Question on mis match speaker ohms

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by roadhog96, Jan 13, 2009.

  1. roadhog96

    roadhog96 Member

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    I'm using a SFDR which has 8 ohm output and the extension cab has 2x12 at 8 ohm each. They are wired in parallel to 4 ohm. Will this hurt my amp, output tranny or? I have read it won't and have read it will. Which is it, there must be someone who is knowledgeable on this subject. I really don't want to ruin this amp.
     
  2. RyanPitch

    RyanPitch Member

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    At the very least it'll make your tubes wear out more quickly. At the very worst, who knows, maybe your tranny will overheat or something.

    In general, I think, you'd rather have the mismatch the other way around (i.e., run a 16 ohm cab). Hopefully someone else who knows more than I do will chime in and confirm.
     
  3. rockon1

    rockon1 Member

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    Dont know about the amp in question . Most (meaning alot but not all) will tolerate a mismatch of one in either direction. A mismatch to a lower impedance Ive read is preferable(ie: 8 ohm amp/4 ohm cab) as theres less chance of "flyback " voltages damaging the OT. Bob
     
  4. RyanPitch

    RyanPitch Member

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    Interesting, that's the opposite of what I've read. I'll have to pull out some books.

    On page 307 of Gerald Weber's book "Tube Amp Talk for the Guitarist and Tech," in the chapter titled, "Everything you always wanted to know about transformers (but were afraid to ask)," it says:

    "If you run a 4 ohm speaker load on an 8 ohm output transformer, this will shorten tube life dramatically."

    He says some more in the book about this topic, but I can't find it right now.
     
  5. slider313

    slider313 Silver Supporting Member

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    Your DR extension speaker out is in parallel. If you use a 4 ohm speaker load it will drop the total ohms to 2.6, which is too low for your tranny. If you rewire the cab to 16 ohms, the total would be 5.3 which would be closer to the 8 ohms the tranny "wants" to see.
     
  6. roadhog96

    roadhog96 Member

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    See this is what I was refering to. Some say to wire it to 4 ohms and some say to wire 16 ohms. It will damage the tubes dramatically at 4 ohms. Which tubes, all of them? Can you be more specific on the tubes. If I wire it for 16 ohms it will be closer to what the trans output is but still not what it should be. What will it do like this? Is there more chance of Flyback voltage damage, not fure what that is but have heard of it before.
     
  7. roadhog96

    roadhog96 Member

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    I did a little research and this is what I found. Go to this web page and scroll down to Ohms and Amplifiers and read the third paragragph. Please translate this to me because I want to make sure I'm understanding things correctly.
    http://www.legendarytones.com/ohms.html

    Here is an artical that I copied and pasted from a another site. Looks like it's harmful either way.
    From: Chuck

