaps some UK brand?
If you are a DIY'er, you should definitely check out his webpage on op-amps:
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His experience certainly agrees with mine in that area.

 

[Peter Johnson]

Douglas Self is a die-hard objectivist.

I dont have a problem with that. Being an EE student, I too am of the opinion that everything can be measured.

However, having said that, I have heard MANY amps with stunning specs (krell and bryston are examples) which IMO sound pretty average compared to another worse speced amp...such as a tube amp.

So the question is..are we measuring the right things?

 

[Larry B]

Peter:

So the question is..are we measuring the right things?

In many cases, apparently not.

Larry

 

[Dennis B]

Peter,

So the question is..are we measuring the right things?

That's exactly what that article was about, even presenting strong evidence of correlation between measurements and subjective testing.

 

[Saurav]

Who knows if we're measuring the right things or not. With audio, I tend to be more of a right brain kind of a guy... I use my left brain at work enough anyway. So, I'm much more interested in emotional involvement with the music, rather than objective observations on the equipment. Which results in me judging equipment based on how well they draw me in and allow me to lose myself in the music. Just the other day, I was thinking about how life is too short to worry about soundstage and imaging. This happened while listening to a Doobie Brothers album, when half-way through I suddenly realized that the main vocal wasn't coming from dead center, and I hadn't even noticed. All too often, I spend less time listening to music and more time fiddling with speaker positioning and toe-in trying to get the soundstage centered and spread out correctly.
To me, a good system is one which brings a smile to your face, makes you jump out of your seat and play air-drums along with the music, and so on. As far as I'm concerned, that's the whole point of an audio reproduction system. As you can see, die-hard objectivist I am not

 

[Tony Baker]

What about amps that have the same power ratings for different ohms? The only example I have seen is McIntosh.

 

[Duke H]

Saurav-Yeah, we musicians do have all the fun! One thing to point out, though...Keyboard amps are supposed to be clean. Guitar amps are ALWAYS supposed to be dirty. I've heard it said by many an ear (usually in pairs ) that some amps, especially Roland 120's, are TOO clean. Many Jazz guitarists want a "pure" tone, though. Even when an amp is on a clean setting, it will still have more distortion (we prefer the term "warmth") than most any HT power amp-I'd compare the overall level to that of a cheap shelf system or boom box. You have two schools of thought on the distortion issue, though. First, you have "processed (ie, solid state and/or digital)" distortion, and "natural" (tube) distortion. Although tubes are prefered, solid state is by far the more common. I like both. For a good example of solid state distortion, listed to "Should I Stay or Should I Go," by the Clash. For a good example of tube distortion, listed to most any Clapton or BB King recording. Digital, or "Modeling" amps suck, IMO. On a different note, I sometimes wonder if "low-end" Pioneer amps are built by musicians...loads of features (check out the 850), and loads of distortion!!! (Rated at, and I quote, "No more than 2%" THD @ 1 kHz.)

 

[Duke H]

Bump

 

[Saurav]

(Rated at, and I quote, "No more than 2%" THD @ 1 kHz.)

Well, there was a post here a while ago about research that had shown that THD below 2% is pretty much inaudible. I don't think THD correlates directly to musical enjoyment though - you have to dig deeper into the specifics of it - is it harmonically related or random? What's the spectrum of the distortion? And so on... I've enjoyed most tube amps I've heard more than most SS amps I've heard, even though objectively the SS amps are the better performers.

 

[Duke H]

Saurav-yeah...the rotating horn is a really cool psycadelic effect. The cabinet is called a Lesley, and has been used by Robby Kreiger of the Doors, Stevie Ray Vaughn, and quite a few others. As far as the THD thing goes, a good rule of thumb seems to be "keep it below 1%." I doubt that 2 % is audible, but at work I have a Pioneer 811 hooked up next to all the Yamahas, and the difference in tone is unbelievable. The Yammies sound SOOOO much better, but the Pioneers have more features (excluding Yammie's DSP technology). I agree with you all the way on tubes. I'm hopping to steal my dad's old stereo tube amp. I'm not sure about the specs, but I'd like to give it a go.

 

[Saurav]

Well, I built my tube preamp from a kit ($300 with my upgrades, $150 for the basic kit), and bought ASL Waves which are 8W tube monoblocks and cost $200/pr. If you have efficient enough speakers with a flat tube-friendly impedance, these are great little amps, IMO. And at $200, they're cheaper than almost every respectable SS amp.

Lesley, that rings a bell.

 

[Duke H]

The examples I listed were guitarists, but the Lesley cab was designed for keyboardists. Most of the distorted tones you hear from keyboards are actually preamp "synth" tones. That's why keyboard amps are clean...so they don't "colour" the tone of the synthesizer.

I actually think I'm going to go with a tube power amp and a solid state preamp. I have JBL and Yamaha speakers, neither of which are extremely efficient. What I'd like to do is use my Yamaha HTR-5560 as a preamp, and use the main preamp outs to feed a 2 channel tube power amp. I'm just wondering if I can balance the output between the fronts (tube) and the surrounds and center (solid state). What do you think?