17 comments on “Undoubtedly you would all like to popularise this

  1. Very good! Please can he also do one explaining matching an amp’s output impedance and headphone impedence. I’m utterly lost and audiophiles are laughing at me…

  2. John Square

    I think this covers the importants:

    In the domestic audio world, you don’t want to match impedances. What you want to do is arrange things so they work.

    If you have a turntable with a moving coil pickup, you probably want to have the preamp input set to something between 10 and 300 ohms (MC cartridges vary a great deal in their innards).

    If it has a moving magnet cartridge, the input impedance should be 47000 ohms (MM cartridges are generally designed so they work best with that value).

    If you have CD/DVC/BluRay/etc player and you use the analog outputs to hook up to the audio system, the inputs of the audio system should have impedances of say 10,000 ohms. Or more. Too low, and you’ll (probably) lose bass, and or get some distortion.

    Your speakers will be 4-16 ohms, ‘cos that’s what they are. If you have an amplifier which is NOT a valve amplifier, just wire it to the speakers and forget about it. If it’s a valve amp, then it’s almost certainly got an output transformer, and should have a choice of (say) 4 or 8 ohm outputs. These figures indicate the impedance of the speakers you’ve connected, NOT the output impedance of he amp. In all normal cases, you want the output impedance of the amp to be much lower than the impedance of the speakers. No ‘matching’.

    Where you do want matching is when running very long wires around the place, like a hundred feet of co-ax to carry an aerial signal to a radio or TV. Then the wire has a natural impedance of 75 ohms, and the things it connects expect this.

  3. What’s with watts? Why do they say how many watts a speaker has? Noise is measured in decibels isn’t it?

  4. John,

    I’m just about to leave, I’ll try to gives longer answer later.

    Resistance is mainly to do with DC, impedance means it has a frequency components. Coils, speakers, are effectively short circuit at DC but increases as frequency increases.

    Capacitors, used in most circuits, more later, are effectively an open circuit at DC and impedance goes down as frequency goes up.

    In audio what you want is a flat response across the frequency range, which is hard to get.

    Having input and out reacting differently to different frequencies causes distortion. It also affect what is known as the power transfer characteristic.

    Must dash, Weymouth bridge waits for no man.

  5. @AndrewC, a Decibel is a relative measurement. Broadly, “+6dB” is useless unless you know what it is relative to, it’s better notated as “+6dBu” or “+6dBv”, or whatever. a dB gives a ratio between a known value and whatever you are working with.
    I suppose you could say “this amplifier has a power output of 30dBw, where w is the relative value of 1W” but that’s a complicated way of saying “power output of 1000w”. (I think)

    It’s made confusing because practical audio engineers – i.e. the ones running mixing desks on stage or in a recording studio – use the dB without bothering to refer to the reference, and just ask for “another couple of dB on the bass guitar please”, but hey, that’s simply a practical application of something that someone else designed / invented / discovered.

  6. @Tim, yes, a decent enough explainer of the basics, but why on earth not actually demonstrate it? The video has a sound bed; so why not say “here’s what happens when you crack up “treble” or “HF” / crank it down; rinse and repeat. With audio stuff there’s little substitute for actually hearing things.

  7. Andrew C – A loudspeaker is an electro mechanical device. You push current through it one way, the cone (or equivalent moving thing) moves out; push the current the other way, the cone moves in.
    So there’s a couple of very important limits: push too much current through, and you’ll exceed the current-carrying capacity of the wire making up the coil; something will melt and your speaker becomes useless.
    Push too much current through, and the cone will try to move further than it physically can, causing (possibly) damage and certainly distortion.
    Push too much current through, and the whole thing will get too hot, and glue will unglue, wires melt, etc.
    So there’s a limit to how much current you can push through the thing and keep it healthy.
    The ‘watts’ rating of the loudspeaker is just that limit, expressed as watts rather than amps, because we have an idea of its impedance (“8 ohms” or whatever).
    Make sense?
    [There are electrostatic speakers too, but they generally get limited by volts not amps]
    As to loudness, there’s also the sensitivity of the speaker, expressed generally as “88db/watt” or some such. As noted, dB is a relative measure, and the 88 here is generally relative to the threshold of hearing – 88dB more power than the quietest thing you can just about hear. For a given speaker, more turns on the coil will create a more sensitive speaker (cos more movement for given current cos the coil generates more magnetic field so you get more push); but it will make the thing heavier, too. Tradeoffs.
    Generally, an 84dB speaker is pretty insensitive and needs lots of power to sing reasonably loudly; a 102dB/w will make lotsa noise at rather low powers. Listening at 100dB/C (C is a frequency weighting specification) for any amount of time is too loud for me.
    A longer treatment is here:
    http://www.psbspeakers.com/articles/Guide-to-Speaker-Specifications
    PSB make very good speakers.

  8. John,

    In answer to matching impedance. you get maximum power transfer when the source (amp) matches the load (headphones/speaker). Any mismatch not only waters power but also causes distortion.

    See maximum power transfer theorem on wiki or elsewhere.

  9. Hurry induced dyslexia this morning made me see the video title as “QE Explained”, so I was quite looking forward to some clarity on that when I returned.

    Hey, now there’s an uncle-nephew challenge – video tutorials explaining economics to Richie. Would those be Remedial or Special?

  10. BlokeInTejas said:
    “Push too much current through, and the whole thing will get too hot, and glue will unglue, wires melt, etc. So there’s a limit to how much current you can push through the thing and keep it healthy.”

    So you can’t turn it up to 11?

  11. Good crikey- that was some comprehensive explaining there.

    We’ve either got a bunch of people who take their listening seriously or a lot of electrical engineers here. Both, probably.

    Either way, I’m all edumacated now.

    If anyone is interested- the devices that raised the question was the Zishan Z1 and KZ ZS5 IEM’s. And I can thoroughly recommend both. Best £40 I ever spent

  12. BiND: That’s true in RF systems where impedance mismatches cause power to be reflected from the load. However driving speakers is more like the electricity distribution network – you want the impedance of the source to be as low as possible so minimal power gets dissipated at the source end and in the connecting cable so most of it ends up in the load.

  13. TG,

    Good point, I’m a radio engineer by training, although we did do one adio stuff but that was 35 years ago.

  14. dB is, indeed, a ratio (10dB being a factor of 10), but dB(A) is a defined measure of sound volume (weighted according to the sensitivity of the human ear, but opinions vary and other, slightly different, weightings are available). So it’s correct (if a bit shorthand) to describe a noise level (e.g. from a pneumatic drill) as 100dB (you need to specify how far you are from the source, in most circumstances).

  15. Yes dBs are a ratio but the real beauty of them is that they are lograthmic and therefore scale nicely.

  16. Hey guys, I’m the nephew!

    Great idea John. I’ve just made a video explaining Headphone Impedance which you can find here:
    https://www.youtube.com/watch?v=DTt2ESqJ8D4

    Thanks njc for spotting that typo! It’s now fixed for all future videos, can’t believe I didn’t spot that…

    Thanks for your feedback guys, and thanks Tim for sharing!

    If you’ve got any other music-related tutorials you think I should do, feel free to leave me a comment on Youtube.

    Cheers,

    Dave

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