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Topic: Impedance ? |
Tim Walker
From:
Marin County, CA (originally U.K)
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Posted 12 Dec 2008 1:38 pm |
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Hi,
Could someone please explain speaker impedance in simple terms ?
I am borrowing a 15 watt univalve - there is the option to use either an 8 ohm or 16 ohm speaker - will one have more headroom than the other. ( I know it will depend on the speaker power rating as well).
Thanks !
Tim |
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Jonathan Cullifer
From:
Gallatin, TN
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Posted 12 Dec 2008 2:39 pm
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| Impedance is an AC resistance, so the higher impedance the speaker, the less current it will allow through (and the less power you'll get out of it). You'll get somewhere around half the power out of a 16 ohm speaker compared to an 8 ohm. All other things being equal, that equates to a 3 db reduction in sound level, which is usually barely noticeable. |
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Mike Schwartzman
From:
Maryland, USA
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Posted 12 Dec 2008 2:54 pm
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Ok Tim...I'll try simple:
The output transformer in the Univalve wants to "see" 8 ohms minimum.
So you can use 1 8ohm speaker or 2ea. 16 ohm speakers in parallel (or as we say daisy chained) to get a total of 8 ohms.
If you use 1ea. 16 ohm speaker you'll likely get half of the amp's wattage.
Most musical instrument speakers are rated higher than 15 watts, so you probably will want lower wattage speakers for maximum efficiency. Some higher wattage rated speakers are efficent with a lower wattage amp...depending upon the particular speaker or speakers.
I have a 12-14 watt tweed deluxe that sounds great with a Weber speaker rated at 50 watts.
_________________
Emmons Push Pull, BMI, Session 400, Home of the Slimcaster Tele. |
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Jack Stoner
From:
Kansas City, MO
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Posted 12 Dec 2008 4:15 pm
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If there is an option to use either 8 or 16 ohm speaker then I would guess that either one will give you the same power output, assuming there is a switch for 8 or 16 ohms.
Otherwise it's what the amp is rated at for the different speaker impedence loads.
(I've never heard the "daisy chain" term for speaker connections, either series or parallel. Must be a relatively new term that is being used for this function). |
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Dave Mudgett
From:
Central Pennsylvania and Gallatin, Tennessee
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Posted 12 Dec 2008 7:38 pm
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The THD Univalve is a tube amplifier. Unlike most solid-state amps, tube amps have a relatively high output impedance and use an output transformer to approximately match impedance to a typical speaker.
Since there are both 8 and 16 ohm outputs, then you should simply use an 8 ohm speaker cab with the 8 ohm output or a 16 ohm speaker cab with the 16 ohm output to get maximum power transmission to the speaker and also to minimize reflections back to the transformer, which can damage it.
If you want to read more on impedance matching, try this wikipedia article - http://en.wikipedia.org/wiki/Impedance_matching - but be aware that this requires some basic knowledge of AC circuit analysis. The section on matching amplifiers to speakers is here - http://en.wikipedia.org/wiki/Impedance_matching#Loudspeaker_amplifiers
To a first order of approximation, there is no significant difference between using an 8 ohm speaker cab with the 8 ohm output versus using a 16 ohm speaker cab with the 16 ohm output. But there are second order effects due to different voltage/current relationships with each of these, not to mention differences in the speakers. Most of this is only noticeable when the amp is pushed hard, and I prefer to evaluate this based on how it sounds to me, and not some theoretical analysis. Either of these should work fine, but you may prefer one or the other. |
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Tim Walker
From:
Marin County, CA (originally U.K)
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Posted 12 Dec 2008 9:16 pm
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Thanks very much - that makes it a lot clearer.
Tim |
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Donny Hinson
From:
Glen Burnie, Md. U.S.A.
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Posted 13 Dec 2008 9:07 am
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| Quote: |
| I am borrowing a 15 watt univalve... |
I certainly hope you're not expecting very much from that amp!  For most pedal steel work, that's an insufficient amount of power. (Though it might work as a practice amp.)
