How to Find Impedance?

[razbo]

Hi, folks. I have re-read a thread where I asked if I could use a multimeter to determine impedance. The answer was basically that it would be close, but I was measuring DC resistance, not impedance yet still would be handy in determining the load of an unknown speaker.

To keep the story short, I opened up the used cab I had purchased some time ago to rewire for a new tube amp I just got. I found 3 (most likely original) speakers with big square bottoms and no markings + 1 Realistic speaker clearly marked 8 ohm, 100 watt.

If I unhook each (original) speaker get DC Resistance ranging from 13.5 to 14.5 on each speaker, can I safely assume these are 16 ohm load? This is my brand new baby (Jet City 50 watt) and I don't want to fry it!

And as a sub question, is there any way I can "McGuyver" up something to accurately test speaker impedance? Like with a 9 volt battery, paper clips and rubber bands?

And a sub-sub question: Is there any way to test the wattage of a speaker?

This cab is a Peavey 4x12 from the 80's, I think, and the serial number is partially illegible, so I can't really look it up. I should just buy a new cab, but I want to hook it up today, and the hurricane in my area has all the music stores closed. So, any help would be appreciated. Thanks!

Edit: I did find this. It's from a chart for car audio cabs, but should be just the same, and my readings would be giving me what I expect, since I really believe the original speakers are 16ohm...

This might help some other newb like me -- unless you knowledgeable folks find it at fault!

The best way to find out the impedance of a speaker is to read it off a label on the speaker. If the speaker is unlabeled, look for manufacturer's literature or spec sheets. If all else fails, you can attempt to measure speaker impedance with an ohm meter set for 20 ohms full-scale.

The ohm meter will read slightly low:
3 to 3.5 ohms 4-ohm speaker
6 to 7.5 ohms 8-ohm speaker
13 to 15 ohms 16-ohm speaker

[jcgss77]

First of all, I would never guess about speaker impedance. Great way to fry an amp!

since you are probably going to need to disconnect the speakers anyway, I would anyways to do it myself with some newer speaker wire, unhook them and test the resistance of each one. That will tell you if you need to wire them in series or parallel.

If you don't know how to do such wiring, check this site out:

www.bcae1.com/spkrmlti.htm

That impedance tester you are talking about is a good old fashioned Multi Meter, unless I am mistaken! And if you are going to buy a new cab, you can send me that old one!

[sumgai]

raz,

Short story long, your assumption is valid, and I'm corroborated by the blurb you posted from another site. The deal is, with one exception (noted below) all speakers made for musical production or reproduction have the characteristic of showing a DC resistance of about 75-85% of the nominal AC impedance..... when measured with a simple multimeter (analog or digital).

The exception would be speakers that are designed for use with 25v or 70v outputs. They have much smaller voice-coils, so they measure quite a bit less, DC-wise. Such units are preferred in places where many speakers must be mounted over a large area, or covering several/many rooms. This might be open spaces like a warehouse or in a park (think outdoor concert), or vehicles like trains, boats and planes. Lots of use for these things, should you wish to investigate, but don't lose sight of the fact that they don't hold very much power - they depend on voltage to work, not current (power).


Sadly, it takes a lot of processing power, or IOW, an expensive piece of test gear to measure impedance. This is because impedance deals with AC, and the very first thing you need for this kind of job is, indeed, a frequency generator. And not a common single frequency generator either, nooooo..... you'll need to sweep the whole audio range, so right there, we see the cost going up, up, up......

It can be done, but trust me, you're not gonna wanna lay out what a decent amp or axe will set you back! That is, unless you want to do this as a pro, in order to try and earn back some of that cash outlay. Good luck there, pardner.

Score:
Inhumane electronics feldergarb - 1, Macgyver - 0.


In essence, your Peavey cab probably works just fine. Your only question will be, what is the ohmage present at the input jack? Measure that point with your meter, and multiply by about 1.2 or 1.25, and there's your answer. If it's over 14, then you're looking at a 16Ω cab. If it's over 6, and under 8, then it's wired to give you 8Ωs. You can figure it out from here, I'm sure, for a 4Ω reading, right?

Readings ranging from 8 or so Ohms to about 13.5 or 14, you might be seeing a "mix-and-match" scenario. That'll clue you in to check each speaker individually, and "find the one that doesn't belong". It's not as easy to do that with an 8Ω setup, and a 4Ω rig is out of the question. If you're not certain, don't think of skipping a step, just open up the cab and measure them all individually.

Having mis-matched speakers isn't all bad. Sometimes a player wants a certain tone, and mixing two (or more!) types/ratings just might be the way to get what he/she wants. It's all part of the territory, and instantly qualifies one for a life-time membership in The NutzHouse! ;D

HTH



sumgai

[Yew]

All this tal kabout speakers has got me thinking about what my Celestion G10 vintage woudl sound like in my tiny terror combo.. Improvement or not? (that thing sounds almost perfect for me as it is though )

[razbo]

Thanks, folks! This is once again my absolutely most trusted site for critical answers! I did more checking against known speakers and consistently got the expected results. Because I had that mismatched 8ohm in there, I couldn't wire the cab right, but I wired a single 16 ohm up for my amp and had some great fun. ...That $40 Kijiji-bought cab has been totally worthwhile as a useful device and learning tool.

Thanks again!

[sumgai]

raz,

Da nada! ;D


~!~!~!~!~!~

yew,

Talk to newey about replacing the stock speaker in a small/cheap combo with a better quality unit. He'll tell you that it's a very worthwhile investment.



HTH






sumgai

[newey]

Yew-

(that thing sounds almost perfect for me as it is though )

I suspect your stock Orange speaker is as good or better than a Celestion. I do recommend speaker changes in cheap small amps, but the Orange Tiny Terror only qualifies as small, not cheap.

Edit:

Doesn't the Tiny Terror Combo already have a Celestion speaker, and a 12" to boot?:

www.geartree.com/p-32608-orantt15c-orange-tt15c-tiny-terror-15-combo-with-celestion-g12h-speaker.aspx

[Yew]

I have the 1x10 version G10-N40 in it, so Im unsure if the G10Vintage is a better speaker or not,