    I have heard various views on impedance mismatches between the amp and the speakers. One is that you should always match the impedance (4 ohm amp = 4 ohm speaker or two 8 ohm speakers in parallel), or you can blow your transformer. The other is that it is fine to mismatch, but you may lose power. Should the impedence match? If yes, then how quickly could you damage your amp when you have a mismatched impedence?
    Chuck, technically, you should always provide a load that is recommended by the manufacturer of the amp. The designer of the amp chose a particular output device (tube) and specified all of the operating voltages for the output stage so the tube would work at its optimum efficiency while delivering maximum power to the load with minimum distortion. Ok, so let's discuss the problems associated with mismatches. When you use a load that is lower than the intended load, the output has to drive the load (speaker) with more current because it is a lower impedance than is expected. Two inherent problems associated with transformers are flux leakage and regulation. Flux leakage is also referred to as leakage inductance. It is related to the current in the secondary, and these problems increase as the current increases. As the current draw in the secondary increases, the primary has a more difficult time transferring the signal to the secondary, so the secondary signal to the load gets squashed, or 'soft-clipped'. This soft clipping is called regulation. While regulation is desireable in a power supply, it is undesireable in a transformer. In other words, in a power supply, if the input voltage or the output load current changes, we don't want the output voltage to change. In a transformer, we want the output voltage to follow the input voltage and not regulate at all. When you put a heavier load on the output than was intended, it will pull the output voltage down, hence regulation. The leakage inductance problem arises because the current from the heavier load causing the regulation to occur reduces the efficiency of the transformer by not allowing the output to follow the input. Transformer designers simulate or view this problem as having extra inductance in series with the primary. The extension of this idea then, is that with the heavier load, you could affect the efficiency of the transformer, alter the frequency response (due to the extra leakage inductance in series with the primary), and cause other distortions to occur. OK, on to mismatching the other way. A speaker is a current operated device in that it responds to the current through it to generate a magnetic field that works against the magnetic field of the speaker magnet to make the cone move in and out. Thinking in very short amounts of time, when the output charges up the voice coil with current, then the signal goes away or gets reduced, the cone system moves the voice coil back to its home or resting position. As it is moving back, it generates a voltage that is fed back up the line into the transformer and appears in the output circuit of the amp. This generated voltage is often referred to as flyback voltage, because we are charging up an inductor, then when we disconnect or stop charging the inductor, the magnetic field in the inductor collapses and induces this big voltage into itself. This big voltage then 'flies back' to the source of the charging current. There is a mathematical formula to determine how big the voltage is and it is related to the inductance of the voice coil, the amount of time it was fed current, and how much current it was charged with. The bottom line is that the voltage fed back to the output circuit is oftentimes much higher than the voltage that was used to drive or charge up the voice coil initially. This voltage gets transformed up by the turns ratio of the output transformer, and in many cases can be over 1,000 volts. What happens then is that arcing can occur between the pins on the output tube socket. Once this has occured, a carbon path forms on the tube socket between the pins. The carbon path allows a steady current to flow between the pins and eventually burns up the socket due to the heat that is generated. For example, it wouldn't be too uncommon to see a transformer turns ratio of 30:1. If we had a voltage fed back from the voice coil that was around 50 volts, 30 times 50 would be a 1,500 volt spike at the plate of the output tube. This is why you often see designers connect diodes in a string between the output tube plates and ground. They are trying to suppress these spikes and dissipate the energy in the diodes rather than allowing an arc to occur at the tube socket. So, when you use a higher impedance load on a lower impedance tap, the turns ratio is higher and resulting fed-back (flyback) voltage gets multiplied up higher than what it would have been with the correct impedance load.
    It's just about impossible for me to answer how long an amp would last under these conditions. It all depends on how the designer took these potential
     
    Last edited: Jan 14, 2009
  8. rockstarjay

    rockstarjay Member

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    You wire the extension cab to 16 ohms. Fenders don't mind a slight impedence mismatch if its lower.

    This is not true of all amp brands (Boogies should only be mismatched higher for instance.)

    Run it at higher than 8 and it will sound horrible.

    I wouldn't worry about the tube life too much unless you run your tubes really hot and constantly dime it.
     
  9. rockon1

    rockon1 Member

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    +1 If your using the internal speaker (and it is 8 ohms) and a 2x12 extension cab with 8 ohm speakers wire it for 16 ohms. As noted the total load for the three speakers would be 5.33. Bear in mind the internal speaker would recieve 2/3's the amps power. If only the extention is being used I'd wire for parallel at 4 ohms. Harder on tubes but safer for the OT.Bob
     
  10. roadhog96

    roadhog96 Member

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    You wire the extension cab to 16 ohms. Fenders don't mind a slight impedence mismatch if its lower. Isn't 16 ohms higher than 4 ohms? Wouldn't 4 ohms be lower?

    This is not true of all amp brands (Boogies should only be mismatched higher for instance.)

    Run it at higher than 8 and it will sound horrible.

    I wouldn't worry about the tube life too much unless you run your tubes really hot and constantly dime it. I remember my Amp Tech adjusted the Bias from 25 to 21 MA he said he likes to run them a little cooler. I don't ever run the amp at 10, 7 or 8 sometimes.
     
