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Old June 9th, 2007, 10:03 PM   #1 (permalink)
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Measuring speaker ohms

I recently got a PA speaker cabinet that has a tweeter and woofer. The cab is wired as 16 ohm. I really just want the woofer. I don't know much about electronics but just want to see if the woofer itself is rated at 8 ohms and then wire it directly to the output jack. I have a multi-meter but basically only know how to measure battery output with it. (Told you I don't know much!) By the way, there's no info on the speaker that indicates it's ohm ratings.

Can I measure the speaker ohms using the multi-meter? Any help would be greatly appreciated!

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Old June 9th, 2007, 11:07 PM   #2 (permalink)
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I recently got a PA speaker cabinet that has a tweeter and woofer. The cab is wired as 16 ohm. I really just want the woofer. I don't know much about electronics but just want to see if the woofer itself is rated at 8 ohms and then wire it directly to the output jack. I have a multi-meter but basically only know how to measure battery output with it. (Told you I don't know much!) By the way, there's no info on the speaker that indicates it's ohm ratings.

Can I measure the speaker ohms using the multi-meter? Any help would be greatly appreciated!
Set the multimeter for resistance and measure accross the speaker leads. You will get close to the impedence of the speaker with that. For example, if it reads 6 ohms you probably have an 8 ohm speaker.

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Old June 10th, 2007, 12:42 AM   #3 (permalink)
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Most horn-tweeter PA cabs without a separate input for woofer and tweeter have a passive crossover - this is a combination of an inductor or small coil and a large bi-polar capacitor.

This splits the incoming speaker signal into high and low portions. Even if you have a tweeter with a voicecoil, it will only present the same impedance i.e. above say 2khz there will be 16 ohms from the tweeter, and below 2khz there will be 16 ohms from the woofer.

To your DMM through the input it will appear as though it is one speaker @ 16 ohms impedance rather than two - they are not like a series or parallel connection when connected through a crossover.


C1 is the cap, L1 is the coil or inductor, C1 stops low freqs wrecking the tweeter, L1 stop high-freqs cooking the voicecoil in the woofer

Most modern cabs have piezo tweeters which have no effective impedance, although they might have a 5-20 ohm 10 watt resistor to present a load through those freqs to the power amp, and to load the tweeter down to match efficiency to the larger speaker.

IOW, take the back off the speaker or pull the woofer out the front if a sealed cab, disconnect it and I'm sure you find it is a 16 ohm speaker with either a 16 ohm voicecoil or piezo tweeter through a crossover.

If you use a DMM to measure it, you will get somewhere between 10-16 ohms DC resistance for a 16 ohm speaker
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Old June 10th, 2007, 01:17 AM   #4 (permalink)
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I took the speaker out, and the tweeter and woofer are simply wired together (with no coils, etc in the circuit). There are two speaker jacks in the back of the cab; I suppose you could daisy-chain the cabs. It's a Peavey cab, model 112 PT PA.

I tried using the multimeter but was not able to understand the reading I got from it. I should probably get a tutorial on how to use an multimeter. As far as I could tell it was showing at 2 or 3 ohms - that seems low.
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Old June 10th, 2007, 01:36 AM   #5 (permalink)
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I took the speaker out, and the tweeter and woofer are simply wired together (with no coils, etc in the circuit). There are two speaker jacks in the back of the cab; I suppose you could daisy-chain the cabs. It's a Peavey cab, model 112 PT PA.

I tried using the multimeter but was not able to understand the reading I got from it. I should probably get a tutorial on how to use an multimeter. As far as I could tell it was showing at 2 or 3 ohms - that seems low.
Voicecoil might be blown or shorted. Piezos don't need a crossover as they usually present a capacitive load which in effect means they just reject low freqs perfectly adequately. Some cheap cabs and floor monitors have no crossover, which means the woofers can be subjected to highs. PVs usually do though.

Did you measure it disconnected? Don't hold the tips against the contacts very long - voicecoils don't like DC voltage.

If you aren't going to use the tweeters, just clip one or both of the wires to it and tape the ends up.
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Old June 10th, 2007, 01:42 AM   #6 (permalink)
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Everything was disconnected. But unhooking the tweeter doesn't necesssarily solve my problem because I don't know what the ohms of the speaker are. I want to use a Crate Powerblock head that has an mono speaker out for 8 ohms.
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Old June 10th, 2007, 03:22 AM   #7 (permalink)
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Set your DMM on DC-ohms, usually on the 0-20 scale depending what it has. Zero the DMM if it has that feature. Touch the probes together and it should read zero. Scratch the terminals with the probes to get a good reading.

How do you know it's a 16 ohm cabinet?

Touch the red probe to +ve and back to -ve. You should hear the speaker crackle and get a reading of 10-16 ohms. Otherwise, the speaker may have been changed. If you only get 2-3 ohms this is a 4-ohm speaker.

Does the speaker look like this?


These came as 4 and 8 ohm units.

If you have two four ohm you could wire the cabs to get 8ohm in series. If they are 8's just run one unless the maker says you can run it at 4 ohms.

Here is a good page on speaker wiring:

Speaker wiring basics
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Old June 10th, 2007, 01:20 PM   #8 (permalink)
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The plate that gives the model number and serial number also states that it's a 16 ohm cabinet.

