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EPI M250 and EPI M180 Speakers and their ohm rating???


redpackman
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OK, I admit, I"m no expert when it comes to crossover electronics.  But go to the Human Speaker page dealing with EPI speakers in the late 70's. 

https://www.humanspeakers.com/e/epimodels.htm#more

Both the EPI M180 and EPI M250 have identical crossover schematics. They both have identical speakers in that configuration if you look at the replacement speakers one can order there.   I have a M250 that I just picked up. Three of the four woofers in that M250 pair rate at 6 to 8 ohms. One rated at 4 ohms. If you will notice  in both the M180 and M250 both woofers are wired in parallel.  If you have two woofers in parallel and both are 6 to 8 ohms isn't your overall speaker ohm rating going to be somewhere between 3.5 and 4 ohms, roughly?  I know, the tweeters will affect the reading some too, but work with me.  The M250 is rated at 4 ohms in the spec sheet which makes total sense to me with two 8 ohm speakers wired in parallel, but the M180 is rated at 8 ohms.  And that's from the same schematic with the same speakers. 

How can that be? 

Furthermore, I'm finding a lot of EPI 8" woofers test out at around 4 ohms all by themselves. Put them into a parallel wiring configuration like the M180/M250 schematic and what do you have? An ohm rating that's dangerously WAY below 4 ohms, isn't it?  And a lot of the EPI 8" woofers on eBay and elsewhere test out at 4 ohms not 8 ohms. I realize that a speaker's ohm rating is not static, but varies with the frequencies flowing through it, but aren't two 4 ohm woofers wired in parallel a real problem unless one's amp can handle ohm ratings down around 2 ohms?  I know, some say they can without being damaged  but.......

EPI must have made their 8"speakers in 8 ohm and 4 0hm varieties. Mixing them, particularly in parallel arrangements like this isn't going to work. 

epi-180.gif.e52bdfa90e2276092636a7e8d9ba61e7.gif

epi-250.gif

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Since the woofers are 8 ohm that means the amp is seeing a static ~4 ohms on each channel.

Most of the EPI 8" woofers I have measured are ~7.6 ohms to 7.8 ohms (7.7 ohms seems the most common). Wiring the EPI woofers in parallel means the amp can now see ~3.8 ohms to 3.9 ohms for each speaker hookup.  
I measured some speakers (EPI, Advent, and AR) using the external hook up terminals this morning and found ALL of these speakers in a static state measure around 4.1 - 4.2 ohms. This includes some M180s I tested.
 
So what's happening here? 
Where does the 8 ohms impedance come into play?
I am assuming that the audio cine wave as it goes positive and negative causes the overall impedance to "look" or "feel" like ~8 ohms to the amplifier?
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On 4/21/2021 at 7:50 AM, Woofer_01 said:

Since the woofers are 8 ohm that means the amp is seeing a static ~4 ohms on each channel.

Most of the EPI 8" woofers I have measured are ~7.6 ohms to 7.8 ohms (7.7 ohms seems the most common). Wiring the EPI woofers in parallel means the amp can now see ~3.8 ohms to 3.9 ohms for each speaker hookup.  
I measured some speakers (EPI, Advent, and AR) using the external hook up terminals this morning and found ALL of these speakers in a static state measure around 4.1 - 4.2 ohms. This includes some M180s I tested.
 
So what's happening here? 
Where does the 8 ohms impedance come into play?
I am assuming that the audio cine wave as it goes positive and negative causes the overall impedance to "look" or "feel" like ~8 ohms to the amplifier?

Your meter is measuring dc resistance. The dc resistance is not the same as impedance. The dc resistance gives a general indication of whether a driver is 4 or 8 ohms nominal impedance. Two 8 ohm drivers in parallel is equivalent to 4 ohms impedance. 

Speakers are rated by impedance, which is relative to the frequency applied to the driver. You can measure impedance, but you need something different than an ohmmeter. Impedance for drivers are best shown on a graph with the x-axis being frequency and the y-axis being impedance, generally ohms.  Such impedance charts can also show the phase shift of the driver. It is not unusual for an 8 ohm nominal impedance driver to have an actual impedance that varies between 5 and 20 ohms over the frequency range.

Nominal impedance is not well defined. In some cases, nominal impedance is the average impedance over the operating frequency range of the driver. Most times, it is an arbitrary value that reflects if the driver impedance is more generally 4 ohms, 8 ohms, or even 16 ohms.

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