Tell me about an "impedance mismatch"?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by wrxplayer, Feb 26, 2009.

  1. wrxplayer

    wrxplayer Supporting Member

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    The specific question is what would happen if someone wanted to run a second cabinet off of a single speaker output using a y cord with a combo amp that has no impedance adjustment? My understanding is it would create an "impedance mismatch" but I don't know what that means.

    Tnx
     
  2. skipm45

    skipm45 Member

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    With a "Y' cable, I'm assuming the two cabs are in parallel, so the impedence would be halved. A rule of thumb here is that a 2x mismatch is OK. Some will argue against this, some will be in favor.
    I've done it, and it worked OK for me.


    Skip
    www.skipzcircuits.com
     
  3. rockon1

    rockon1 Member

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    Yes it would be an impedance mismatch. The amp want to have a certain load-usually 4,8 or 16 ohms. Most amps will tolerate a 100% mismatch-double or 1/2 what the amp is set at. It might help if you gave us a bit more information. What amp? Whats the impedance of the amp? Whats the impedance of the cab? Or is this just a hypothetical question? Bob
     
  4. wrxplayer

    wrxplayer Supporting Member

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    It is not a hypothetical. I bought a used Alessandro Working Dog Doberman (which sounds great). After I bought it but before it arrived I emailed Alessandro asking if there was an outlet for a second cab and also asking what the stock impedance was as this info isn't available online. He responded it is 8 ohms and that there was not a second speaker outlet and that one would use a Y cord to run a second cab. I followed up asking if the speaker in the second cab should also be an 8 ohm speaker. His reply was "there is no impedance switch so if you run a second cabinet there will be an impedance mismatch." He later wrote that I could run a second cab but that the amp was designed to operate as a combo. Which gets back to my first email and his reply which suggested if I wanted to run a second cab I'd use a Y cord but as added later that there would be an impedance mismatch.

    Impedance mismatch sounds like a bad thing. I'm confused. Is running a second cab this way to be avoided? If I was going to do it what kind of speaker would I use?
     
  5. Lespaulsignature 74

    Lespaulsignature 74 Silver Supporting Member

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    If you want to run an extension cabinet and match the required 8 ohms you would have to have two 16 ohm loads in parallel. The speaker/speakers in the combo would have to total 16 ohms and the external cab also at 16 ohms, this would give you an 8-ohm load! But you would always have to use the external cab in order to achieve the 8 ohm load.
     
  6. rockon1

    rockon1 Member

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    It seems like he isnt concerned with an impedance mismatch otherwise he would have said straight away not to do it IMO. Most amps will tolerate a 100% mismatch ie; in your case 4 ohm load into an 8 ohm amp if the cab you use is 8 ohms. If its 16 ohms them the mismatch is even less at 5.33 ohms. Bob
     
  7. GearHeadFred

    GearHeadFred Member

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    I think the OP is asking what the risks are of an impedance mismatch..

    It puts more strain on the output tubes and output transformer. If this extra strain pushes them beyond their limits, they can be destroyed..

    As some have said, there is usually enough "wiggle room" in the design of amps to tolerate this.. but there are many variables (how loud do you play, what type of tubes are they, how hot is it?)

    Hope this answers your question.
     
  8. amp-head

    amp-head Member

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    A LOT has been written about impedance mismatches, however there still exists a great deal of confusion on the subject.

    The benefit of matching the output impedance of the amp to speaker cab is power transfer - theoretically maximum output power will be achieved when the impedances are matched. Having said that, it really won't make much of a difference volume-wise if you mismatch impedances and cause a power transfer drop when using a guitar amp. So, that's not a problem.

    The most common concern is: will I blow up my amp if I don't match the cab to the amp output impedance?

    The answer is: not likely.

    In lower power amps - say under 50 watts - that use good quality output transformers I say mismatch to your heart's content. The likelihood of damaging tubes from high current or output transformers from high voltage is remote - besides that would really only come into play if for many hours in a row you played your amp at full volume or close to it - does anyone ever do that?

    With higher powered amps I would be more concerned, but if the mismatch range is from 4 - 16 ohms (any combination of 4, 8 and 16 ohms) and the amp is not going to be continuously dimed for long periods it is again unlikely that damage will occur.

    Notwithstanding the above, there are situations where OTs and/or tubes have been damaged due to impedance mismatches. In such a case I would be looking at the quality of the OT as the root cause of the problem.

