ohms etc

c c johnson · Post by **c c johnson** » 10 Sep 2001 7:30 am

As you can readily tell I am completely lost when it comes to electicity;well, and other things too. I have an amp which says --- into 2 ohms. When I buy a speaker do I have to buy a two ohm speaker or does the speaker ohm make any difference. Ok you guys out there quit laughing. thanks Jack CC

Glenn Austin · Post by **Glenn Austin** » 10 Sep 2001 9:20 am

It sounds like a specification for the power output of the amp into a 2 ohm load, like say 2000 watts into 2 ohms or 1000 watts into 4 ohms.Some amp makers put this on their amps to make people think that they are really loud, but it is very misleading.Could you post what kind of amp it is? Ideally the ohms of the speaker should match the amplifier in order to transfer the maximum amount of power. I think it's safe to say that you could run a 4 or 8 ohm speaker off of this amp, just don't run it too loud to start with! As long as the speaker impedance ( ohms ) is higher than the output impedance of the amp, there shouldnt be a problem I don't think 2 ohm speakers even exist!

c c johnson · Post by **c c johnson** » 10 Sep 2001 10:34 am

thank you Glenn, this is a generic homemade amp head that I picked up at a yard sale. I guy was playing a lead guitar through it and a 15" spkr enclosure which was fully enclosed so I could not get to the spkr. It was as loud as my nash 400 and the speaker did not crack under "Girl from Ipanema" If I read you right you believe any 15" spkr would be allright with the vol at small club level? cc

Mark Cohen · Post by **Mark Cohen** » 10 Sep 2001 11:25 am

The speaker impedance, measured in Ohms is a crucial spec on your rig. If the impedance is too low, you risk damaging the power amp; too high, and the amp doesn't have enough power.

Even if you don't know the electronics, bear with me on this:

If you have more than one speaker, and wire them in parallel, the impedances can be calculated as:

<center>Final Impedance = R1xR2/(R1+R2)</center>

For example, two 4 ohms speakers in parallel yields a 2 ohm load (4x4/(4+4)). A pair of 2 ohm speakers is a load of only 1 ohm.

The reason that a low impedance can damage a power amp is that the amp provides voltage and current to the speaker. Most amps are "voltage sources" meaning that they try to deliver a constant voltage. Ohms law says that the current is equal to the voltage over the impedance (impedance can be thought of as a fancy word for resistance). Thus the amp must transfer twice the current if the impedance is reduced by 2. This actually doubles the power requirement for the amp, as the power is equal to the voltage times the current.

Message: A proper match between your amp and speaker is very important. If you don't really know - be careful about fooling around. I can answer questions by e-mail, if you like.

(I'm proud of my Peavey listing...)[This message was edited by Mark Cohen on 10 September 2001 at 12:27 PM.]

Glenn Austin · Post by **Glenn Austin** » 10 Sep 2001 1:26 pm

Electronics theory aside.You need to measure the impedance of the speaker. Shove a cable into the jack on the speaker cabinet and measure the ohms between the tip and sleeve of the plug.If you don't have a volt ohm meter, you could get a mechanic at a garage to measure it. An 8 ohm speaker will measure about 8 ohms(give or take), and oh yeah, this only works if the speaker is in good shape. If your speaker works now, why do you want to buy another one?

c c johnson · Post by **c c johnson** » 10 Sep 2001 1:39 pm

Glenn I don,t have a spkr for it now. The guitar player would not sell the spkr. The amp says 2 ohms and spkrs in the catalogs all say 4and 8 ohms so thats why I asked the ?? cc

Jack Stoner · Post by **Jack Stoner** » 10 Sep 2001 2:21 pm

From your description, I'd go with a 4 ohm speaker. It sounds like it's rated down to a 2 ohm load, but I've never seen a 2 ohm speaker. If you had two 4 ohm speakers in parallel you would have a 2 ohm load.

c c johnson · Post by **c c johnson** » 10 Sep 2001 3:24 pm

Thanks so much for all the replies. You guys and this forum is great. CC

Paul Graupp · Post by **Paul Graupp** » 10 Sep 2001 8:08 pm

At the risk of appearing Nit-picky or something, let me put my two cents in here.
You cannot measure the Impedance of a speaker with an Ohm meter. Impedance (Z) is a derived or calculated value determined by the manufacturer. It requires four factors and these are operating parameters.

Since a voice coil is an inductor you need to know inductive reactance (Xl) and the wires will have some distributed capacitance so you need to know capacitive reactance (Xc). You need to have a frequency to determine either of these values so it must be driven or operating.

The fourth value is the DC resistance that you CAN measure with a standard Volt-Ohm meter. While it's true that both Z and R are expressed as ohms, they are not the same even if the DC resistance you can measure safely is close to the designated impedance of a unit.

Finally a word of caution. Never try to measure the resistance of an operational speaker. If you do this, I'd have a fire extinguisher near by and a spare amp you can use while this one is being repaired.

Regards, Paul[This message was edited by Paul Graupp on 10 September 2001 at 09:10 PM.][This message was edited by Paul Graupp on 10 September 2001 at 09:12 PM.]

Terry Downs · Post by **Terry Downs** » 10 Sep 2001 9:30 pm

It is true as Paul said, that you cannot measure the impedance of a speaker with an ohm meter. You are measuring the DC resistance of the coil. However, in most cases, the DC resistance is about 75-85% of the impedance. Since at least 95% of all speakers you will ever likely see are either 4 ohm or 8 ohm, you can determine the speaker impedance with an ohm meter. Most 8 ohm speakers will measure about 6-6.5 ohms on an ohmmeter. A 4 ohm speaker will be 3.xx ohms. There are a few 16 ohm speakers that are used for tube guitar amps, and a few speakers with impedances below 4 ohms used in automotive applications.

The lower impedance speaker you load an amplifier with, the more power you get out...up to a point. This point is either where the damping factor (reciprocal of the output impedance) limits the power OR the amp reaches its thermal limit. The latter is the most common. If the amp spec states that it delivers XX watts into 2 ohms, it implies that it can handle a 2 ohm load. A 2 ohm load almost always comes from two 4 ohm speakers in parallel. To get the most power out of commercial power amps these days, you generally need to have a 2 ohm speaker configuration.

Another clarification..there is no such thing as a "match" of the speaker impedance to the output impedance of a solid state power amplifier. This is a carry over from the tube amps with transformer outputs and from RF electronics theory. The goal of the solid state power amp designer is to get the output impedance as close to zero as possible. It is necessary for this type system to have a maximum voltage transfer characteristic. This is why you must limit how low the speaker impedance is, so the power amp dissipation can be contained.

So to answer your original question, you probably can't buy a 2 ohm speaker, but you can parallel two 4 ohm speakers to get 2 ohms. You will probably get the maximum output with 2 ohms. If you just want a single speaker and you use a 4 ohm, you will be operating the amp cooler and will have less than the maximum power.

If you have Excel, I have example curves of a typical power amp power output as a function of load impedance in my musicians workbook below.

http://nightshift.net/musicians_worksheet.htm

Regards,

------------------
Terry Downs
http://nightshift.net
terry@nightshift.net