Impedance Rant

Discussion in 'Amp Central Station' started by JamesFlames714, Oct 18, 2021.

  1. mad dog

    mad dog Friend of Leo's

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    It is a confusing topic to non amp techs. I do understand what is safe - though not recommended. Even so, I have yet to play through a mismatch (speaking tube amps here) between amp and speakers that sounds as good or better than the recommended impedance match. So I always match amp and speaker or cab. It sounds better to me.
     
  2. Dukex

    Dukex Tele-Afflicted

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    They make impedance matchers for that. You can play any amp through any cab.

    Z-Matcher 100w Impedance Matcher – Weber Speakers (tedweber.com)
     
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  3. chezdeluxe

    chezdeluxe Doctor of Teleocity

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    Why would Fender amps like my 63 Deluxe have an extension speaker out if a 50% impedance reduction was dangerous?

    Did Leo goof on this?
     
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  4. W.L.Weller

    W.L.Weller Tele-Afflicted Silver Supporter

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    Because the OP (and other working musicians responding to this very thread) sometimes have to use speaker cabinets provided by the venue. Whose rated impedance isn't known until the musician is standing in front of (well, behind) it.

    And sometimes, looking at the cabinet isn't enough. I have a speaker cable with alligator clips on one end that makes it easier to test DC resistance (a quick and inaccurate stand-in for measuring AC impedance, close enough for government work, as the man said).

    Of course you can just hold bare wires to the volt-ohm meter's test leads. (I suppose a true pervert could make a cable with a speaker connection at one end (1/4" phone or Speakon) and test lead connections to your model of multimeter at the other end)

    Anyway, testing the DC resistance of unknown cabinets is probably worth the 40 seconds. So is having a head with a multi-tap output transformer.
     
  5. dsutton24

    dsutton24 Doctor of Teleocity Ad Free Member

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    Point taken, but that's not really very helpful. The 'other working musicians' responding to the thread only added to the noise.

    If you truly have to use a cabinet in whatever situation, then the impedance doesn't matter. If this is the case, then why even ask the question? If the speaker cabinet is 16Ω, and the amp is rated for 4Ω, are you really not going to plug in and play?

    What really is 'close enough' as you say? It depends on the amp, and to some degree, luck.

    The correct response as to what speaker impedance to use with a given amp is to follow the manufacturer's guidance. That's a technician's answer.
     
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  6. telemnemonics

    telemnemonics Telefied Ad Free Member

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    The idea that sometimes a working musician HAS TO use an unknown cabinet provided by the venue is kind of a straw man.
    Similarly asking the question without telling us WHAT AMP makes it similarly straw based with no meat to cut into.

    If one has a great expensive sensitive vintage tube head and is "forced" to use a random cab, the owner of the expensive tube head DID have the option of buying a little 100w class D 1x10 combo amp rather than the sensitive expensive tube head with no speaker.
    (I'm not sure who these players are that play venues where cabs are not allowed, yet still feel they must bring a tube head with single fixed impedance. Choose your tools, don't say somebody forced you to blow your OT.)

    OR they could keep a little 1x10 cab in the car then test the impedance of the provided cab and if it's way off, use their little spare cab.
    OR bring a Quilter microblock/ superblock in the gig bag.
    OR just buy the damn expensive tube amp as a combo rather than a head, if they play venues that don't allow guitars players to bring speaker cabs.

    What the missing info means to the discussion is that WE DON'T KNOW if your Monoprice head will run fine at 32 ohms using the straw man mystery cab at the mythical venue that doesn't allow guitar players to bring cabs.
    As long as we're discussing science but omitting science facts, might as well make the straw man extra big!
    I don't get the idea that a player with such high standards that they must have a tube head, then cares nothing for what speakers it runs through. Or are some players forced to buy tube amps now?
    Is there a shortage of SS amps?
    And are all the clubs putting 16ohm and 32ohm cabs in their backlines now?
    What might really happen?
    The OP has an oddball 16ohm tube head, and the venue has a 4ohm or even a 2ohm cab?
    Or the OP has an old 4ohm Fender head and the venue has 16ohm Marshall cabs?
    Then why not include that info at the start?
    Seems the OP did a bunch of research before buying his "new head" that shall remain unknown.
    With a distressed sense of impedance needs and a choice to not bring a cab, did you choose a new head with a multi tap OT???

    Basically any older player has probably knowingly or accidentally run a tube amp into the wrong impedance cab, or forgotten to change the impedance setting on the head, or had a speaker blow in a 2x12, or didn't even know about impedance and just ran their Twin into a second cab making the impedance wrong.
    The quote of Ted Weber as saying you must never run your tube amp into a cab of lower impedance?
    That seems like a misquote or out of context, given a million Fender combo amps with ext speaker jacks used with an additional cab.

    As for this thread question?
    There is just no way to say for certain that every tube head can safely run into every cab at every impedance for any length of time and at any volume.
    Straw man.
    Player beware!
     
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  7. JamesFlames714

    JamesFlames714 TDPRI Member

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  8. MilwMark

    MilwMark Doctor of Teleocity Ad Free Member

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    Does your head not have a selectable impedance on the back? Most have inputs and/or a switch or dial for at least selecting 8 or 16 (which are the large majority of cabs anyway).
     