    So, the possibility of a problem exists, but in the real world of tube amp usage I believe it to be small.
     
  9. Keyser Soze

    Keyser Soze Member

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    Just to add another generalization to the mix.

    A mismatch of lower total impedance is generally safer for your amp than one with a higher total impedance. Lower than spec impedance basically just works the output section harder than usual. Higher than spec impedances can cause nasty things called flyback spikes, over time these can short out your output transformer.
     
  10. wrxplayer

    wrxplayer Supporting Member

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    Another question: If the amp is 8 ohms and is driving an 8 ohm speaker (combo) and I want to use a Y cord to run a second speaker, what ohm speaker should the second speaker be if I want to minimize risk to the amp?
     
  11. GearHeadFred

    GearHeadFred Member

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    I think the best answer is 8 ohms. You will still have a mismatch, but it will be a lower one, which is preferable... plus, if you choose another value (16 ohms, for example), you will have the additional problem of one speaker being sent more power then the other, and volume balancing might become an issue.
     
  12. rockon1

    rockon1 Member

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    True about the imbalance of power with 16 ohm ext cab. 2/3 of the power will go to the 8 ohm speaker. That said there will be less of an overall mismatch at 5.33 ohms using a 16 ohm cab. At any rate the extension cab should be 8 or even 16 ohms -4 ohms will create too much of an overall impedance mismatch. Bob
     
  13. mooreamps

    mooreamps Senior Member

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    Output transformers used in guitar amplifiers have secondary taps ; marked by impedance. 4 ohms, 8 ohms, 16 ohms. It is done this way, such that, it allows the user to load the output transformer to the loud speaker / or loud speakers. . . As such, the reflected impedance going back to the power tubes , through the primary coil of the output transformer , remains at the intended level. The intended level is defined by the designer which depends on what type of power amp is being used.

    For example, proper loading for one EL-84 used as the power amp, is about 5 K ; assuming the load on the output transformer is matched to the speaker load. If the speaker load is different, then you get a different reflected impedance from the primary coil back to the power tube ; either higher or lower than the 5 K reflection.

    Now, what the other guys are saying is, and I have seen this in my own experience, if there is a mismatch between the speaker load, to the output transformer, it does not seem to matter too much. These are audio frequency amplifiers, and the power tubes can tolerate a certain range of output loading.

    Now for some advanced thinking here. Output transformers work just like power transformers. Power transformers have tapped secondaries, output transformers have tapped secondaries. A center tap on a power transformer provides a lower output voltage, just like the 4 ohm tap of an output transformer provides a lower driving voltage. You can use the 4 ohm tap to drive anything, and nothing in the amp should become over stressed. However, as the 16 ohm tap provides the highest output voltage from the output transformer, I would use care to maintain a 16 ohm load on that 16 ohm output tap.




    -g
     
    Last edited: Mar 1, 2009
  14. donnyjaguar

    donnyjaguar Member

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    I just wanted to emphasize what another poster said. The amplifier will be most efficient at its design impedance. By placing a second cabinet in parallel the maximum power of the amplifier will be reduced. This is generally the opposite of what you want. However, the net result may indeed be more volume anyway.

    Its generally a safe bet unless the amplifier is so poorly designed its running on-edge normally.
     
  15. jay42

    jay42 Member

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    It's an EL-34 amp:

    http://www.alessandro-products.com/working_dog/html/doberman.htm

    Don't mismatch up. Put another way, don't attach a 16 ohm load to this thing. I avoid mismatching any amp with EL-34s. If you're going to do this a lot, pick up a 100W Z Matcher from BBQ-boy.

    What's going to happen with a downward mismatch is that you won't get all 40W clean happening. It's inefficient to run at a mismatch. You might get 30W instead. By adding speakers, you might believe that it will be louder, but that really depends on the speakers. If they're all identical -- those in the combo and those in the extension cab, it won't be louder. If the combo speakers are 97dB models and your extension cab has 103dB speakers in it, it will be louder.
     
  16. wrxplayer

    wrxplayer Supporting Member

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    It seems as though the best thing for me to do is run the amp as a combo as using a second cab might have risks and will likely alter the amp's character. Although I did purchase another (unloaded) cab after George Alessandro advised that I can use a y cord to add a second cabinet, it appears that it wasn't the best piece of advice.

    Thanks for all of the advise. Check the emporium in another day or two for my new-soon-to-be-old cabinet. My wife told me it arrived today.
     

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