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Should a Good Amp Double Its Power?

Discussion in 'AV Receivers' started by Chu Gai, Jul 27, 2004.

  1. Chu Gai

    Chu Gai Lead Actor

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    What do you think? Why? Scenarios?
     
  2. Dave Milne

    Dave Milne Supporting Actor

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    You mean:

    "Should a good amp double its power when the load impedance is halved"

    And gosh, Chu... I'd have thought you already had an opinion on this. My answer is a definite "maybe". If cost is no object, then sure, build in enough power supply and output current capability to "double down" a couple of times. Any more is ridiculous overkill. I'd like my 250 wpc Bryston 4BST to do 500 @ 4 (it only does 400), but if it could do 2000 wpc into 1 ohm, it would be of dubious value since the deepest impedance trough of my speakers is about 3.2 ohms.

    When you throw cost into the equation, then the answer gets even more negative. If I'm joe average with a 150 wpc receiver and 8 ohm speakers (say, 6 ohms miniumum), I don't want to pay a 30% premium to be able to say that, on paper, I get 300 wpc at 4 ohms.
     
  3. Yogi

    Yogi Screenwriter

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    On paper yes. In reality probably not unless you have super conductors employed in the circuit paths and the components had zero resistance and zero dissipation. Manufacturers overstate their higher impedance ratings so the amps would seem to double its power when the load impedance was dropped in half. For example the Levinson 336 measures at:

    [email protected]

    [email protected]

    [email protected]

    So its not really doubling its power as the load impedance is halved but when you look at the spec sheet it says:

    350W/700W/1400W @ 8/4/2 ohms seeming like the amp does double its power with half impedance.

    So IMHO no amp can really act as a true voltage source no matter how massive its power supply or how many output devices are utilized. There are always power losses associated with the internal wiring/components etc.

    But I am sure you already knew that Chu.[​IMG]
     
  4. John Garcia

    John Garcia Executive Producer

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    double down, show me the money baby!

    To me, all it is showing is that the power supply is substantial and can handle a more demanding and/or lower impedance loads. I haven't seen too many that truly "double down", but many that are close. Is it a useful spec in determining if an amp is good or not? I don't know.
     
  5. Kevin C Brown

    Kevin C Brown Producer

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    For the price, here's one of the best ones I've seen:


    Review over at Stereophile.

    I don't think it says anything about sound quality per se, but it does say something about the robustness of the power supply.
     
  6. Robert Cowan

    Robert Cowan Supporting Actor

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    im going to answer with a definite yes.

    i saw this for one reason, my speakers like to drop low for bass, and they start at 4 ohms. so, for my speakers, they need not only a lot of power, but they need to have that power ready to go even down to 1 ohm.

    and if you do the "simple" math, it "should" double... power is VAC^2/R. so, by that formula, if the resistance is halfed, the power should double. speakers have a dynamic resistance profile, and if their resistance halfs, i think they should get double the power then. just my opinion.
     
  7. Chu Gai

    Chu Gai Lead Actor

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    Just wanted to throw it out for discussion Dave without interjecting my own thoughts.
     
  8. Yogi

    Yogi Screenwriter

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    But you will be giving your thoughts at some point? I would like to hear what you think.
     
  9. Chu Gai

    Chu Gai Lead Actor

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    I was more interested in hearing people give their thoughts Yogi. Get some discussion going with people who don't always participate.
     
  10. Wayne A. Pflughaupt

    Moderator

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    Perhaps we are making presumptions about the way amplifier designers design their amps.

    For instance, do they set out to make an 8-ohm amp and then say, “Hey, let’s see what it will do at 4-ohms?” Or do they design a 4-ohm amp and then say, “Just for grins let’s see what it will do at 8-ohms. Well, looky there, it put out even more than the half-power we expected!”

    I only ask because I really don’t know what approach amp designers come from, but if it’s the latter, what are they to do? Do they just publish a lower half-powered 8-ohm spec, so it will look like the amp will “double down” at 4-ohms? Or do they take advantage of the better-than-expected 8-ohm figures and publish them?
    Mathematically speaking, it will get double the power when resistance halves – up to the point where the amplifier maxes out. That might not be double at the per-load maximums, but at below that, it will be. Who consistently runs their amps at maximum power anyway? Who with any sense, that is?

    Much of this is barely relevant anyway. We all know that amplifier power ratings are ascertained with a static resistor load, not the reactive loads they get from speakers.

    That said, I’m really more interested in other specs such as linear response, dynamic range, S/N ratio and THD than whether or not an amp will double its power with lower impedances. To me the primary advantage of low-impedance stability is the knowldege that you have a more robust and reliable amplifier for all practical purposes.

