Clarification on impedance mismatches?

Discussion in 'The Workbench' started by What?, Sep 10, 2020.

  1. What?

    What? Well-Known Member

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    I thought impedance mismatches were the other way around (amp 1: cab 2, for example: amp 8 ohm, cab 16 ohm), but I read this recently, which has me scratching my head.

    https://tedweber.com/lets-talk-speakers-q-a/
     
  2. Jethro Rocker

    Jethro Rocker Well-Known Member Silver Supporting Member

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    That just seems wrong to me, for the tube amps. We have had Santiago say it is otherwise. Experience from many a member says otherwise. hell, the manual for my little Mesa said otherwise.
    Personally I'm gonna go with what has been conventional wisdom for years.
    Long term failure of what? OT or tubes? Playing at high output is hard on tubes too. But we do it.
     
  3. anitoli

    anitoli Well-Known Member Gold Supporting Member

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    You left out his technical explanation which sounds right:

    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.
     
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  4. pedecamp

    pedecamp Well-Known Member

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    Thats all wrong, I have proof from a burned up amp years ago. Impedance must match or be higher in the cab, tube or ss. :yesway:
     
  5. What?

    What? Well-Known Member

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    It's not clear to me. I somewhat understand what Ted was saying, which sounds right. I always read it being the other way around elsewhere, but it is never followed by any real explanation of why.
     
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  6. IOSEPHVS

    IOSEPHVS Well-Known Member

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    For valve amps:

    If the speaker(s) impedance is higher than the amp impedance, power is reflected back into and can destroy the output transformer.

    If the speaker(s) impedance is lower than the amp impedance the amp must work harder to achieve the same output power which will stress and possibly destroy the power tubes.

    The best course of action is to always follow the manufacturer's instructions.
     
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  7. Marshallhead

    Marshallhead Well-Known Member

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    Exactly - just match the impedance and be done with it.

    anyone that says it’s fine to set the amp lower than the speaker load , or vice versa, should only be giving that advice if they’re prepared to personally guarantee to whoever they are advising that the amp will suffer no damage AND pay for the repair if it does go up in smoke.

    If you’re not prepared to stand by such advice, then you should not be giving it!
     
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  8. neikeel

    neikeel Well-Known Member

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    There are other considerations like the primary impedance and the tubes you are using. I have used 100w OTs that were designed for four EL34s that have 4/8 and 16ohm taps with 1.7k primary. You can use the same OT with a pair of EL34s at 3.4k primary if you don't use the 16 ohm tap and reassign the 8ohm tap as 16 and the 4 as 8. That way you match the primary impedance to the tubes and to the load.
    I prefer to match the impedances personally according to the tube type and expected load.
     
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  9. Fusedbrain

    Fusedbrain Active Member

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    At lower volume, a mismatch one step in either direction is usually not a problem.
    Running the amp at high volume or into an attenuator is probably not a good idea for either mismatch.
    Best to just match the load to the amp, although the scenario Neikeel layed out above is pefectly valid.
    In that case though, you need to know for sure what transformer is in your amp, and what the specs are.
     
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  10. SkyMonkey

    SkyMonkey Well-Known Member

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    The Santiago mismatch shouldn't be confused with actually running a load at the wrong impedance.
    It only applies to amps that have dedicated pairs of jacks.
    i.e. for speakers across the range of 16 ohm (single), 16 ohm (pair) & 8 ohm (pair).
    The single 16 ohm jack is never used in the Santiago mismatch.
    This would include the JVM, DSL40CR/100HR, and Astoria amps.
    If the amp relies on switching to match the speaker jacks to the load (e.g. DSL401) you cannot do the Santiago mismatch.

    Plug a 16 ohm load into one of the 2x16 ohm jacks (=half a 2x16 ohm parallel load).
    Plug an 8 ohm load into one of the 2x8 ohm jacks (=half a 2x8 ohm parallel load).
    The two half loads add back up to a whole load and (according to Santiago) there is no actual mismatch seen by the amp.
     
