Where does the power lie?

Discussion in 'AV Receivers & Amplifiers' started by S, Jul 12, 2001.

  1. S

    S
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    I've always been a bit sceptical of manufacturers' figures for power rating but I was surprised to read in the Hardware Review section that HCC had independently confirmed Sony's 5 * 90W RMS rating for the STRDB930. I'm surprised because the power consumption for the unit is only 290W!

    I thought the 90W figure was probably a peak value but Sony also rate the unit as 5 * 100W DIN 1Khz into 8 Ohms, and I thought the DIN rating effectively implied a continuous output albeit with quite a high allowance for total harmonic distortion.

    Can anybody make the figures add-up or have I just solved the world's energy problems? ;)

    S
     
  2. Guest

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    For A.V amps these tests are generally done with one or two (at the most) channels driven ONLY! Again generally speaking any THX ultra certified amp should deliver a minimum of 100 watts per channel all channels driven simultaneously (all be it under fairly undemanding load conditions). It's a common trait of manufacturer's particularly with budget models to state "X" watts per 5 (6/7 even) under real world conditions when all channels are being driven that figure will be drastically reduced. Think of it more that the transformer is capable of sustaining that current for any two of the five channels while the output devices on each individual channel are current rated as to be able to deliver that output if required. With more up-market models THX ultra specified but more particularly power amps these generally speaking will deliver their specified ouput all channels driven.
     
  3. Reiner

    Reiner
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    On a more technical note: capacitors can be used to "buffer" power and hence increase the output power demanded for a very short time, as well it is highly unusual that all 5 channels would require e.g. 100 Watt each at the same time.

    What you actually have discovered is the 'problem' with budget amps: the power supply is under-dimensioned (obviously for cost reasons) and most units cannot deliver the promised power output, or like Steve says, only with 2 channels driven simultaneously.

    The DIN rating at 1kHz is IMHO totally useless, a reliable rating (which can be used for comparision) would be xxx Watt @ 8 Ohm (20Hz-20kHz with a THD of < 0.1%

    Any other ratings are either useless or (intentionally) misleading, especially those stated at 4 or 6 Ohm. 100 Watt at 4 Ohm sounds great, but it's only 50 Watt at 8 Ohm.

    However, keep in mind that power is not everything to consider when purchasing an amp.

    [ 13-07-2001: Message edited by: Reiner ]
     
  4. Guest

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    As Reiner say's should you be remotely interested the capacitance stage of an amp wlll be part of a manufacturers rated output (more often than not) when you see terms like PMPO (peak music power output) and the like, an amp claiming 200 watts pmpo is most likely only able to run perhaps 20 watts continuously, although this sort of thing is more common with midi or p.c type hi-fi's (???) not normally with established a.v products. Pretty much the be all and end all of most amp's output is down to the ability to develope current by the main primary and secondary windings of the transformer (some have multi tapped secondary windings to supply low level signal components)
    Tend to disagree with Reiner in so much as with multi-channel amplification generally the output (or the ability of the transformer to sustain the output) plays a pretty major part in the overall ability of 5.1 etc performance. (sorry mate) unless you are very careful about matching efficient speakers, obviously i realise that there are many many other contributing factors but give me a hefty high current power-amp every time.
     
  5. Ars longa, vita brevis

    Ars longa, vita brevis
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    look at the power ratings on the revies on this website, i find them a little more informative.

    for example the review of pioneers thx 908 receiver says it can only do about 65w without distorting, whilst the similarly power quoted yamaha a2 has less distortion at 95w. Sony's 930 and 940 have the same power output (i'm not sure if i'm using the correct terms here, i dont think i am) on paper, but not in real terms.
     
  6. General Skanky

    General Skanky
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    Also, for an amplifier to drive 'difficult load' speakers it will have to be able to produce a good current delivery.
    It gets complicated, but to give you an example, NAD amps generally are rated as 'only' 60 watts let's say. But they have huge reserves of power to drive speakers along, especially in the short term.
    An amp may be rated let's say as 100 watts per channel with low distortion etc, but it may have power supply protection limiting its output. Hence a higher power amp may sound less capable than a lower power one. Fun isn't it? :rolleyes:

    I'd advise a good read of the tech. guru Paul Miller from HiFi Choice when he does tech. reviews on kit each month. Very enlightening!!!!!
     
  7. S

    S
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    Thanks for all the replies. Much appreciated.

    I'd suspected the situation with my '930 was as SteveEX described it, namely that the power is pooled - any individual channel is capable of delivering the rated figure but not at the same time as the others.

    I'd definitely agree with the need for an amp with plenty of current grunt. Speaker impedances vary a lot with frequency - dropping to almost zero with some designs. Reservoir caps help tide things over during the dips but there's no substitute for an amp with an inherently high current drive. I know one hi-fi enthusiast who reckons you should be able to weld with a decent amp!

    I'm not sure that I agree with some of the comments on the different power ratings (sorry Reiner). I think the DIN rating is useful because it implies a continuous output (10mins I think?). Power ratings at impedances other than 8 Ohms can also be revealing. Lower impedances are harder to drive for the same output voltage so it doesn't follow that the power rating of an amp at 8 Ohms will be less than its rating at 4 Ohms. In fact it's the other way around. A decent amp should be able to deliver the same power into a 4 Ohm load as it can into an 8 Ohm load. An under-specced amp on the other hand will have a 4 Ohm rating (if one is even quoted) which is significantly less than its 8 Ohm rating.