    Regards,
    Wayne A. Pflughaupt
     
  11. Robert Cowan

    Robert Cowan Supporting Actor

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    wayne, you dont know by this forum, but im a bit of a hobbyist that builds amps in free time.

    all the amps i build are ohm-doubling, regardless of resistance. this of it this way... it might be able to do 100watts peak into 8 ohms, but you are right, you dont operate at the max. but what if you operate at 50%? the same rules apply...

    amp designers dont say "this will do x at 8 ohms, let see what it will do at 4". they know this from the start. an amp is specifically designed to be most stable at certain resistances. some, and IMO, the best, dont care about the resistance.

    but, most amps will have peak power at 8 ohms, because speakers are rated at 8 ohms for the most part, so it looks best.

    in regard to what you are saying about reactive loads and such... it matters a GREAT deal. if you can measure a static load at 8/4/2 ohms, and it doubles at each, then you have an amp that can mathematically provide the power that the speaker will demand at less resistance.
     
  12. RobertR

    RobertR Lead Actor

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    I tend to agree with Robert. The amp should not be limited in its power output at lower resistances.
     
  13. Shiu

    Shiu Second Unit

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    I agree with Robert Cowan, based on the I^2 (current square) relationship, half the resistance (impedance), double the power. Most receivers could not deliver the "double" power at lower impedance because the power supplies are not designed to handle the high current. Some of them (e.g. mid level Yamaha, HK, Denon) can handle "high" current, but the power supplies are not robust enough to sustain high current. They can handle high instantaneously power demand but could not claim the "double" power on "continuous rms" basis.
     
  14. Kevin C Brown

    Kevin C Brown Producer

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    One limitation is still the AC line, and whatever else you have plugged into it.

    It's easy to say that the power *should* double from 8 to 4, and from 4 to 2 ohms, but with real test results from real amps, this is still rarely the case even for "expensive" "high end" reputable amps.
     
  15. Shiu

    Shiu Second Unit

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    True, they are expensive, rare, but they are there........
     
  16. Robert Cowan

    Robert Cowan Supporting Actor

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    i think for arguement's sake, we should assume your ac line has enough current to deliver...

    if you have a TV, bi-amped HT, dual subs, etc... your AC line will suffer if your amps can double-down or not.

    amps that double-down are simply designed to do that. they are done so with a hefty power supply in mind, as well as ability to drive lower impedences. if an amp isnt very robust, its designers might limit its output at lower impedences, to stop it from overdriving itself, and keep it stable. chip amps do this with an internal circuit.

    typically, you will only see class A amps that double down because of their high bias and large power supplies. but, there are some cheaper amps out there that will double down.

    and as far as "real world tests", if an amp is designed to do so, it SHOULD test the same way. the amps im almost finished with, in real world and in test scenarios, will double down to 1 ohm or so. even though they are rated at about only 50 watts into 8 ohms, they will double to about 250 watts at just under 1 ohm. this was the designers (not me) intention. it is stable down to a short, but will not provide extra power beyond its supply of course.

    the speaker is a reactive load, and if the amp has a linear impedance/power curve (consistantly doubling down), then the speaker's erratic impedance curve becomes irrelevant because the amp theoretically reacts the same no matter what load its presented with (given the equation above).

    please ignore all spelling errors [​IMG] i didnt proof-read.
     
  17. mackie

    mackie Supporting Actor

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    Shoot - I just want to know what makes an amp a good and what makes one amp better than another one. I don't think price is the determining factor.
     
  18. Robert Cowan

    Robert Cowan Supporting Actor

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    the sound is what makes an amp better than the other [​IMG] and depending on your speakers, the power rating too.

    there are no numbers that determine what an amp will sound like. ive seen amps with high damping factors, low damping factors, high amperage, low amperage, etc... it doesnt matter. the amp just sounds good or it doesnt.
     
  19. mackie

    mackie Supporting Actor

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    there are no numbers that determine what an amp will sound like. ive seen amps with high damping factors, low damping factors, high amperage, low amperage, etc... it doesnt matter. the amp just sounds good or it doesnt.

    That's what I thought too...
     
  20. Robert Cowan

    Robert Cowan Supporting Actor

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    ive seen amps with poor numbers that sounded awesome. a pathos tube integrated comes to mind... it was driving a pair of PSB GoldI's. it was like 20 watts per channel, and my father owns these speakers, and they didnt sound good until he bi-amped them with 200x2 per side! very very strange.
     

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