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How much power needed.

Discussion in 'AV Pre-Amp/Processors & Power Amps' started by Kobus, Aug 17, 2005.

  1. Kobus

    Kobus
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    Hi,

    What size power amp will be optimal for my B&W 803 speakers?. They are rated to 250W.

    How does this anyway work, when can an amp be regarded as to small and when will it be regarded as to big?.
     
  2. alexs2

    alexs2
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    You'll find a few threads here relating to B&W 800 series speakers and their power requirements.

    In general,all of the 800 series are a moderately difficult load,and quite demanding in terms of current delivery,especially at low frequencies,which can be a difficult job for many amplifiers.

    I use a set of Krells with my 805s,and have found a few other similarly rated amps which struggle with them....my own advice would be to look at not less than 100W per channel,and preferably more,from an amp with very good load tolerance,such as Krell,Bryston,Chord etc,all of which will match your speakers very nicely.
     
  3. Kobus

    Kobus
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    Thanks,

    Is load tolerance just a charactaristic of an amp or is it a spec that one can look for?
     
  4. alexs2

    alexs2
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    To an extent it often ends up as an unpublished characteristic,but you can look for things like an amp doubling it's power output as the load impedance halves,ability to drive less than 4 ohm loads(which would be very useful for most of the 800 series),and high current output.

    To give an example,my amps are rated at 100W into 8 ohms but will double output as the load halves all the way down to 1 ohm,where they deliver 800+W at approximately 45A.

    I'm not suggesting that's what is required,but it illustrates the point reasonably well...all of the amps I've quoted have very good load tolerance and current delivery,and would match the speakers very well sonically also.
     
  5. la gran siete

    la gran siete
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    Can we safely assume Alexs2 that the only amps that do this desirable doubling of power into more difficult loads are expensive ones like Krell, Chord and Bryston and what actually makes them do this? Larger transformer?
    I wonder if there are any 70s Jap amps that perform that task? And what make they might be- Sansui perhaps.There is a site that sells a few of these amps/ receivers ie http://www.audiogold.co.uk/catalogue/default.php?cPath=5_14
    http://www.audiogold.co.uk/catalogue/default.php?cPath=5_9
    http://www.audiogold.co.uk/catalogue/default.php?cPath=5_8
    Of course I havent a clue what those products are like but do wonder if they would extricate the last piece of bass from an 8 ohm pair of speakers.
     
  6. alexs2

    alexs2
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    In general,the ability of an amp to double its power into decreasing loads etc has a lot to do with good power supply design,and in turn,providing the output stages with voltage and current that dont fall dramatically as the load impedance falls,and power demands increase.

    In some amps,this is provided by relatively massive transformers,with oversized reservoir caps etc,and in others,massive regulated supplies and so on....Chord adopt a slightly different approach by using switch-mode supplies,but again the answer is a very stable,high current power supply,which equates to a lot of cost,whatever the solution.

    A lot of modern amps are capable of either doubling output into 4 ohm loads,or at least going a good way towards this,but very few will do so into 2 ohm loads or less.

    Being honest,a lot of those old Japanese receivers and amps were capable of good power into reasonable loads,but really struggled when the going got rough,although many were beautifully made.
    One that springs to mind was an Accuphase design rated at 100W with(for the late 70's)amazingly low distortion figures etc,but it sounded awful....very flat and sterile,and really didnt like reactive loads.

    My(rather long winded)point is that really good,high powered amps are rarely cheap,and a lot of that cost is in the power supply.
     
  7. la gran siete

    la gran siete
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    iIassume by cheap you mean anything less than £1000 .In that site they have an Audio Research D130 power amp and an Inca tech power amp going for less than a grand but they dont give any spec. Do you know anything about those products?
     
  8. alexs2

    alexs2
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    The Inca's I was never very impressed with personally,but I have always liked Audio Research gear...very well made,reliable and nice sounding....their valve amps are superb.

    This little review from the 90's may help...

    http://www.audiophilia.com/hardware/d130.htm

    BTW,I didnt put a price on performance,but it will be difficult to find amps under the £1k that you've quoted that are capable of that particular trick....it doesnt mean there arent a lot of good sub £1k amps around.
     
  9. Kobus

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    Interesting, thanks for the informative stuff.

    1. Is there an optimal size amp for say a 250 watt speaker?
    2. Can an amp get to "big" for a given speaker (I am sure), but at what point?

    Thanks
     
  10. Steve.EX

    Steve.EX
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    You can never have enough power imho.
     
  11. la gran siete

    la gran siete
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    The review doesnt appear to rate it very highly. :(
    If the bulk of budget / mid fi amps are aimed at 8ohm speakers then one is looking at terble and midrange performance but not much bass extension for it is in that area thast a speaker drops to 4 and then 2 ohm which then results in the amp having to ratch up its current delivery which the cheaper amps simply arent capable of. Would that be correct?
    Given that dealers want to sell their cheaper amps that would not be information they would willingly give unless they though the punter had the readies to splash out on a plus £2000 amp
     
  12. Kobus

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    So more power than the speaker's rated power will can not damage the speaker.

    Also is it then fact, in line with what la gran siete (what is that) says, that most receivers (not power amps) will underperform severely with bass, espesially with speakers in the league of B&W 803 etc.
     
  13. Steve.EX

    Steve.EX
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    If you are asling for my opinion then the 803's will not be best served on all but the very largest receivers, and then receiver will unlikely thank you for the further additional load other speakers present. I myself would not consider anything other than the sort of amps mentioned above.
    It is far easier to harm a speaker via underpowering than simple overdriving, which is almost unheard of in a domestic enviroment. The larger the stable supply the better the transients, and control of will be, amongst other things.
    You have some nice speakers it would, in my opinion, be something of a shame to "Yamaha" them up - nothing other than persoanl opinion mind.