  11. roadhog96

    roadhog96 Member

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    1 If your using the internal speaker (and it is 8 ohms) and a 2x12 extension cab with 8 ohm speakers wire it for 16 ohms. As noted the total load for the three speakers would be 5.33. Bear in mind the internal speaker would recieve 2/3's the amps power. If only the extention is being used I'd wire for parallel at 4 ohms. Harder on tubes but safer for the OT.Bob This is exactly what I have been doing, using all three speakers. Would I be better at just unplugging the speaker in the amp and running the cab with the 2 speakers at 4 ohms? or would I be better off wiring then cab at 16 ohm? if I go with 16 ohm does that mean I should run just the cab or can I use all 3 speakers. Which is the best senerio? I just read on some site that gave several options for cabs and amps with several output and ohm configurations and it that going from 8 ohms to 16 ohms would be a safe mismatch but they never showed a 8 ohm to 4 ohm option. Question?, The total load for the three speakers would be 5.33 if wired in parallel in series it would be 12 according to the chart below. You said to wire it 16 ohm if I was using 3 speakers. which would make it 12 instead of 5.33. Wouldn't it be closer to 8 ohm by wireing it in parallel? Man I'm getting confussed.


    Number of Drivers.......Series......Parallel........Series+Parallel

    2 x 4 ohm.....................8.............2..................n/a
    2 x 8 ohm...................16.............4...................n/a
    2 x 16 ohm..................32............8...................n/a
    3 x 4 ohm...................12..........1.334..............6 or 2.667
    3 x 8 ohm...................24..........2.667............12 or 5.337
    3 x 16 ohm.................36..........5.334...........24 or 10.667
    4 x 4 ohm...................16.............1...................4
    4 x 8 ohm...................32.............2...................8
    4 x 16 ohm.................32.............4..................16
     
    Last edited: Jan 14, 2009
  12. rockon1

    rockon1 Member

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    Actually if they sound good all three running (disregarding the power imbalance) running the internal 8 ohm and the cab at 16 ohms is less of a mismatch at 5.33 ohms than just running the cab alone at 4 ohms. Running it at 4 ohms as slider313 stated with the internal speaker is definately too much of a mismatch at 2.6 ohms........ Or you can try the 4 ohm cab by itself.

    You would have to try the cab wired at 16 ohms to see how it sounds since when you were running the cab at 4 ohms and the internal speaker the cab was getting 2/3's the power (as well as running an unsafe 2.66 load).

    Just remember 16 ohm cab ,8 ohm internal speaker= 5.33 and close enough.
    Cab wired for 4 ohms if used alone. Ive always been told by trusted members here this is safer than 16 with an 8 ohm amp.

    Heres an equation for parallel wiring .

    Total Load = (S1 x S2) / (S1 + S2)
    Where:
    S1 = the impedance of Speaker load # 1
    S2 = the impedance of Speaker load # 2


    8(internal speaker) x 16 (cab)= 128 / 24(total of 8ohm internal and 16 ohm cab)=5.33
    8(internal speaker) X 4 (cab) = 32 / 12 (total of 8 ohm internal and 4 ohm cab ) = 2.66 Bob
     
  13. roadhog96

    roadhog96 Member

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    Just remember 16 ohm cab ,8 ohm internal speaker= 5.33 and close enough.
    Cab wired for 4 ohms if used alone. Ive always been told by trusted members here this is safer than 16 with an 8 ohm amp. Bob
    Bob I appreciate the help but I'm missing something here. According to the chart I posted it says that 3x8 ohm is 12 if wired in series or 5.33 in parallel right?
     