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  11. Jethro Rocker

    Jethro Rocker Well-Known Member Silver Supporting Member

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    The manual for my Mesa stated this was a safe mismatch and even gave examples. Always was a higher impedance speaker in the lower jack.
    They manufactured thos and have no issues stating it in the manual. I have no issues doing so as well. While ome strives to match, it seems that on occasion this is a safe way to do ot, butnot the reverse as indicated in OP post.

    Yes agreed completely. This goes to the above statement as well. Santiago also stated that runjjng a 16 and 8 parallel at 5..3 ohms was fjne in a 4 ohm socket. He designs Marshall amps. Not internet lore. So again, I will go with that.
     
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  12. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    although, you are conflating 2 different things...

    Santiago has said that mismatches are highly overblown. It's OL & sudden drops, like when a speaker fails, cable fails, connection, etc.

    The multiple tap thing is another thing altogether, & as he has said - no mismatch, when done that way.

    pay attention to the 2nd paragraph:

     
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  13. Jethro Rocker

    Jethro Rocker Well-Known Member Silver Supporting Member

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    Yes I know. I was not talking about the multiple taps. I was talking about the mismatch. There is another instance where he mentioned specifically running 5.3 in the 4 ohm out.
     
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  14. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    sky was talking about the multi-tap thing...

    multi-tap - not a mismatch, if you do it accordingly
    mismatch - running different output to load
     
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  15. Jethro Rocker

    Jethro Rocker Well-Known Member Silver Supporting Member

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    Yep. Santiago talked about both.
    In any case, I am not telling others to mismatch in this way, simoly that according to OPs post, there is another train of thought on this and my belief is that the generally accepted theory is correct, partly based upon manuals plus Santiago's wisdom. People can do whatever they want, they are adults, I am not telling people what to do either way.
     
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  16. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    notice though, he calls it the mismatch "myth".

    He has said similar backing statements throughout the forums. Dr Z made the Joe Walsh amp, based on a Fender amp that purposely mismatched impedance. These guys (Ted) Weber, Santiago, Dr Z, all know their stuff...

    They design amp's & then build them, testing them along the way. They do all the no-no things that people on internet forums say not to do, but, they do it for a living...

    What do I know though? :shrug:
     
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  17. myersbw

    myersbw Well-Known Member

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    Ok, guys...Dogs of Doom & others are correct. You can't take "theory" into practice as a general truth when amp designs are all over the place. A transformer is just that...it "transforms" impedances. And, the actual impedance at the output is directly proportionate to the tube load impedance the tubes present to the primary winding. If the transformer is "beefy" or overrated enough, it won't make a lot of difference if it's one impedance tap up or down. And, how you use it is entirely relevant.

    Try diming a Marshall Major and then kick in a fuzz...kaboom! But, take a small VHT Special 6 and pop about any tube that fits in there and add any flavor of speaker and...wow! Why? The latter was designed to take abuse...the Major? To kill your eardrums if everything was perfect.

    However, for the sake of the "unknown" output design, one notch above or below the design isn't overly bad unless the designer built to the edge of OT specs...and THAT is not good practice given how they should know we abuse these things.

    Now, all that said...the BEST design is the output jack that shorts the OT secondary if a plug is accidentally pulled. Fender did some of these. Why? Because it is much safer to overload and blow a fuse than to go infinite impedance and flyback voltage destroys your OT in the process.

    Rock on, folks!
     
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  18. mickeydg5

    mickeydg5 Well-Known Member

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    I never talk about anything.

    Still it is a reason why I do not bother reading other people's shit, including Weber.

    Bottom line is follow your manual or the warranty is VOID.
     
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  19. myersbw

    myersbw Well-Known Member

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    mickeydg5, you're no fun! Live a little! However, that's one thing I really loved about the Special 6 "manual"...do what ya like, you won't blow up this amp! lol
     
  20. mickeydg5

    mickeydg5 Well-Known Member

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    Did you mean VHT Special 6?
    I saw no indication of do what you want with that amplifier.
     

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