A dumb question about Ohms

Discussion in 'Archived Threads 2001-2004' started by Guy Martin, Dec 7, 2002.

  1. Guy Martin

    Guy Martin Second Unit

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    Folks-
    I feel pretty dumb about asking this, but I don't really understand Ohm ratings that well. I'm looking at upgrading my receiver and a couple of the models I'm looking at are designed for speakers with 6 ohm impedence. But my Energy Take-5s have 8ohm impedence. Is it bad to use a 6ohm receiver with 8ohm speakers?
    - Guy
     
  2. Cire

    Cire Stunt Coordinator

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    It's fine Martin. If it was the other way around, that would have been the potential problem.

    I use a 4ohm setting to match my Take 5.2s, which has 8-ohm nominal impedance, and impedance swings of up to nearly 4-ohms, depending on the frequency of the signal fed to it.
     
  3. Guy Martin

    Guy Martin Second Unit

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    Thanks Cire!
    - Guy
     
  4. RussKon

    RussKon Stunt Coordinator

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    the receiver companies are using the 6 ohm rating to make their receivers look like they have more power.

    100 watts @ 6 ohms is less power than 100 watts @ 8 ohms

    marketing - pure and simple!
     
  5. Neil Joseph

    Neil Joseph Lead Actor

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    My receiver is designed for 8 ohms and I have been using 6 ohm mains for years with no problems and no heat buildup in the receiver. I would dare not drive 4 ohm speakers though as this would put unneeded strain/load on it.
     
  6. Andrew Walbert

    Andrew Walbert Stunt Coordinator

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    Ohm ratings are fairly straightforward to work out, once you understand the basiscs. In a parallel circuit, resistance (ohms) drops, because the current has more possible paths to travel. In a series circuit, the resistance increases due to having a longer and harder path to travel. Therefore, if you run two 8-ohm speakers on the same channel in parallel, you drop the load to 4-ohms. In a series circuit of two 8-ohm speakers, the load jumps to 16-ohms.

    The lower the resistance of the load, however, the more current flows through, increasing the strain on the amplifier. If I remember the formulae right (please correct me if I am wrong), P(watts) = I(current) x E(voltage) and E(voltage) = I(current) x R(resistance). So P = I^2 * R. Therefore, if the receiver is rated for 100w @ 6-ohms, it is pushing approximately 4 amps on the line. But with 100w @ 8-ohms would only have to push about 3.5 amps.

    Most home amplifiers can handle an 8- or 6-ohm load, and some will handle a 4-ohm load for awhile, but that will often void their warranty. Car stereo amps, on the other hand, often are built for at least a 4-ohm load, with most doing 2-ohms or lower. This is because a lot of people run multiple speakers (subs especially) in parallel to increase the power to them.

    Note: If any of the above is not correct, please feel free to hit me over the head with a cluestick. It's been a couple of years since I studied this stuff.
     
  7. GeorgeTW

    GeorgeTW Stunt Coordinator

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    You are correct. I would just add that because current and resistance are exactly oppposite forces, they areindirectly proportional. Whatever one does, the other does by the exact opposite amount.

    Current (I) = Voltage divided by Resistance (Ohms).

    Since Resistance is the denominator of a fraction, increasing this lower value decreases the numerator.

    example. I = V/R : 4 = 8/2

    If we decrease the value of 2 to a One (8/1), then I = 8.
    If we increase the value of 2 to a Four (8/4), Then I = 4.

    Since Current is part of the Power equation (I x V = P), you can surmise that I & P are directly proportional (meaning that Resistance in Ohms is still an opposite force to both).
     
  8. Andrew.P

    Andrew.P Auditioning

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    This is wrong. If you change 'power' to 'voltage it would be right. Watts is the unit of power. 100 watts is 100 watts.

    By breaking down the power rating into the two components - voltage and current, you can start to see the difference between an amp rated for 6 ohms and one for 8 ohms. All amps will have a limit on how much voltage they can supply before they clip. They also have a limit on how much current they can supply before damage to the circuitry occurs (hopefully it shuts itself off first).

