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Why do you need a very powerful amp?

Discussion in 'audio' started by mandryka, May 20, 2019.

  1. George J

    George J Herefordshire member

    My old [and now late] friend Bertie Heywerd retired aged 69 as principal double bass player at the ROHCG. He was a superb player even in old age, but he could not deal with Jazz "to listen to," though he could vamp with the best!

    When we shared a music stand [I also used to play double bass] in the orchestra, he was tremendous fun to rehearse next to, and if something went wrong in the orchestra [or even us] he would smile and say,

    "Good enough for Jazz!"

    ATB from George
    Ian M and GML like this.
  2. Darth Vader

    Darth Vader From the Dark Side

    There is a bit more to it than that oversimplification. True that 20Hz to 20KHz is the accepted range of human hearing but those are single tones. I think you said that you read Chemistry so did you not study Fourier analysis at Uni? In a nutshell waveforms of which there are plenty in music and our surroundings can be broken down into a series of sine waves. You'd be surprised at how high some of those frequencies go. In the case of a fast rising square wave such as a drum rim shot the sum of the odd harmonics go up (in theory) to infinity this is the extreme case but other waveforms are equally surprising.

    This means that in order to represent a waveform without distorting it you need an amp with a wide frequency bandwidth. If it were limited to the human hearing range then fast rising waveforms would lose their shape and rather than a clean steep slope would tend to round off at the top and instruments would tend to lose their natural timbre.

    If someone just listens to compressed 'pop' then it probably doesn't matter but for those of us that enjoy live music it does.


    naimplayer likes this.
  3. JensenHealey

    JensenHealey pfm Member

    That assumes that the human ear listens to the 'shape' of the waveform arising.

    AFAIK it does not. It performs a kind of frequency analysis by having 'hairs' that respond to particular frequencies - in effect doing a live Fourier transform. As such it has no way of knowing, or responding to, any energy at frequencies beyond the highest responding hair, which of course deteriorates with age - as well I know.
    sean99 likes this.
  4. chartz

    chartz pfm Member

    I use a powerful Technics SE-A3 to drive 94 dB efficient Cabasse speakers.
    Small amps won’t drive the stiff 12 inch bass speakers properly.
  5. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    Oh they will but not to as loud a volume as a big amp.

    I note several comments along lines of "I have big woofers/heavy woofers so only a powerful amp will do" this is completely wrong!

    Remember that many low power amps are low power because they are cheap! Don't compare a £130 25WPC integrated with a £1000 80WPC amp and then decide the difference is down to the power output!
  6. Jim Audiomisc

    Jim Audiomisc pfm Member

    However, it is ... suprise... more complicated than that. :)

    e.g. our ability to hear high or mid frequencies is affected by the presence of LF.

    Practical example of this was some years ago when many TV listeners complained about the BBC News playing 'drums' under (sic) the announcements at the start of a news broadcast. The BBC tried to dismiss this and said it was poor audio kit at the user end. But it also happened to people using decent audio systems.

    The reason is that the HF 'hairs' are at the *external* end of the inner part of the ear. So the LF has to get past them. This shakes them and their 'substrate' about and can disturb their ability to hear the HF they are meant to detect.

    Both the 'hairs' and the substrate also change their behaviour in response to being shaken. The sensors are actually active.

    This is one cause of tinnitus. Some hairs get a feedback gain 'helping them to move' that is so strong that they start vibrating even when there is no sound input. IIRC some audiologists can therefore put a mic in your ear and detect the tinittus because of the noise this makes!

    However the more usual problem is that the 20-20k ain't a flat response. Partly due to amp rolloff. Partly due to factors like interaction between amp output impedance and speaker input impedance. In some cases this can have quite a marked effect. Particularly in old single-rail amps with an output series d.c. break capacitor.

    Again, NOT an effect shown up by the standard measurements magazines make.

    I should confess that despite liking - and using - the QUAD 34 preamps I laughed out loud when I saw what one did to an LF squarewave. (And, yes, this also has an effect on the power requirements for the poweramp!)

    All this cheerfully skates past the standard review measurements of a power amp...
    George J likes this.
  7. SteveG

    SteveG pfm Member

    I've been doing some switching around of speakers of late and one of those changes had one set of speakers going from being driven with a 200W per channel Krell to being used with an 11W Audio Note valve amp. I was expecting the less powerful amp to deliver less bass and less bass control and was a bit surprised that I didn't. At normal listening levels (up to really quite loud) the difference in power wasn't noticeable in any meaningful way. What was more obvious was that both amps were considerably better with those speakers than the previous 35W per channel Naim was - so it wasn't just related to power.
    lindsayt and Arkless Electronics like this.
  8. Ian M

    Ian M pfm Member

    Suppose your you’re taking the output from a MC phono amp directly into a power amp, do you not need more power and how much?
  9. chartz

    chartz pfm Member

    Don’t get me wrong, I tried many amps with those speakers, and they are just not happy despite their efficiency.
  10. cooky1257

    cooky1257 pfm Member

    As a rough guide.....Take a speaker's anechoic sensitivity, add 3dB for a pair and another 3dB(ish) for the room, so +6dB, then reduce by 6dB for every doubling of distance ie @2m,4m,8m etc.
    For a 90dB/w/1m pair, sitting at 4m away the real world listening level at 4m for 1w = 84dB ,10w = 94dB, 100w = 104dB.
    For those rare 30dB crest factor cd's....I'd suggest for max levels of 100dB a 40w/ch amp min for 90dB/w speakers, 80w/ch for 87dB/w, 160w/ch for 84dB/w speakers....
  11. mandryka

    mandryka pfm Member

    We discussed JR 149s in the conversation that started this thread, with a former MF engineer who I believe has a good reputation. He commented that to get the best out of them you need a very powerful amp. He liked the speakers very much, by the way.

