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Can someone clarify Impedance and Ohms, watts for me?

Discussion in 'Sound Science' started by pancakeplease, Dec 3, 2011.
  1. pancakeplease
    Hey, anyone want to take me to school on a particular electrical issue? :)
    Question 1: 
    I never took electrical engineering in college... just a bunch of biology so this is all new to me. 
    From what I understand, Impedance is measured in the unit "Ohms", which when it comes to audio equipment, refers to how much resistance the speaker requires?/puts out? (i think it's puts out but I'm not sure). And the amplifier/receiver needs to match the Ohms for it to be able to power it? All I know is that 8 ohm is the most common and "weakest" or "least demanding" of an amplifier, correct? And when you get down to 4ohms, it requires much more from an amp. But requires much more WHAT? Does it require more watts to power speakers with LOWER impedance? How do watts factor into this?
    In my real life scenario, my bookshelf speakers are 8 ohms. My receiver/amplifier is rated at 85w PER channel equally distributed to all 5 channels and 1 LFE/Subwoofer channel for a total of 4XX watts. The 85w is rated at 8 ohms. Here is an excerpt from my amp: 
    Power Amplifier Section
    Rated output                       *THD figures are power amp stage values. ​
    Front...........................................85 W + 85 W (8 ohms, 20Hz - 20kHz, 0.05% THD) ​
    Center........................................ 85 W           (8 ohms, 20Hz - 20kHz, 0.05% THD) ​
    Surround ................................... 85 W + 85 W (8 ohms, 20Hz - 20kHz, 0.05% THD)​
    What I don't understand is why does the watts go UP when the impedance is lower? Meaning why does it go up to 120w for 4 ohms? Shouldn't it be lower? The amplifier is able to put out 120w for 4 ohms but only puts out 85w per channel for 8 ohms? Doesn't make any sense....
    If one were you use lower impedance (6 ohm or 4 ohm) speakers with the above rated power specs, how would one calculate whether or not it would be safe to do so? 
    Question 2:
    I have logitech x-530 5.1 computer speakers. These are the power ratings:
    1. Total FTC power: 70 watts RMS 
      Left/right: 7.4 watts RMS x 2 (into 4 ohms, @ 1 kHz, @ 10% THD) 
      Center: 15.5 watts RMS (into 4 ohms, @ 1 kHz, @ 10% THD) 
      Rear left/right: 7.4 watts RMS x 2 (into 4 ohms, @ 1 kHz, @ 10% THD) 
      Subwoofer: 25 watts RMS (into 4 ohms, @ 100 Hz, @ 10% THD)
    2. Total peak power: 140 watts

    If I were to plug these into my receiver at the same time as my bookshelf speakers, how will I know if it's too much? I tried it and it works fine but I'm not sure if it's safe electrically speaking. Don't want to blow anything. 70w @ 4ohms.. should be a LOWER # of watts @ 8 ohms? 
    Cliff notes: 
    Why do Watts go up when Ohms are lower (I.E. 85w @ 8 ohms vs. 120w @ 4 ohms) when lower Ohms indicates more resistance and requires more power from the amplifier? 
  2. LimeANite
    You've got it backwards - higher impedance means more demanding (in general).  If you feed the same current into something with an impedance of 4 ohms and something with an impedance of 8 ohms, the 4 ohm device will receive more power.
    In electrical terms, ohms are a measure of resistance - the ease (or lack thereof) with which an electrical current can pass through a device.  Higher resistance means that it's more difficult for the current to pass through.
    Edit: As for the specs, I'm not particularly well-versed on their exact meanings, but I'm pretty sure that the power ratings on the receiver are maximums, not constant power levels.  Someone more knowledgeable on the subject should probably confirm this first though.
  3. AzraelDarkangel
    Right, ohms is a measurement of how much something resists or "impedes" the flow of electricity. Higher ohms means it resists the flow of electricity more. Speaker amplifiers are usually made for speakers of between 4-8ohms which means they are designed to be most efficient in that range. That can usually drive somewhat higher impedance safely but not very efficiently however driving lower impedances than recommended can damage the amp because of the higher return current causing the amp to heat up and maybe overheat (someone with more engineering knowledge can explain much better).
    Don't be too worried by an amps wattage rating. It's simply a matter of the efficiency rating of the speakers. For example if the speakers are rated at 88db (1w/1m = 1 watt input measured from 1 meter away) than 2 watts input will mean the speakers can play at 92db (double the watts = adding 3db to the maximum loudness of the speakers)  4 watts = 95db, 8 watts = 98db, 16 watts = 101db, etc. As you can see it doesn't take many watts for most speakers to play pretty loudly. Most common speakers are usually rated between 86db and 90db (for 1 watt).
  4. LimeANite