    Steven
     
  14. The Dude

    The Dude
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    Hi Kobus,

    If you haven't settled on anything in the meantime, one of my Aaron No3 will almost certainly be moving over to Capetown in December... you're more than welcome to a demo with your 803s... I'd be interested to hear the combo myself, whichever way you decide to go... :)
     
  15. Nic Rhodes

    Nic Rhodes
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    These speakers REALLY need power to drive them, follow Alex or Steve's advice and you won't go far wrong. This is something that you can't compromise on, I have seen MANY examples where B and W owners don't use enough and are not happy. The more the better in my book.
     
  16. rags

    rags
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    I have 803S speakers and use a Parasound A51 with them. The amp drives them very well. Its rated at 5x250rms @ 8 ohms and 5x400rms @ 4 ohms with a current capacity of 60 amps peak per channel and is a beast :D
     
  17. Thunder

    Thunder
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    I used to own a pair of Musical Fidelity XA200 monoblocs, which were very powerfull and refined for the money. They used to retail at £1000.00 per pair and can generally be picked up for about £650.00 to £700.00 for a mint pair. They were rated at 200w into 8ohms and 400w into 4ohms with a peak current output of 40amps. They could be an affordable solution to power hungry speakers :smashin:
     
  18. alexs2

    alexs2
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    Good point...they've always been very decent amps,well specc'd and stable,plus a nice smooth sound.
     
  19. Steve.EX

    Steve.EX
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    Rags has stated his amp is capable of xxxx amps (peak i presume) PER channel. Although this figure is not exactly a hard and fast guideline to an amps ability what is of note is that many "lesser" stereo power-amp specs are based on a "total" current output rather than a figure per channel. Just something to be "aware" of.

    Steven
     
  20. Steve.EX

    Steve.EX
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    I personally would look to either newer or other older models of the MF range if that is your poison. I have no doubt that the 200's continue the (almost always) predictably pleasing quality of sound, i have often extolled the virtues of some of the MF pre and power performance. However, if i recall correctly (as there is something rattling around at the back of my mind) the 200's were found to be wanting at dirty end of the hard labour, which your BW's will certainly present IMHO.

    Steven.
     
  21. Steve.EX

    Steve.EX
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    Second hand MF F range power amps can be had for 1-2K (old but very powerful, respected range) or indeed the newer large 3 range can be picked up for similar money second hand.
     
  22. alexs2

    alexs2
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    A set of good points there Steve,esp those relating to the peak and sustained current output of amps,plus single and all channels combined.

    I think that most of those mentioned so far are capable of good sustained current output,rather than short term peak outputs,which many do reasonably well,although far fewer are capable of sustaining e.g.45 A continuously.

    Another old wheeze that some manufacturers use is to measure not only short term output,but also peak to peak,often giving figures which look good,but simply arent sustainable.

    The older F series MF amps are still quite sought after,although they can be had for reasonable prices via the usual sources now.
     
  23. Pat Marcus

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    As someone with an entire 800 series analogue system, I am quite familiar with the power demands of these cabs. As you will see from my sig I use modified 804s as fronts which are just as difficult to drive (my Dealer actually thinks they are harder) but have had a good play with 803s in both their original and current 803S incarnation. They are a wonderful speaker that will get better and better with each upgrade in amp. Please dont think of putting them with a receiver or anything other than top notch amplification. Be prepared to spend at least to near the value of the speaker (£3500) on the amplification, if required. After much auditioning, I chose a meridian 559 which will happily make an 802 sing and works very well with an 803 (or 804). They can be had for around £2500. I have also heard and loved the 803s partnered with Naim amplifiction at around the 3k mark which adds pace and energy to their lovely liquid sound. Ive also heard them on bryston stuff (great gear but not a good match), chord stuff (an excellent combination), Audionet stuff (also very good) and have heard from others that they work very nicely with Gamut amplification. Newer Audio Research kit has more "bite" and should suit them. The older stuff might be a bit too laid back. Ayre Acoustics are big bucks but getting great reviews from the cognoscenti. Check them out, too !
     
  24. General Skanky

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  25. Pat Marcus

    Pat Marcus
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    Full of good stuff, General Sir.
     
  26. General Skanky

    General Skanky
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    But as much as it pains me, the credit has to go to a certain Nic Rhodes.

    He is far too clever really. :rolleyes:
     
  27. Nic Rhodes

    Nic Rhodes
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    I believe BK aimed that thread at full range speakers. Use a sub etc and you can reduce the numbers. The figures quoted cover all the freq range. I believe he still stands by what he wrote back then.

    He spend almost 2 years looking at the amps in question and eventually liked the big Brystons on his Kef Ref4.2 but went active in the end. Genelecs I believe and he thinks this is a better way forward. In fact others in that thread have followed his lead ;) He feels this is both better than 600w into Ref 4.2 but also cheaper.

    The ref 4.2 now are his rear speakers :) He still builds amps for fun, though he specialises in low power ones as he thinks they are 'more fun'. It does give him a certain insight into how much power is needed as he has made many mistakes over the years, but he won't tell you about thoses :) He has a decent variety of speakers to test these on as well.

    Taught him all he knows :lesson: Don't like him much though...... :rolleyes:
     
  28. General Skanky

    General Skanky
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    Scary what people remember on these forums isn't it? :)
     
  29. Steve.EX

    Steve.EX
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    Nope.
    The Battlestar thread on TV and whatever forums is as i understand the word to mean - scary.

    Steven
     

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