Since you'll have so little power available, try it both ways, and use the tap that gives you most of what you want, sound and volume-wise. |
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T. C. Furlong
From:
Lake County, Illinois, USA
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Posted 14 Dec 2008 8:10 am
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As long as the topic is IMPEDANCE, I thought it might be good to put out some facts. The impedance rating of a speaker really is called nominal impedance. Nominal means "named" impedance. Actually, the impedance that the amplifier output "sees" is all over the place and dependent on which frequency the speaker is reproducing. I have measured impedances of many speakers and it's not uncommon to see an 8 ohm (nominal) speaker have a measured impedance of 4.5 ohms and 30 ohms depending on frequency. Here is a graph from a speaker the manufacturer calls an 8 ohm speaker.
At 200 Hz the speaker has a measured impedance of 4.95 ohms. Hardly 8 ohms as the manufacturer would have you believe. In fact, the amp will only see 8 ohms at 47 Hz, 96 Hz and 3.5KHz. Put two of these babies in parallel and when you play a note that has a fundamental of 200 Hz, your amp will see just under 2.5 ohms not 4 ohms as you would think. Play a note with a strong fundamental of 75 Hz and the speaker will have the amp see 17.5 ohms. Also, the impedance will change depending on which cabinet it's being used in, primarily at the low frequencies.
For most of the range that the speaker would be used for steel guitar, this should probably be called a 5 ohm speaker. They call it an 8 ohm speaker only because it hadn't gotten as low as the next nominal impedance which would be 4 ohms. I always try to be conservative in my use of impedances. In my SPLIT amps, I use 8 ohm speakers even though the amp is rated for operation down to 4 ohms. If the speaker sees 5 ohms, no worries! The only thing that happens is that the available power increases. Another benefit is that things tend to remain more linear and the amp runs cooler.
I ran into a retired gentleman at a steel show who had worked at JBL as a speaker design engineer. He told me that they would put the exact same cones into a 16 ohm and a 4 ohm speaker and that they were just labeling them differently. He made the point that depending on the purpose of the speaker, you could argue that the nominal (named) impedance was appropriate. Most amplifiers are fairly forgiving when the output is loaded below the recommendation. But if you go too far below, it will cause too much current flow and trigger protection or worse.
TC |
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Lynn Oliver
From:
Redmond, Washington USA * R.I.P.
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Posted 14 Dec 2008 11:39 am
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TC, did you mean to write "coils" instead of "cones?"
From the Weber speaker site:
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| You'll often see a speaker or other device rated at a 'nominal' Z, or impedance. The word 'nominal' comes from the Latin word 'Nomen' which means simply 'name'. An example of where you may have heard this term used in another context is during a space shuttle mission. During the initial ascent, you'll often hear the astronauts say "all systems nominal", or "mission nominal". What they mean is that everything is going as planned, as written, or as described. In the case of a speaker, we are calling, or naming the device a certain impedance. The electrical quantity of impedance is made up of resistance, which doesn't change with frequency, and reactance which does change with frequency. So, impedance is the combination of the two at a particular frequency. Remember in the movie Wizard of Oz when the Scarecrow finally got a brain, he immediately started reciting some bizarre formula about "The sum of the squares of the sides of a right triangle...."? He was reciting the Pythagorean theorm for right triangles. We can use that formula to calculate impedance. Think of a flagpole with the sun casting a shadow on it. The height of the flag pole would represent the reactance, and a line between the base of the flagpole and the point on the ground away from the flagpole where the shadow stops would represent the resistance. If you connected a string between the top of the flagpole and the point on the ground where the shadow stops, the length of the string would be the impedance quantity and would be longer than either of the other two. So, what am I getting at? A speaker that is said to be 'nominally' 8 Ohms will have a resistance lower than 8 Ohms. So, a rule of thumb is that if it measures lower than a commonly used name, such as 8 Ohms, but not lower than the next lower commonly used name, such as 4 Ohm, then you would call it the higher name, or 8 Ohm. Many 'nominal' names have been used over the years, including 2 Ohm, 10 Ohm, and 15 Ohm. 4, 8, and 16 seem to have been standardized though, for the past 30 years or so. The main reason for the variance between several coils, all of which would be called 8 ohms, for instance, would be because each might have a slightly different DC resistance due to the length of the coil, size of the wire used, etc. In each case, though, the DC resistance would be in the range of 5.5 to 6.5 DC Ohms, so it would be called an 8 Ohm device. Another method for naming the impedance of a device is by actually measuring the impedance, or AC resistance of the device with special test equipment. Many have used 400hz as the test frequency, while others have used 1,000hz as the test frequency. Some have derived the name from an impedance plot such as the one shown in Figure 1 below. They declare the nominal impedance to be the impedance at the 'first dip after the first peak'. Notice the large peak at the speaker resonance around 100hz. Then, it drops dramatically, dips, then starts going back up. It would be the impedance at the lowest point of the dip that would be the declared 'nominal' impedance. It's an interesting exercise in using an impedance bridge, but the old rule of thumb we discussed earlier works just fine for declaring a nominal impedance. |
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T. C. Furlong
From:
Lake County, Illinois, USA
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Posted 14 Dec 2008 7:33 pm
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Lynn, yes the voice coil is what determines the DC resistance but the cone with the coil attached, inside the magnetic gap is what is actually measured for impedance.