    Thanks for clarifying that part.  I had a feeling I was missing something in my explanation.
  5. Asr
    Ohms are a unit of impedance, which is electrical resistance - i.e., 4 Ohms is less resistant than 8 Ohms. Wattage ratings go up from 8 Ohms to 4 Ohms because it takes more current (measured in amperes) to drive speakers at 4 Ohms.
    Watts (W) are a unit of power = voltage (V) multiplied by current (I, or A). Hence, voltage has an inverse relationship to current.
    The wattage typically almost doubles from 8 Ohms to 4 Ohms because the 4 Ohm speaker requires more current (i.e., more amperes) to satisfy its low resistance.
    The bottom line is that current is just as important as voltage, and 8 Ohm and 4 Ohm speakers have slightly differing demands - the 4 Ohm speaker requires more current to achieve volume while the 8 Ohm speaker won't require as much to reach the same volume. Having enough current on tap on a 4 Ohm speaker is hugely important - a lack of available current can actually damage speakers. A lot of amplifier manufacturers don't publish their peak current specs. Parasound is one of the few that do, and you'll find the spec listed as "current capacity" (see the JC1's specs here for example: http://www.parasound.com/halo/jc1.php).
    The numbers alone don't mean much, especially on amplifiers. With speakers it's extremely important to look at the speaker specs first - as their impedance is a design decision, and it's necessary to use an amp that can output enough power at the speaker's impedance.
    These are the specs from Monitor Audio's PL200 speakers for example:
    4 Ohms
    117.8 dBA
    250 W
    100 - 250 W

    As you can see, Monitor Audio is nice enough to tell you what sort of amp power is needed to drive these speakers - 100W minimum at 4 Ohms. Of course it's a good idea to use an amp that far exceeds the minimum though.
    Using 4 Ohm speakers will cause an amp to produce more wattage, which also means more heat output, and you'll find that extremely powerful amplifiers are usually designed to combat heat in some way - typically with tons of heatsinks.
    The Plinius SA-103, for example, outputs up to 220W @ 4 Ohms. That's a huge amount of power. This amp will drive the Monitor Audio PL200 properly too. [​IMG] It will get very hot if using it to drive any 4 Ohm speakers. That's why it has a huge chassis and tons of heatsinks:
    The specs on amplifiers are also usually maximums. During typical listening, the power output will never even approach the maximum. It'll increase when music gets loud and decrease when music gets quiet. Most amps are designed to have more than enough power than you'll need for this reason - so they won't run out of power at anything below ear-bleeding volume levels. And on 4 Ohm speakers, very loud volume bursts will require instantaneous current - i.e., the peak current spec will come into play for loud music. If there's a lack of available current, the speakers will get damaged.
  6. AzraelDarkangel
    An over simplified conceptual way to view it too is too compare an amp to your cars engine. The amp is made for a certain range of ohms like your car engine is made for your car. You don't want to sit still and rev your engine really high for long which is like having very low load on your amp. Both could overheat under those conditions. You can pull a small trailer behind your car but not a huge tractor trailer which is equal to a higher ohm load on your amp than it's rated for. Too much of a load can cause problems too. But with most speakers there should be no problems.
    Now as far as amplifier power goes. Even if you have plenty of power for your speakers to go as loudly as you like you can still get improvements from more powerful amps. Sort of like how your Volkswagon Rabbit may have enough power to go at highway speeds yet it may not be so easy to pass other drivers at those speeds but if you Have a Corvette ZR-1 you have plenty of power on hand even if you never go faster than regular highway speeds. This relates to the control an amp has over the motion of the speaker drivers.
    Awesome explanation and example Asr!
  7. kingpage
  8. wuwhere Contributor
    I had a pair of 1-ohm Scintilla speakers, my mono-block KMA-160 Krells did not get hot. But I did burn one of the Electron Kinetics mono amp.
    Yes, you can damage your speakers when your amp clips.
  9. pancakeplease


    Thank you all. This helped a lot and so did that link above this post. 
    Does anyone know how to calculate (roughly) the conversion to use 4 ohm speakers on an amp/receiver? My receiver says nothing about being restricted to 8 ohm speakers, just that it is rated at 85w/channel @ 8 ohms for a total of 430w. I believe it says somewhere around 120w/channel @ 4ohms but I'd have to double check. How would one go about figuring out if this will work with speakers? My current 8 ohm speakers are rated at 20-100w so the original 85w rating would work. Looking above at my first example, if I were to hook up my computer speakers (logitech x530s 5.1 speakers) that are 70w @ 4 ohms, what's the compatibility on that? It seems super small and since they're computer speakers each using around 7.5 watts (4ohms) it doesn't seem like it would damage my amp but I'd like to know if there's a way to calculate so I can compensate for the increase in demand of my amp by the 4 ohm specification of the computer speakers. 

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