    I’ve not had a chance to connect the Krell to the 149s but it’s something I’ll try to do soon, and I’ll report back. It’ll be very interesting to see how the compare with the Spendors, which sound like . . . real music.

    This is why I need a thingy to be able to swap speakers at the flick of a switch - if anyone has one they don’t mind lending me please say.
  12. ToTo Man

    ToTo Man the band not the dog

    I discovered by accident many years ago that a peak in a loudspeaker or headphone's frequency response significantly impairs my ability to hear the frequencies beyond that peak more so than the frequencies below it. It's almost like a masking/obscuring effect. e.g. if a speaker has a broad enough peak in its response in the midrange then it subjectively sounds rolled off and lacking crispness and air in its top octave, to my ears. I attributed this to way our brains process sound, sort of similar to how turning the bass down makes the music sound more trebly even though objectively the amount of high frequency energy remains the same in both cases. Our brains seem to focus on the most dominant sound at the expense of others.
  13. Tony L

    Tony L Administrator

    I’ll be very interested how you get on there. I’m slightly sceptical as they are an easy load and I’d have thought 15 Watts would have a B110 banging into the backstop on certain bass notes. I’m with Jez here on amp quality over power output, e.g. to my ears the 10 Watt Leak sounds far bigger and more powerful than any 50 Watt solid state amp I’ve tried with them so far. I’ve never tried playing very loud as I never would with a little nearfield speaker, but my suspicion is I could bottom them out on techno etc with the 10 Watt Leak.

    PS I remember this scenario with Naim kit and Kans. I went from a Nap140 to 2x135s and if anything the available volume headroom actually decreased as it was just so easy to bottom-out the B110s on certain dance 12” singles with the more powerful and dynamic amps.
  14. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    Seems there's pretty much nothing me and he agree on then!
  15. John Phillips

    John Phillips pfm Member

    Quite right but the distance factor, as published in a couple of professional papers I know, averages 3 dB loss per doubling when in a room rather than the 6 dB free space figure.

    Also you may need to take the sensitivity figure, if provided by the manufacturer, with a pinch of salt. It is sometimes accurate but often inflated by 1 dB and I have seen reviewers claim 3 dB exaggeration.
  16. mandryka

    mandryka pfm Member

    I am sure this has been asked and answered a thousand times before and if it’s just too annoying that someone is asking it again, I’m sorry. But what is amp quality? How do you measure it? What do I look for in an amp to know that it’s a quality amp?
  17. Tony L

    Tony L Administrator

    That’s how I understand it too, plus I thought the 3db room-gain parameter was only on the table for wall proximity (half or quarter-space) speakers. I’ve never known whether to factor it in when doing the ‘math’ with my. 149s which are about 10” from the wall.
  18. mandryka

    mandryka pfm Member

    I think this touches on something quite interesting. The 149s were designed for close listening and that’s how you use them, bean bag and all. And I tried that for a very short time and I can see what you are getting so enthusiastic about for sure. But . . .

    But . . . I want my speakers to fill a room, I don’t want to be listening near field.

    I’ve got some sympathy for where you come from. If it was mastered with 60s equipment then it’ll probably sound good with 60s equipment; if it was designed for near field monitoring then it’ll probably sound best as a near field monitor . . . It’s a sort of hifi HIP (historically informed playback), hi fi authenticity - following the designer’s intentions stands a good chance of getting good results.
  19. cooky1257

    cooky1257 pfm Member

    I did include +3dB for 'the room',ie reflected sound you add this to your starting/1m figure.. I've seen the effect line arrays have in undermining the inverse square law(by 3dB) but not direct radiator/point sources.
    Indeed, the averaged over xxhz to xxhz bandwidth or @1khz are just 2 ways the books are cooked, there's been much of an interweb kerfuffle regarding the disparity between the new Tannoy Legacy Arden's 'meagre' 95dB in comparison with the System 15 DMT's 101 dB sensitivity figures given they are essentially the same driver-the disparity is Tannoys pro spec was 'in room and for a pair'...

    PS. Interestingly I just measured one channel only of (tolerable level of)pink noise at 1m/2m/4m and got 91.5/86.4/82.4dB, a -9.1dB drop over 4m instead of 12dB..adding the notional +3dB for the room(ish) reflections to the 1m figure ie 94.5dB gives us the -12dB drop at 4m so not a bad rule of thumb.
    Last edited: May 23, 2019
  20. Tony L

    Tony L Administrator

    I don’t know, though I have certainly heard it. The most striking example I can bring to mind was comparing my Nap140 to a friend’s Musical Fidelity P170 into first generation AE1s, which are real amp-suckers. Despite on paper being similar spec the Naim just could not drive these speakers, it sounded thin, small, gutless and just terrible. The MF was way more powerful and gave a good indication of what the speakers could do. The owner later bought a Krell KSA100 and with that the AE1s sounded amazing. I’m not talking simple volume here, it is the ability to really drive seriously inefficient and reactive speakers like the AE1.

    I would love to try a big Krell or whatever with the 149s, but I have never felt they were hard to drive at all. A Quad 303 or 10 Watt Leak sounds lovely and neither is a powerful amp by any stretch. The measurements reflect this too, their impedance doesn’t go below 8 Ohm, so they are easy but inefficient. The inefficiency just doesn’t matter as you’d never want high volume from a 5” bass mid in a sealed box! Want louder or further away? Buy a bigger speaker!
    mandryka likes this.

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