Educate me about ohms (4 ohm speakers w/ 8 ohm amp?)

Discussion in 'Audio/Video & Home Theater' started by Alienwho, May 2, 2009.

  1. Alienwho

    Alienwho Diamond Member

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    I just bought the YAMAHA RX-V565 @ Newegg for $329 because it has all the features I want. Then I realized that I believe my old Klipsch Promedia 5.1 Satellites I'll be using for the next little while are 4 ohm while I believe the Yamaha is 8 ohm.

    Will it still work or will it hurt anything?
     
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  3. frostedflakes

    frostedflakes Diamond Member

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    Your amplifier will deliver a lot more power connected to 4 ohm speakers than 8 ohm, so you could potentially overload it. Honestly I think it depends on the amp, higher end ones can usually handle lower impedance speakers whereas lower end models can't. I'd consult the owners manual first and then Google to see what success others have had using 4 ohm speakers with that amp.
     
  4. Tiamat

    Tiamat Lifer

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    If the amplifier does not have enough heat dissipation, it will warm up much more quickly running the 4ohm load due to more current flowing through. If the heat dissipation is inadequate, it will overheat.

    That being said, if you listen at moderate to low volumes, you may not have problems.
     
  5. Alienwho

    Alienwho Diamond Member

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    Thanks for the info. From what I read this amp runs very cool, and I don't plan on blasting it.
     
  6. bobdole369

    bobdole369 Diamond Member

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    In any event, since the ohms listed on a speaker are a "characteristic impedance" - it matters not that much.

    A speakers impedance changes based on the frequency that you feed it. For example:

    At 35hz (very deep bass) - might be something like 28 ohms, while 500hz (bass guitar) might be 12 ohms, 1200 hz (tenor/alto) might be 8 ohms, and 4000hz could be 3.1 ohms. The numbers I gave out are made up to illustrate a point.

    The difference between an 8 ohm and a 4 ohm speaker doesn't really become apparent unless you are blasting the amp near its limit. Then it might warm up more and possibly trip protection circuitry if its a 4 ohm load on an 8 ohm amp. Essentially you are losing power because of the mismatch.

    As said before, don't blast it and you'll be fine.
     
  7. dawgtuff

    dawgtuff Member

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    bobdole369 is right. Manufacturers take an average impedence through the frequency range for that speaker. However, in multiple speaker setups,especially using different brands of speakers, a damaging low impedence can happen. Don't run high volumes, and you should be Ok.
     
  8. EddyKilowatt

    EddyKilowatt Junior Member

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    The actual output impedance of your amp, by design, is actually a tiny fraction of an ohm -- effectively a voltage source -- so don't have any concern about it being 'mismatched' for your speakers. The lower-impedance speakers will just draw more current from the amp (Ohm's Law), and more current means more heat. That's a concern, but it's not an Instant-Death sort of concern.

    Like the other folks said, check the amp manual for recommendations. Don't go crazy with the sound levels, at least not right away (or while under chemical influences), and keep an eye or rather finger on the amp heatsink temperatures. Output fuses might add peace of mind while you are feeling things out, and you can remove them later if you have sound quality concerns.
     
  9. devilchrist

    devilchrist Member

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    actually..

    read this.

    http://www.roger-russell.com/wire/wire.htm

    My personal take on the ohm of the speakers is,, 8ohm just give you ability to run same thickness wires longer without quality degredation. that is why home theater speakers are all 6-8ohms and car speakers are 4ohms.

     
  10. lhwidget

    lhwidget Junior Member

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    What's being discussed there is a little more subtle. A high impedance speaker system will be less sensitive to an additional 0.5 - 1.5 O increase in its impedance. Typical 10 to 30 foot cable lengths of 16 ga speaker wire will add 0.08 to 0.24 O to the speaker's impedance. This won't be enough added impedance to change the woofer's bass response.

    The higher current drawn by the lower impedance speaker system will be the thing to watch for. If you don't play movies loudly, I doubt it will cause problems.
     