    S
     
  8. Charlie Whitehouse

    Charlie Whitehouse
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    An 'ideal' power amplifier should double its power output as the load impedance it is driving halves. That means it has to deliver twice the current, assuming the voltage remains constant. Unfortunately, at the lower end of the market, the power supply is where most manufacturer's skimp. So the cheaper amps can rarely achieve this power doubling.

    My big Krell is rated at 200wpc into 8 ohms, but it can happily drive 400wpc into 4ohms, 800wpc into 2 ohms and 1600wpc into 1 ohm!! All continuous!! The power supplies and output stages are ridiculously overspecced to achieve this, hence its huge weight (50+kgs) and price. My speakers are nominally 4 ohm impedance, but this dips to around 1 ohm at 20kHz, hence the need for a chunky amp. :cool:

    As for arc-welding, there are some apocryphal stories of this being demonstrated, but personally I wouldn't dare try it! :eek:

    Cheers,
    C.
     
  9. General Skanky

    General Skanky
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  10. S

    S
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    You're quite right!

    I should have said in my original reply that it doesn't 'necessarily' follow that an amp's rating at 8 ohms will be less than its rating at 4 ohms for exactly the reasons you describe. Serves me right for trying to compose a coherent reply at near midnight on a Friday!

    Unfortunately for me, the likes of Krells, ATCs etc. which can maintain constant drive voltages over such a wide range of impedances are a bit out of my bracket. I usually reckon if an amp can deliver the same total power into a 4 ohm load as it can into an 8 ohm one then it's not doing too badly (although obviously this doesn't translate into the same acoustic power).

    One other little trick I've seen is for manufacturers to inflate power levels by quoting at ridiculously high levels of THD (like 10%). It seems to be more prevalent in the PC audio world but it's worth watching out for.

    Cheers

    S
     
  11. Reiner

    Reiner
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    Some missunderstandings to be cleared up (perhaps caused by my poor English or way to articulate myself ;) ):

    Power is important, especially for AV systems but what I meant it's not all to look out for. As well the damping factor plays a considerable part in judging/rating an amplifier.
    Or in other words: it has to be quality amplification (see the comments to PMPO and NAD amplifiers).

    The DIN rating at 1kHz is IMHO useless as it's measured at one frequency (1kHz that is) only, however music signals cover from 20Hz to 20kHz so the rating should be measured across the whole f-band.

    Power rating at different Ohm levels: yes, a amp rated at 8 Ohm should be able do double the power if the impdedance halves, but to rate an amp at 4 (or 6) Ohm is just missleading as most are IMHO rated at 8 Ohm (assuming all would be measured across 20Hz-20kHz with a THD <0.1%), or at least they should be.
    I firmly believe this is done intentionally (woah, this got 150 Watt (fine print: @4Ohm) but this one "only" 100 Watt (@ 8 Ohm).
    As said above the latter should be able to give 200 Watt at 4 Ohm, depending on the power supply, and most likely it can give at least 150 Ohm @ 4 Ohm, too, and hence the first amp is not to be seen as "better" as the manufacturer wants us to believe, in fact it could be worst (in terms of power output and perhaps other specs).

    Check out some specifications (brochures etc) and you will see what I mean, some makes are well known for 'questionable' ratings (though some of them are improving, too).
     
  12. Guest

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    Re: Damping.
    I have quite a bit of research on this matter and indeed spoken to several technical dept's of the more elevated amp manufacturers (Naim, Cyrus even Rotel etc).
    And it would semm the hifi fraternity is somewhat split on this one there semm to be those who fervently believe the damping factor (which is ascertainable by a simple calculation using specific factors of an individual amp) to be v. important and others myself included who think that it can play a part in performance but is a lesser of several key factors.
    An amp with a relative damping factor of say a 1000 with a rated output of only 50 watts will not necessarily sound "tighter" than an amp with a D.F. of 150 with a stable output of 1000 watts infact was is agreed upon is that the "real world" difference's between D.Factors of say 150 and 500 are really quite small.
    Certainly a large damping factor would be considered practicable if using speakers with large drive units (12,15 and 18" perhaps) but in more modest speaker (6" or less) i believe this to be far less relevant.
    Speakers being what they are having several electrical properties (impedance, reactance etc) have a far more audible reaction to amplifier properties such as size and quality of mains transformers, size and quality of the amps capacitors, again quality of output devices used (transistors etc), capacitance (the amps reaction to) of speaker cable used etc etc.
    In short while damping may(?) be a relevent part of amplifier design i believe you would find it someways down the list on design criteria.

    Regards

    SteveEX
     
  13. Guest

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    To simplify:
    An amp's Damping Factor is simply the sum total of other (essential) design parts.
     
  14. Guest

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    A low Damping Factor by no means signifies a "poor" amp.
    A high Damping Factor by no means guarantees a "good" amp.
     
  15. Guest

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    For those of you without a life (me?) :
    Damping factor = loudspeaker impedance divded by the amplifier output impedance.
    i.e speaker 8 ohms divided by amp output impedance of 0.1 ohms will give a damping factor of 80.
    So rather obviously the damping factor is relevant only to the speaker actually driven. Manufacturers always give the D.F. under an assumed 8 ohm load.
    Looking back through my paperwork it would seem that anything over 50 (ish) can be assumed as "good" as as previously stated huge increases in D.F do not necessarily (and sometimes not at all dependant on amoung other things type of speakers driven) have an absolute effect on perceived "sound".

    I will shut-up about it now!
     

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