Speaker mismatch in SS amps

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by alderbody, May 18, 2006.


  1. alderbody

    alderbody Member

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    As far as i know speaker mismatch is not so much of a problem in Solid State amps.

    But is it like that?

    I have a 20W (SS) Marshall and i want to connect it's speaker terminals on my twin's 2x12.

    The Marshall has an 8Ohm speaker and the twin has 4Ohm total load.

    Of course i can always disconnect one of the two 12" and have an 8Ohm load, but i want to know how safe it is to run it on 4Ohms, either for the amp or for the speakers.
     
  2. tonezoneonline

    tonezoneonline Member

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    Mismathching the load on a SS amp is asking for trouble.
     
  3. alderbody

    alderbody Member

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    Hmmm.... Could you be more specific?

    Ok, i got it. I shouldn't do it...

    but what's the trouble you're "talking" about?
     
  4. da-boogieman

    da-boogieman Member

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    I have not seen a solid state output that would have a problem with a higher load impedance. Since they don't have OTs (no "flyback" voltages) and are current outputs (vs tube voltage), higher impedance should be OK. Lower impedance is where you could quickly get into trouble. Higher current increases output transistor power dissipation. I would stick near the manufacturer's recommended impedance on the low end unless you have access to someone experienced who can review the design for you.

    If someone knows of a solid state amp that is overstressed by a higher impedance load, I'd like to understand the circuit details.
     
  5. bob-i

    bob-i Member

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    In my experience it's LOWER loads that cause stress on SS amps. The increased current can stress the power supply and cause starvation on the output devices and clipping. In extreme cases you can see transistors fail but in most cases it's the power supply that has the issues. The clipping can damage speakers.

    I've run many SS amps at much higher impedances all the way to open with no issues.
     
  6. donnyjaguar

    donnyjaguar Member

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    This is a pretty easy thing to test to see if its okay. As a general rule-of-thumb, solid state output devices shouldn't get any hotter than a cup of coffee (or tea if so inclined). I would suggest playing it for a while and checking with you hands. If you can't hold a finger to the heatsink where the output devices are attached, its running too hot. The option is to play at a lower volume or through only one speaker. Speakers are rated at nominal impedances. That means "in name". The actual impedance across the frequency spectrum can vary quite a bit both lower and higher. For instance when you go up in frequency the inductance of the voice coil actually causes the impedance to increase. When you hit the speaker's natural resonance it will also increase. The latter is effected by the cabinet also. Technically the output impedance of a solid state amplifier is much, much lower than 8 Ohms. If you know the damping factor of the amplifier you can calculate this impedance. For example, if is is 100, then we know that the output impedance will be 8/100 Ohms. That's pretty low. When you go to a lower impedance the power should technically double (for the same voltage output). In some amplifiers it does, but in others it will drop off. The reason for the dropoff could be either the power supply can't cope with the additional current, the impedance of the output devices has enough resistance that some of this power is dissipated across them, thus making them run hotter, or both. Most musical instrment amplifier designs (and I'm speaking in generalities here) are robust enough to handle this kind of abuse.

    DJ
     
  7. drbob1

    drbob1 Silver Supporting Member

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    Just for fun, is there any marking on the power amp about "minimum impedance"? or can you google that? That'd tell you the minimum it was engineered (and guaranteed) for-usually 1/2 of the supplied speaker (so you can use an extension cabinet).
     
  8. plowkraut

    plowkraut Member

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    I wouldn't risk it. Buddy of mine ruined a SS Marshall using the wrong impedance.
     
  9. alderbody

    alderbody Member

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    I actually tried it a long time ago, but i never cranked the amp.

    I remember that when i heard this "clipping" mentioned above, i immediately killed the project to protect my speakers and my amp.

    I think i'd only try that again with the one speaker out, to match the rated impedance.

    Thanks for all your replies!
     

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