  11. rdp6

    rdp6 Senior member

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    May as well give an example speaker gain vs frequency plot (I like Bode) and evoke the maximum power transfer theorem while we're at it. My EE experience doesn't carry to audio amplifiers. Is 4 vs. 8 Ohms really the characteristic impedance? If so, then how significant are the effects of SWR on the amplifier given a mis-matched load? For giggles, what is a typical DC (pure) resistance of a 12" woofer voice coil?

    Of course, you might run two sets of speakers in series to have an 8 ohm load for fun.
     
  12. Tiamat

    Tiamat Lifer

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    If your EE experience/training doesn't carry to audio amplifiers, you didn't receive correct EE training :laugh:

    You just have to sit down and do your own thought experiments.

    Nevertheless, 4 vs 8ohm nominal impedance doesn't tell you much at all. It is near worthless.

    The metric that is worth something is Impedance vs. Frequency curve of the speaker. That is, both the phase angle and magnitude are presented. Just because a speaker has impedance 8ohm doesn't mean it is an easy load to drive. If the phase angle is 45 degrees out of phase, the amplifier will have to dissipate 4x the heat while outputting only half the power. I'm not an EE by any means so if my math is wrong, correct me.

    Many speaker, unless they are first order crossovers, will have swings in the phase angle as well as magnitude all over the place.

    If the amplifier is rated/designed for 8 ohm loads and the speaker, (for example) presents a 4 ohm, -45 deg. impedance at 80hz; and you play a loud and prolonged 80hz sine wave, you will probably have some problems with the amplifier overheating.


    In the real world, I have not seen many non-electrostatic speakers present difficult loads above 120hz. If you have a subwoofer and/or cutting out the lower bass frequencies, the amplifier will not break a sweat.
     
  13. lhwidget

    lhwidget Junior Member

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    It's a subtle effect, adding ? 1 to 5 O in series with a woofer won't just lower the driver's output for a given voltage, it will also increase the woofer's low frequency response. I've never thought about the cause very deeply, I just got used to adding coils' DCR to the circuit when designing crossovers (the increased resistance increases the electrical Q and total Q of the driver).

    Basically, the motion of a driver's voice coil in a magnetic field is caused by current flowing in the voice coil. The voice coil's movement also generates a voltage that is out of phase with the driving voltage. The current caused by this back EMF acts to damp the coil's motion at low frequencies (it actually changes with frequency). If the resistance of the circuit the back EMF flows through is increased (by adding a high wattage resistor bank in series, or a long light gauge speaker cable), the less this counter flowing current will be able to damp the cone's movement, and the driver's output will increase. Also, with woofers (all drivers actually, but it is more pronounced in large drivers), the impedance increases at higher frequencies, reducing the impact of making small (1 to 5 O) changes in the circuit.

    Typical drivers rated @ 8O nominal impedance will have a DCR of 6 to 8O. Typical 4 O drivers will have a DCR of 3 to 4 O.

    And yup, two 4 O systems in series will do as you say. They may sound odd or great, it depends on how the sound from the two systems combine.

    Tiamat,
    Actually, the phase of the current varies with frequency. It starts at about 45° @ DC, goes to 0° at resonance, swings negative from resonance to about 100 Hz, and increases to +45° at around 10 kHz.

    If I could figure out how to attach pics this would be easier...
     
  14. rdp6

    rdp6 Senior member

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    I'm well corrected about loose language about my EE studies preparing me for work with audio amplifiers, I meant that my personal experience is limited enough to get me asking questions.

    It seems that sometimes there is a gap between electromagnetics theory studies and associated analytical problem solving and seeing a physical manifestation of the properties in the discussions. It's easy to see that the reactive component is the coil, yielding a (frequency * inductance) + DC resistance for overall impedance. That's easy enough, now we have back emf as the voice coil moves through the environmental magnetic field.

    At this point for me it is time to use some typical values and write out (that's how I work) some math to see what comes out. I am interested in audio amplifier and speaker design as a hobby. I can see me just finding more and more questions as I get into it.

    The vast majority of my memorable lab experience was with control systems and interfacing lots of different things to a PIC24.