    Using V = I * R and P = I * V (V voltage (volts), I current (amps), R resistance (ohms), P power (watts)).

    Combining the two we get P = I^2 * R

    For the 100W at 6 Ohms amp we get
    I = 4.08 amps
    V = 24.48 volts

    For the 100W at 8 ohms amp we get
    I = 3.53
    V = 28.24

    So if you connect 8 ohm speakers to the 6ohm rated amp the 24.48 volts is the limiting factor. (If the amp tries to go over this it will clip).
    V = 24.48
    I = V/R = 24.48/8 = 3.06
    P = V*I = 74.9 watts.

    Unless I've made a mistake, you'll get at least 74.9 watts by using 8 ohm speakers with a 6 ohm before clipping occurs. It could differ depending on if the DC voltage supply levels are higher or lower than that 28.5 volts calculated.


    Oh and to answer the person on car speakers. You're not quite right. The reason for the lower impedence (2 ohms vs the common 8 in home amps) is that they use the car's DC electrical system for power, which is of course a nominal 12 Vdc (as compared to the twenty-some volts calculated above). Yes there are charge pumps which could convert that to a higher voltage, but then you'd have to deal with the noise those buggers make. Much cheaper and cleaner to just buy the heavier speaker cables and use lower impedence speakers.

    Why don't home amps use 2 ohm speakers? Biggest reason is probably because the transformer would have to be bigger for the extra current (at lower voltage) and thus more expensive. The higher impedence of 8 ohms also makes the cable resistance less of an issue for longer runs.

    Hope that helps.
     
  9. Yu-Ming

    Yu-Ming Auditioning

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    Can I please add my own newbie question to this thread?

    My Monitor Audio Silver S8's have 6 Ohm impedence while it states in the manual that my Pioneer VSX-D2011 amp is best matched with 8 Ohm speakers - basically the opposite situation to Guy's original post. The amp does have a 6 Ohm setting for lower impedence speakers.

    Reading the above, it seems like the main issue in this situation is the ability of the amp to supply enough current at high volumes. My questions are

    1. The D2011 can be configured to Bi-amp the fronts (using the two spare rear channels if you are using a 5.1 system) - will this help?

    2. Is impedence only likely to be an issue when pushing the amp hard? Ie if I listen at moderate to low volumes is it safe to assume that the impedence mismatch is not going to affect sound quality?

    Thanks for your wisdom!
     
  10. Patrick Sun

    Patrick Sun Moderator
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    If you turn up the volume too loudly, you simply run the risk of clipping the amp, which would, of course, affect the sound quality.

    So, you'll need to determine how loud you can go and still keep the amp operating in its normal range before it starts clipping the waveform amplitude and sends some bad stuff to the speakers (clipped waveforms simulate squarewaves which have very high frequency harmonics which can fry a tweeter in no time).

    Low impedance speakers do draw more power from an amp, and it does have an effect on how loudly you can go before the amp can't produce enough current to produce the amplitude for the waveforms to be played at a loud volume level through the speakers.
     
  11. GeorgeTW

    GeorgeTW Stunt Coordinator

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    Why don't you buy a 2 Ohm, 1 watt resistor for each of your speakers, and connect it in series with the amp?

    I am kidding of course. When speakers spec an impedance rating, it is usually centered around a middle frequency, say for example, 1 KHz. Check your docs to know for certain.
    Technically, no one can say their speaker is rated at ONE impedance value across the entire audio range. Ohm's Law won't let them.

    Why?

    Because Frequency is a component of Impedance.

    The formula for Impedance:

    Imp. = Square Root of (Resistance squared + Reactance squared)

    The Reactance is the combined total of Inductive and Capacitive Reactance(coils and caps), both of which have Frequency as a component of their value:

    Inductive Reactance = (6.28)fL :
    Freq. increases, Reactance increases proportionately

    Capacitive Reactance = 1/(6.28)fC :
    Freq. decreases, Reactance indirectly proportional

    If the frequency shifts, so does Impedance.
    It's Ohm's Law. Therefore, the rating given by the manufacure is only valid at a certain Frequency, which if not otherwise stated, is usually centered around the middle, or 1 KHz.