  14. rockon1

    rockon1 Member

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    Its easier to think of the cab as one speaker either a 4 ohm speaker or a 16 ohm speaker. So either you have an 8 ohm (internal) and 4 ohm external in parallel for 2.66 ohms or you have an 8 ohm (internal) and 16 ohm external in parallel for 5.33 ohms.Bob
     
  15. roadhog96

    roadhog96 Member

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    OK I think I'm starting to understand it somewhat, it's all about the way something is explained to someone and your doing a good job. Applying what you said and looking at this chart makes more since now. Now getting back to what you said earlier Quote above; 8 ohm amp/4 ohm cab) as there is less chance of "flyback " voltages damaging the OT. Bob So if I wire the cab 16 ohm with the amps speaker at 8 ohm I risk a chance of "flyback" occuring, is this right. Wire the cab to 4 ohm and there is less chance of this happening. It's sounds like this might be a catch 22. Wiring it 16 ohm with the amp speaker at 8 ohm sounds like it might be more dangerous.
     
    Last edited: Jan 14, 2009
  16. slider313

    slider313 Silver Supporting Member

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    No. If you use the 4 ohm extension cabinet you run the risk of damage. Remember, speaker in your amp IS 8 ohms and the extension speaker jack is wired in parralel. Using the 4 ohm cab is NOT a good idea. Wire the two 8 ohm speakers in the cab in series for 16 ohms. If you change the internal speaker in your amp to a 16 ohm and run the 16 ohm cabinet you will have a perfect match.
     
  17. roadhog96

    roadhog96 Member

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    I guess I forgot about he extension jack being wired in parallel. This is confussing to someone who isn't familar with electronics. I was actually advised to wire it parallel to 4 ohms by the good people at the Fender amp forum, boy somebody goofed. I have 3 mint 67 Fender/Jensen C12NA speakers. One came in the amp when I bought it. The 2 in the cab are new to me, they are a matched set and I paid a very good price for them. When I bought them, I new it wouldn't be a perfect match for the DR amp but I was told it would be OK to wire them in parallel and be safe at 4 ohm. I hope to someday pick up a Fender amp head for the cab, they have a 4 ohm output so that was the reasoning behind buying the 8 ohm speakers. I just thought it would be nice to fatten up the tone of the Deluxe Reverb with this cab till the day comes I can afford the amp head. I have a mint 66 Fender/Jensen C12NA speaker also that I originaly planned to have a custom cab made that matched the dimensions of the DR amp so that it could be stacked if need be and that speaker was going to be for that project when ever I can swing it. I can rewire the cab tomorrow to 16 ohms and try it. I just remember reading that it stands the risk of flyback voltage damage at 16 ohms.
     
  18. rockon1

    rockon1 Member

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    No evidently Im not doing a good job explaining lol!- your not quite on the same page. Your total ohm load being lower than the amp rating is safer (less chance of flyback voltage to the OT)than higher than 8 ohms.
    The 16 ohm cab and 8 ohm internal speaker combined will be lower than the amp "expects" to see.

    16 ohm cab with the 8 ohm internal speaker= 5.33 ohms

    Bottom line is if your intent is to run the cab and internal speaker this is your only option. Bob


     
  19. roadhog96

    roadhog96 Member

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    I will try to get it re-wired tomorrow. I'll be honest with you, I understand more now than when I started this thread but it hasn't all sunk in yet. My worst subject in school was math, I hate formulas. I can do just about anything mechanical, it's second nature to me. The math problem is why I never became an electrician. I wired my home and it passed building code with flying colors. I built a workshop, wired it, and put in heating. I've restored vintage cars. To me doing is a lot easier than trying to imagine how to do something from a text book. I want to learn more about amps but I can see it's going to be a lot tougher for me than all my other experiences. I don't think I can thank you guys enough for all your help. This means a lot to me, I don't want to ruin this amp, it's all I have for now. I'm retired now and on a limited income, so it takes a little getting use to. Can't buy as often as I use to. Thanks for everything. I will report back after it's done and tested.
     
  20. zippo

    zippo Member

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    If you use a 16 Ohm cab in addition to the internal speaker, you will increase the potential for a heat-related failure (and the external cab will only get 1/3 of the power anyway.)
    With many amps the increased risk is negligible, but a SFDR is already pushing the tubes pretty hard.
    If you need more low end I'd suggest getting a good 8 Ohm cab and using it in place of the internal speaker. It could be cheaper in the end.
     

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