    Anyways, the electrical characteristics being Z = jwL + R, but the physical construction of a speaker being rather spring-like, there must be a 2nd order transfer function. Cabinetry can restrict airflow, adding damping (scales with amplitude) Ok, from this tf we can get a frequency response, which is simple to convert to an impedance vs frequency curve.

    None of that tells me what this means for my mis-matched amplifier. Certainly they are designed to dissipate heat, as all my amps have large chassis with heatsinks and plenty of ventilation. Perhaps 4 and 8 ohms are industry accepted target impedances (at what frequency?) that everyone can use for their design goals.

    Is there anything to be gained by using a capacitor to offset the increasing reactance as frequency increases?

    Ok, I went away and looked for some typical parameters and found this: http://www.epanorama.net/docum...speaker_impedance.html
    Looks like I'll poke around there a bit.

    Thanks Tiamat and lhwidget for the interesting responses.
     
  15. Howard

    Howard Lifer

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  16. lhwidget

    lhwidget Junior Member

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    Glad to help, I'm an ME and still slogging through the math for filter design. You'll find that your intuition was correct, a sealed enclosure will yeild a second order roll off, and ported enclosures will yield fourth order roll offs.

    2, 4, 8, & 16 ohms are just the nominal standards for classifing drivers and speaker systems. In real speaker systems, it's not uncommon to see impedances starting at 6 to 7 O, peaking at 20 to 100 O (at woofer resonance) back to 7 O after resonance and wondering up and down with rising freq as the next driver's impedance and the crossover's influence begin.

    Want to rate an amp's output? I think that's why 2, 4, 6, 8, & 16 O are the industry standards now. Its too complicated to hit a moving target...

    Howard's mention of Zobel networks is interesting. It's amazing how flat the impedance curve can be made (after resonance) by adding a capacitor and resistor in series across a woofer's terminals.

    If you're really going to jump in with both feet, you might want to consider a design program that will model drivers', crossovers', enclosures' and complete systems' responses (impedance, freq response, and phase). I use a program called SoundEasy.

    I mention this because CAD programs will allow you to control not only the freq response of the drivers in a system with classic filter designs, but also allow you to see and control impedance problems by either changing the filter components or adding specific elements only as needed. The hardest loads I've seen typical speaker systems place on amps is caused by bad passive crossover designs which create very low impedance response (< 1 O) at some point. You'll see this with a good CAD program before you cook an amp.

    Howards list of links is the current cream of the crop. DIYaudio.com is one of the better forums for amplifier info. To his list I would add Rod Elliott's excellent site. He helps noobs like me understand analog electronics, and takes amplifier design as far as most people want to go. (He's also very down to earth and engineering oriented, for me, quite refreshing.)

    http://sound.westhost.com/index2.html

    Nelson Pass's site is good for amp design also.

    http://www.passdiy.com/articles.htm

    Good luck & have fun!
     
  17. rdp6

    rdp6 Senior member

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    Thanks a bunch. I'm suddenly having flashbacks of root locus and fun with compensator design in MATLAB.
     
  18. Patrick Wolf

    Patrick Wolf Platinum Member

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    What if a person did the opposite? 8 ohm speakers on a 4 ohm amp? I'm wondering if I could connect a pair of these to the ProMedia sub. I ordered an AVR but it won't arrive about a week after I get the speakers.
     
  19. lhwidget

    lhwidget Junior Member

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    It's not an issue going the other way, as you want to. Basicly, low impedance speakers will allow more current to pass through the amp's output circuit at any specific drive voltage. An amp designed for 4 O loads will have its maximum output voltage set low enough to keep the output transistors in their Safe Operating Area (thermally, e.g., not overheating). Using a higher impedance speaker (8 O instead of 4 O) just lowers the output current requirement and the amp will operate a little cooler. The down side is the system won't play quite as loudly. Unless you're gaming with really loud sound, or play your music at high levels all the time, I doubt you'll notice.
     