  9. Mexitele Blues

    Mexitele Blues Tele-Afflicted

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    The vintage OTs were stout enough to handle the lower impedance from parallel speakers. However, the Fender reissue schematics I have seen show that the extension jack switches both speakers over to a different tap off the OT.
     
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  10. jsnwhite619

    jsnwhite619 Friend of Leo's

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  11. unfamous

    unfamous Tele-Holic

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    Class D circuitry....this is not what people usually mean when they say "solid state".
     
  12. jsnwhite619

    jsnwhite619 Friend of Leo's

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    My understanding on it - and could very well be very flawed - is that like he says above in the video, higher impedance is kind of like putting a governor on it that limits current draw. ClassicTone's "exact copy" 5f1 OT was a 17k winding. If that was an exact clone of an original, I bet nobody ever burned out anything for those models!

    You often see the water pipe/hose analogy about impedance. Like most things electrical and in amps, my first take is/was usually bass akwards from how it works. But, as the designs evolved and got higher voltage and more power, what changed in this area? The Princeton Reverb kept an 8k OT and tiny PT, but the Deluxe got a 6.6k and bigger OT and offered more power. 6k 6L6 Tweed amps turned into 4k Blackface amps -- higher voltage, and lower impedance got higher output. Lower OT impedance offers less resistance and allows more current to pass through the circuit is my understanding, the opposite being true for higher impedance. So, a lower load has more chance of burning something out, a higher load is going to reduce efficiency and headroom.

    So, we've got a river and a wooden slat dam that normally flows 8 slats high, and a watermill designed to run at exactly the river/current flow with 8 slats in the dam. If you add 8 more slats and have 16 in there, water will eventually flow over and may work the mill, but it's not going to run at peak operation - lower efficiency. If you remove slats and let the river run with only 4 slats on there, it's going to let a lot more water/current through. That mill will run like hell for a little while, but it's probably not going to like it or last.

    I am definitely not trying to lay down the gospel on it, but that's how this redneck looks at it.
     
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  13. Mowgli

    Mowgli Tele-Meister

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    Speaker coils are inductors and when AC current is passed through them they store and release energy. They also have DC resistance like any conductor.

    Transformers are also inductors.

    When people mention flyback they're referring to the sudden withdrawal of applied voltage to an inductor that has stored energy and that stored energy can be released quickly in a huge voltage spike. It's the rare voltage spike and accompanying current burst from the flyback event that can harm the wires in these inductor coils theoretically. But the thinnest wires are most susceptible (like a fuse). Flyback is rare but potentially significant; another reason to match impedances if possible.
     
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  14. Spooky88

    Spooky88 Tele-Meister

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    Great thread, comments here are priceless.
     
  15. dvh2

    dvh2 TDPRI Member

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    Just try to run with the manufacturers' specs. No worries.
     
  16. Old Deaf Roadie

    Old Deaf Roadie Friend of Leo's Gold Supporter

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    I have been plugging things in for better than 40 years following ^this^ method, and have never blown anything up due to an impedance mismatch on any type of amp. Never have I ever. Excessive power? Absolutely, but never impedance. Maybe it matters in a laboratory, but not a stage.
     
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  17. Old Deaf Roadie

    Old Deaf Roadie Friend of Leo's Gold Supporter

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    To be honest, I originally clicked on this thread because I thought someone was going to present a case against Ohm's Law & I could use a laugh today.
     
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  18. zeke54

    zeke54 Tele-Afflicted Silver Supporter

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    Years ago I gigged my '73 Princeton Reverb through a 4 ohm Bandmaster cab . Did this many times , didn't kill the amp .
     
  19. Mowgli

    Mowgli Tele-Meister

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    Someone else mentioned that the older Fender OTs could handle mismatches up to 50% lower than stated (8 ohm output into a 4 ohm load).

    That's been my experience, too, but I always had a tinge of fear that excessive current would hurt the tranny so I never pushed it. Eventually I acquired a used Weber matching box to ease my troubled mind.
     
  20. Blue Bill

    Blue Bill Poster Extraordinaire Silver Supporter

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    My 2 cents: The difference between resistance and impedance is based on the notion that some components or circuits are reactive, some are non-reactive. A reactive circuit has some level of capacitance, impedance, or both.

    A non-reactive component or circuit has no capacitance nor impedance; for example, a resistor, by itself, has no reactive qualities. Non-reactive components have the same resistance for DC or AC current, it is the same for 10Hz, 1000Hz, etc.

    Reactive components, like capacitors, inductors, transformers, and speakers, convert energy from alternating current into either static charges, or magnetic fields, charging and discharging with the "rhythm" of the frequency of the alternating current.

    The term impedance refers to the resistance in a reactive component or circuit.

    The resistance of a reactive circuit changes with the frequency of the alternating current; a speaker can have different resistance for bass notes, midrange notes and treble notes. DC current can be described as a current with infinitely low frequency, in effect, zero Hz.

    The point is, measuring the resistance of a reactive component, like a speaker, with a DC multi-meter, does not give an accurate reading of the impedance of the speaker. The rated impedance of a speaker refers to the resistance of the speaker coil at a specific reference frequency.

    In the graph below you can see the impedance of a guitar speaker, versus frequency. The resistance of the speaker is at 8 ohms only at a couple places, below 10 Hz, and around 250 Hz, the dozen or so notes on a piano, below middle C.
    Figure1_Test Bench Celestion G12H.jpeg
     
    Last edited: Oct 24, 2021
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