I agree with Ted Weber's account of the history of speaker impedance except that we don't know which method is used for determining the nominal impedance of a speaker that we are about to match up with an amplifier.
TC |
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Dave Mudgett
From:
Central Pennsylvania and Gallatin, Tennessee
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Posted 14 Dec 2008 8:50 pm
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Of course, any single number "impedance" is nominal - unless a linear circuit is purely resistive, it's impedance is a complex-variable function of frequency - typically expressed as real and imaginary parts or magnitude and phase. All of this is even more complicated if there are nonlinearities involved, and nothing is remotely linear except in a restricted range.
Technically, the optimal matching complex impedance frequency response of a linear amplifier is the complex conjugate of its complex output-impedance frequency response - http://en.wikipedia.org/wiki/Maximum_power_theorem. I agree that this is never achieved in practice, which is why I suggested that output transformers are used to "approximately match" impedance.
As far as JBLs go, every JBL 4-ohm speakers I've had tested approximately 3.2-3.6 ohm DC resistance and 8-ohm versions had 6.7-7.1 ohm DC resistance, so I'd say that those speakers were not just relabeled. I can't remember a time when a labeled 4-ohm speaker didn't have a DC resistance just a bit below 4 ohms, and proportionally for an 8-ohm speaker, regardless of brand, but I suppose it happens sometimes. I'm not sure what the point of meausuring electrical impedance "inside the magnetic gap" is - the amplifier sees the load connected via the speaker wires.
I agree that amps have to be fairly forgiving of moderate impedance mismatch, since, practically speaking, it's impossible to get the optimal match for all frequencies anyway. But I'd be wary of using a 16-ohm nominal speaker when 4 ohms nominal is expected, for example. The lowest frequency on E9 pedal steel is B around 124 Hz, the low E on guitar is around 83 Hz, and a lot of playing is above a typical low-frequency impedance peak and within the range where the nominal impedance is not so far off. It's one thing to have a big mismatch occasionally for a few notes here and there (like would happen with the occasional very low note on C6 near the impedance peak), but I would not want to constantly barrage it like that unless I knew the output transformer was very robust indeed. My experience is that some tube amps - like old Marshalls, for example - can be pretty sensitive to wrong speaker nominal impedance. I'm more worried about reflections back to the output transformer.
I have a THD Flexi 50 - it has a pretty honkin' output transformer for a 50-watt amp, so maybe this isn't such a big deal for that brand.
Just my experience, YMMV. |
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Lynn Oliver
From:
Redmond, Washington USA * R.I.P.
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Posted 14 Dec 2008 8:52 pm
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| Then it sounds possible that one cone could be used in speakers with different nominal impedances. |
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Donny Hinson
From:
Glen Burnie, Md. U.S.A.
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Posted 15 Dec 2008 8:52 am
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| Yes, there are cones, and there are voice coils. A cone can have many different voice coils attached, and therefore can be made to work with different amps. For most intents and purposes, changing the voice coil is what changes the (nominal) impedance of a speaker. The dynamic impedance can be changed by a myriad of factors - even the cabinet in which the speaker is installed. |
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