    I really wish my keyboard was capable of math symbols.
     
  12. Yu-Ming

    Yu-Ming Auditioning

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    Okay, thanks for the help folks!
     
  13. alvaro Castle

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    Then what's going to happen to my new VSX 49TXi running a 4 Ohm M&K speakers? Would I run the warranty if I decide to play at high levels?
     
  14. GeorgeTW

    GeorgeTW Stunt Coordinator

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    your new unit should have some sort of protection circuitry. If it shuts down while you are using it, add a 4 Ohm, 1 Watt resistor in series with the positive lead of each speaker.
     
  15. Paul Murphy

    Paul Murphy Stunt Coordinator

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    so in english im guessing that all means that the more Ohm's you have the better. If you have less resistance in each speaker what exactly is that going to do. Will it create better sound?
     
  16. GeorgeTW

    GeorgeTW Stunt Coordinator

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    Less Ohms in each speaker runs the risk that Impedance will reach 0 (short) much quicker, and Maximum Current will flow through, burning out everything in its path.

    There is nothing wrong with using 4 Ohm speakers if your receiver/amplifier is designed for it, but exercise some moderation. Products, such as those made by NAD, advertise high-current for extremely short durations (less than a quarter of a second). If you want to continously rock the Casbah, read the fine print for warranty coverage.
     
  17. Andrew Walbert

    Andrew Walbert Stunt Coordinator

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    Well, think of it like this: if your ohm load is too low, it means that current is moving way too freely and things will start to burn up (literally). If your ohm load is too high, then current won't move much and your speakers will barely make a sound. Superconductors (materials that move current almost flawlessly) have impedence of near 0. Insulators, however, have impedence that is considered "infinite". Ceramics, rubber, kevlar, and air are common insulators. That's why rubber is the most common material used to insulate wiring, because the current will flow through the conducting element instead of the insulating element. Electricity always follows the path of least resistance (ohms).
    So, no, having higher impedence isn't always a good thing, nor is having lower impedence always a good thing. You have to have the right amount of current getting through to ensure peak performance from your components. Amplifiers will overheat and burn up if pushing too low an ohm load, of course, a very bad thing. Yet if the ohm load is too high, they will be unable to deliver enough power to create sound.
    Er, I'm rambling here aren't I? [​IMG]
     
  18. Chuck Watwood

    Chuck Watwood Stunt Coordinator

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    I have a question to ask about this thread. I have just upgraded my system to mostly 8 ohm speakers, Polk RTi series, and my rear surrounds are Polk RTi 38's which are 4 ohm speakers according to the manual. My Onkyo receiver suggests setting all speakers to 4 ohms if even one speaker is a 4 ohm speaker. Since I changed the setting, I don't notice any difference in sound quality or any clipping. I assume that clipping is just the speaker cutting out? Any comments are appreciated.
     
  19. Chuck Watwood

    Chuck Watwood Stunt Coordinator

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    I forgot to add that my receiver is the Onkyo TXSR800. It has setting for 6 or 4 ohm impedance.
     
  20. Aaron Rosenfeld

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    Ok, I just took a college final over this less than 6 hours ago and am quickly trying to forget it, but just for anyone who still might not understand, I'm going to try do this without any equations. The less resistance (ohms) you have, the more current you will have flowing to your speakers. Current is the amount of electricty you have. Therefore, the less ohms, the more electricity, the louder your speakers will be. Since it is the reciever that provides the electricity, and the speakers that use them, if your reciever can work with less ohms (more electricity) great, but if your speakers aren't rated for less ohms it won't really be drawing any more power so it doesn't matter. What really matters is when you have low resistance speakers asking for a lot of power from your reciever that can't support it.

    The basic rule of thumb is:
    Low resistance capable reciever, high resistance speakers : Fine
    Low resistance speakers, non-low resistance capable reciever: BAD
     

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