  20. MUDD1761

    MUDD1761 Junior Member

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    Never drop your ohm load below what the amp is rated. Unless of course you want to bring it to my shop for repair. you can however go as high as you want with the ohms. 8 ohms or above for an 8 ohm amp. heres a rule for ohms if you take two 8 ohm speakers and parallel them + to +, - to - you Divide the ohms in half ending with 4 ohms. If you series them + lead to one speaker +, - lead to the other speaker -, and jump the - & + between the two, you get double the ohms at 16 ohms. same rule applies for any two alike in ohms. you can also get fancy and parallel two and parallel two more and series them together to end back at 8 ohms. Hope that educates you on ohms.
     
    #19 MUDD1761, Jul 10, 2011
    Last edited: Jul 10, 2011
  21. MUDD1761

    MUDD1761 Junior Member

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    One more thing on ohms the lower you can get the ohms without going below what the amp is rated the better sound and drive you will get out of your amp. I cant stress enough about never dropping below the amps ohm rating it might work for a day it might work for a year but you will blow the finals in your amp sooner or later. so its best to stay above the ohm rating on any amp
     
  22. Soundmanred

    Soundmanred Lifer

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    Thread necro. It's two years old.
     
  23. Harvey

    Harvey Administrator<br>Elite Member
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    You're about half right and half dangerous. :eek:

    A given amp will deliver more power, but it will NOT necessarily provide better sound when driving a lower impedance. The formula for power is:

    P = E&#178;/R

    Power = the square of the voltage divided by resistance. That means, when the impedance is divided by two, an amplifier providing a given output voltage will be delivering four times the power into the load... IF the power ratings of the devices and the heatsinks are adequate to handle that much power.

    As you note, connecting two similar speakers in parallel will present a load of 1/2 of the nominal impedance of one of the speakers. So yes, you'll get more power, but that doesn't say anything about the distortion of the system. Lowering the load impedance lowers the damping ratio, which is the ratio between the load and the electrical source impedance of the amplifier, itself. Typically, a higher the damping ratio (higher speaker impedance) results in lower distortion.

    Furthermore, most amplifiers exhibit higher distortion with greater output power due to thermal and other physical characteristics of the output devices.

    Then, there's the matter that the impedance of a speaker is not a linear resistance across its range of operation. At any given frequency, it is a complex value determined by the inductance and capacitance of the speaker coil, the natural resonant frequency of the mechanical speaker mechanism, the acoustical properties of the speaker enclosure and a number of other factors in addition to the DC resistance of the coil.

    We haven't even touched the far more complex math of dealing with switching amplifiers (class D, class H, etc.), rather than linear amps, and though it's not germain to the OP's question, the game changes again when dealing with transformer coupled amplifiers, such as tube amps, where the amp and transformer are designed to operate over a much narrower range of loads.

    The actual resistance of a speaker that is nominally rated at 8 ohms can vary from well below to tens of ohms greater than its rated impedance. This graph shows the impedance curves of three dynamic speakers.

    [​IMG]

    Quoting from the source:

    You're right about not loading an amp beyond the manufacturer's specs because exceeding those ratings can be destructive to the amplifiers, but you're wrong to advise anyone that, without considering a lot of other factors, simply driving a lower impedance will result in "better" sound.
     
  24. JackMarse

    JackMarse Junior Member

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    A shyster years ago at a boutique stereo shop sold my parents Dynaudio 4 ohm speakers with an English integrated amp (don't remember the name). With the volume loud but not too loud the amp would go into protect mode and be very hot.

    Regarding the advantage of 4 ohm speakers, you will only hear an advantage if you listen to dynamic music such as classical and jazz. For pop musics 8 ohm is all you need.
     
  25. bigi

    bigi Golden Member

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    I have speakers rated at 6 &#937;.

    My amp can be set to either 4&#937; or 8&#937;. Which one should I use to get the best out of both in terms of sound quality?
     
  26. Auric

    Auric Diamond Member

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    Consult manual but it may read something like: "If the impedance of any speaker is 4 ohms or more but
    less than 6, set the Speaker Impedance to 4 ohms." Ergo, for 6, set to 8.