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Impedance matching

Discussion in 'Home Cinema Speakers' started by BenedictGilman, May 10, 2005.

  1. BenedictGilman

    BenedictGilman
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    I've just irritatingly bought an amp and speakers from Richer Sounds only to find that they might not be compatible. The Amp says that it needs the speaker sets connected to have 16 Ohms impedance, but the speakers apparently only have 4-8 Ohms Impedance. I was hoping someone on the forum could give me some advice on my options, I gather there are various ways of dealing with this.

    Wiring in series?

    Bi-wiring?

    Adding an additional resistor / transformer?

    The amp is a marantz PM7200 and the speakers are Mordaunt Short Avant 902's. If anyone can suggest any options I'd appreciate it.

    I g
     
  2. MPK

    MPK
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    That sounds weird. The Marantz website says the following on the PM7200: "It delivers 2 x 95 Watts (RMS into 8 Ohms), rising to 155 watts with 4 Ohm speakers". I've never seen an amp that requires 16 Ohm speakers and would be extremely surprised if a Marantz stereo amp would do so.
     
  3. Reiner

    Reiner
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    Would work, but you would need to purchase a 2nd set of (identical) speakers.

    Makes no difference in terms of load to the amp - the total impedance the amp "sees" remains the same.

    You will find that 100 Watt resistors are not really small and make good heaters while I am not sure you find audio-grade transformers for this purpose. ;)

    Anyhow, the back of the amp says 8-16 Ohm but 16 Ohm if System 1 and 2 (A & B) are used.

    So I reckon you are fine with your speakers as long as you use one pair only.
     
  4. severnsource

    severnsource
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    The need for careful impedance matching is a blast from the past. Valve amplifiers needed to match the speaker impedance fairly closely in order to get maximum output from the amp, and high power output was expensive to achieve.

    Any half decent semi-conductor amp doesn't care much about speaker impedance. If the speakers are high impedance the maximum power output will be reduced, if the speakers are too low an impedance there is a very remote possibility of damaging the amp if it is not well designed. However any modern amp should work perfectly happily with 4-8 ohm speakers. 16 ohm speakers were also only common in the valve era, the last ones that I can recall were the LS3/5as that were designed in the late 60s.

    Of course if the amp has 2 sets of speaker terminals that can be used together (for feeding speakers in 2 rooms say) then the speakers would be a parallel load and the effective impedance would be halved, so the amp makers would then be erring on the side of caution.

    Bill
     
  5. BenedictGilman

    BenedictGilman
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    Would that make each speaker as powerful / more powerful than they are currently?

    Damn, I was planning on hooking two pairs up, that's part of the reason I wanted the Marantz amp because it had two speaker sets :(
     
  6. DAMOON

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    Just makes sure there is plenty of cooling and not too much on top
    The lower impeadence load will just draw more power and limit the ultimate volume level you can acheive with out distortion or cliping.
     
  7. BenedictGilman

    BenedictGilman
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    :confused: So it does. They're not making this easy, god knows what they really mean, looking around for speakers there just aren't any 16 ohm speakers. They don't even have an email contact address :(
     
  8. BenedictGilman

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    Cool, I think the next door neighbours will impose more of a restriction on volume than the technical limitations anyway. Besides, I have a club amp and speakers I can use if I want to be really noisy, I just wanted something really good quality for my own enjoyment.

    At only moderately anti-social volumes it should be okay though?
     
  9. BenedictGilman

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    I certainly haven't been able to find any 16 Ohm speakers, I reckon they're just being extra careful :chin: Only one way to find out I guess :nervous:
     
  10. BenedictGilman

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    If I had two speaker sets would it be better to wire them in series through one speaker channel or in parallel using both speaker sets?
     
  11. alexs2

    alexs2
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    As long as the impedance of both sets in parallel does not drop below 4 ohms(at which your amp is rated to deliver up to 155W),then you should have no problems.

    Most decent modern solid state amps will manage a 4 ohm load quite happily,although a large number begin to struggle quickly with lower loads than that,hence my comment about ensuring your speakers are 8 ohms or above if running in parallel.
    Also worth bearing in mind is that a number of speakers rated at 8 ohms do run to lower impedances for areas of their frequency response,and some may actually be better rated as 6 ohm speakers.
     
  12. dmurfet

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    This maths works but think there are probably a million reason why itr would be foolish but here goes anyway....

    Put two speakers in series - instead of parallel - and daisy chain them - hence 16 ohms...

    Extra speaker cable would probably put up the resistance a bit too...

    <hides>

    :oops:
     
  13. Reiner

    Reiner
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    I don't understand what you mean by "powerful"?

    In parallel is normally preferred - but note that the impedance will drop as alexs2 pointed out.
    If you wire in series (which increases the impedance) you should use identical speakers.
     
  14. BenedictGilman

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    I think they were rated as 4-8 Ohm speakers able to take up to 150W so presumably if I don't use the full extent of the Amp's range (which I can't afford to due to neighbours) then hopefully it should be okay in parallel.
     
  15. alexs2

    alexs2
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    As all the above posts have explained,by running in parallel,you will halve the effective impedance,thus reducing the load from 4-8ohms,down to 2-4 ohms.

    Irrespective of running your amp at high levels,if the impedance does drop to 2 ohms for significant parts of the frequency spectrum,then it will certainly tax your amp a bit.

    As I'd said above,most amps will happily run into 4 ohm impedances,and ideally should be able to double their power output into that,assuming a good enough power supply,output stage,and heatsinking,but very few will be able to run loads much lower than that without trouble.

    It may be worth your while giving MS a ring and getting their opinion on the true impedance of their speakers,as a number of manufacturers may cover their liabilities by specifying a wide impedance range.

    It is also very likely that your amp's protection circuitry would shut it down should it overheat,although that couldn't be guaranteed....not much an answer I know,but as I said,very few amps can be guaranteed to be able to run very low loads without trouble.
     
  16. pragmatic

    pragmatic
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    The question is, why do you want 4 speakers, unless its for multiple rooms alot of people would argue that you will spoil the stereo image and loose detail through having more speakers blasting at you. More does not always mean better.
     
  17. MPK

    MPK
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    I agree with gazbarber. Using the A and B speakers only makes sense if they are in different rooms. Otherwise you should get a multichannel amp with DTS Neo6 or PLIIx if you like surround sound.
     

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