Power Amp Speaker Impedance Woes

Discussion in 'Live Sound' started by michaelwithersone, Sep 4, 2008.

  1. Hi there - I recently purchased a cheap 100watt stereo power amp from maplins for use in my guitar rack system(I got fed up of lugging my 20kg tube power amp around!!) .

    Unfortunatley my poweramp has 4ohm stereo speakers outputs and my speaker cabinet loads at 16ohm when wired in stereo. I have read that this is considered a safe impedance mismatch when using solid state amps but it will have some adverse effects, with lowered output volumes and there will be some high end frequency cut. Should I be concerned about this?

    4ohm into 16ohm seems like quite a big mismatch and I know if I connect my cabinet in mono then the cabinet impedance drops to 8ohm. This would obviously mean connecting only 1 output of the poweramp and having one output totally unloaded which iam pretty sure will be unsafe even if I turned the power volume to 0 for that channel. Does anyone know if this is definatley the case?

    Thanks for taking the time to read my post - and thanks in advance for any help!

  2. Greener

    Greener Guest


    Sorry I mean, pardon?

    Your amp is rated up to 4 ohms, you will be safe.

    If you bridge to mono one channel is not not used. This is the bit I'm whatting about.
  3. This was the answer I was looking for! I used to the world of Valve/tube guitar amps where if you mismatch the impedance you generally get shafted so I thought it was better to check!

    When it bridging to mono does it just switch the other channel off or will it put the full 100 watts into the channel that is being used? Also will running the amp in mono change the output impedance of the amp?

    Sorry for all the noob questions - the manual and technically specification with my amp is next to useless!

    Thanks Mike
  4. Greener

    Greener Guest

    Bridging puts the full 100watts into one channel. Although, really it should bridge 2*100w because you have two amps, one left and one right.

    The output impedance of the amp is not what you're thinking about.
    Basically the amp ohmage rating is a stability rating, when you run at 16ohm you get lower currents than when you run at 4 ohm. Your amp is 4ohm stable so you can run 4ohm, 8ohm, 16ohm or 40,00000000000,000 ohms... No worries.

    Just don't go trying to push a 2ohm car sub without a fire extinguisher or somnat.
  5. GeckoMusic

    GeckoMusic Guest

    Does not compute. :D How many speakers do you have in your cabinet and how many ohms on each?

    16   8   4    2  <- ohms each
    64  32  16    8  : 4 speakers all in series
    16   8   4    2  : 4 speakers, 2 series, then those two in parallel
     4   2   1  0.5  : 4 speakers all in parallel
    32  16   8    4  : 2 speakers in series
     8   4   2    1  : 2 speakers in parallel
    .. if you have two independent, channels each 16 ohms, on your cabinet I guess that would make scene... If that is the case, then wire them as 8 ohm parallel for double power!
  6. Greener

    Greener Guest

    Gecko, I totally understand what you're talking about but I can't hook sense from that table. Lol. :)

    For series R = R1 + R2 + R3...
    For parallel 1/R = 1/R1 + 1/R2 + 1/R3...
    Where R is the total resistance and R1 R2 R3 etc are the individual speaker ohms.

    Wait, I just figured your table out. Groups of 4 and 2.
    It's late. :p
  7. GeckoMusic

    GeckoMusic Guest

    Table edited for clarity.

    Thanks Greener :)
  8. Codemonkey

    Codemonkey Distinguished Member

    Dec 11, 2007
    Scotland, UK
    "get shafted"

    LOL I love that word.

    Along with many other fantastic words.
  9. Boswell

    Boswell Distinguished Moderator Resource Member

    Apr 19, 2006
    Home Page:
    I think you are confusing two different concepts here.

    The first is paralleling of loudspeakers. Wiring two 16 Ohm cabinets in parallel gets you an 8 Ohm pair with the power split between the two. This 8 Ohm load would connect to one channel of the amplifier and the other channel would be left open circuit. This does no damage. However, a better arrangement would be to use one 16 Ohm loudspeaker on each channel and to send the mono signal to both amplifier inputs.

    The second is bridge operation of amplifiers. In this configuration, you would wire the loudspeakers in parallel (to get 8 Ohms) and take one loudspeaker wire to the + output of one channel and the other wire to the + output of the other channel. For this to work, either the amplifier must be switchable to bridged mode (channel B invert), or you must feed phase and antiphase drives to the amplifier inputs. This is generally not a problem when the amplifier is being driven from balanced outputs, as a simple XLR pin 2-3 crossover achieves the desired effect.
  10. taxman

    taxman Active Member

    Sep 22, 2006
    As to making the bridging the stereo amp to mono, I checked on how to do it to my Rotel. It requires you to change three internal jumpers.

    My guess is Mike's amp can't be bridged mono.

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