Dacious, based on your comments I'm beginning to think it's a 4 ohm speaker. The Crate Powerblock that I want to use with it is 8 ohms with a single output or 4 ohms per side, so I guess I could use one of the 4 ohm sides. Does this sound right?

The speaker doesn't look like the picture you supplied - the mounting ring is perfectly round and the magnet on mine is smaller. Also, the frame on mine is all black.
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Old June 10th, 2007, 09:48 PM   #9 (permalink)
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OK, just checked out the Powerblock spec. Yes, you can run it stereo, one side into each 4-ohm speaker. Those black speakers may be PVs - they are one of the few audio companies to make their own drivers and not just buy Eminence or similar. They should have a number embossed in the rim like a 226-47775 or similar varying in digits - this is the EIA number that identifies the maker.
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Old June 10th, 2007, 10:10 PM   #10 (permalink)
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Dacious, can I run just one side of the 4 ohm stereo out? Ultimately I just want the Powerblock running through one 12-inch speaker.

FWIW the only numbers I see are stamped in ink on the back of the magnet and appear in three rows as follows:

70777006
121620PA
67 8448

By the way, I really appreciate all your help!
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Old June 10th, 2007, 11:21 PM   #11 (permalink)
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It looks to me like the Crate is a bridge-capable stereo power amp from the specs. Bridged it can run 8 ohms minimum out of the 'mono' output. Either stereo output can take 4 ohms minimum. I would be inclined not to run it in 'bridged' mode at less than 8 ohms, as that is the rating on the back. Some solidstate amps have protection circuitry to shut down one side if no speaker connection is present - I would not recommend that with the Crate, IMO, as it appears not designed for it. It might just smoke.

Your speakers are Eminence indicated by the 67 in 67 8448 - probably made something like 1984, 48th week. As the PV cabs are 1970's they must be replacements. After the 1980s most speaker makers standardised on 8 ohms for PA, as 4 ohms tend to be prone to frequency response issues, and 15-16ohms is an archaic standard rarely found except in British speakers made for Marshall and Vox today.


Speaker details here.
Weber speaker codes page
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Old June 11th, 2007, 12:05 AM   #12 (permalink)
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You are a wealth of information!

Since the enclosure is 16 ohms as wired standard, can I just run the Powerblock 8 ohm mono output into one of the jacks? Remember there are two at the base of the enclosure. I don't mind using the tweeter if it makes things easier. I guess to be safe I would need to know if these speakers are stock. If so would the above scenario work? I would like to get the 150 watt output of the bridged mono option.

Thanks again!
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Old June 11th, 2007, 12:34 AM   #13 (permalink)
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It sounds like you have two 4 ohm speakers. Forget what the plate says. Once it had a 16 ohm speaker. The crate site is pretty specific - they aren't sophisticated amps. It is looking for 8 ohms minimum out of bridge mode. If you go smaller than that, you are operating it out of design spec - it will be attempting to run into something that looks to it dangerously close to a dead short. If so it would get very hot in the output. For a small amp enclosure without a fan - well, I wouldn't do it.

Your only alternatives:
  • to running both speakers out of stereo - 1 each side
  • wired through a series-mono box to get 8 ohms
  • wire at least one cabinet series/parallel to get 8 ohms or 2 ohms.

    Not that useful - you'd rarely use 2 ohms, which is what 2 x 4 ohms are in parallel.
  • get an 8ohm, 150watt minimum driver or drivers, and sell your 4 ohm Emis on ebay.

People do look for 4 ohm drivers - a 12" 4 ohm speaker is ideal for a Champ, or a pair wired series for a 8-ohm Deluxe extension speaker. Advertise them here on TDPRI to sell or swap.





I'd imagine the tweeter is a piezo, which has virtually no impedance so can't be used to 'fool' the amp. Even if it did (is an old style voicecoil unit), tweeters handle only small outputs, 20-30 watts which is the power in the high-freqency part of 100-200 watts of fullrange sound, so in series with a 100 watt+ 12" would quickly blow from the low freqs and power. Plus it would sound twanky.

The reason for running a crossover is to keep low freqs from the tweeters and hi from the woofer. Running them together is not good practice or practical.

Quote:
NOTE: I get a lot of people asking how to wire 2 speakers up together and maintain the same impedance as one of the speakers, for example taking 2 8 ohm speakers with the end result as an 8 ohm load. It is impossible - your only options are to wire 2 8 ohm speakers in parallel and get a 4 ohm load, or 2 8 ohm speakers in series and get a 16 ohm load (please note that things of this nature are possible with cross-over networks, however, only if the 2 speakers will be responding to different frequencies). It is possible to maintain the impedance with 4 speakers, but not with 2. 3 speaker and 6 speaker systems are discussed at the end of this document
From here
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Old June 11th, 2007, 01:13 AM   #14 (permalink)
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So my best (i.e. safest bet) for using the equipment that I have (the Crate and one enclosure) is to run one side of the 4 ohm output into the 4 ohm speaker that I have in the enclosure? I can settle for 75 watts output that would equal what I would get from one side of the Crate. Is it okay to run only one side of the Crate?
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Old June 11th, 2007, 01:43 AM   #15 (permalink)
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No - as I mentioned above I don't think the Crate has sophisticated protection circuitry that will protect an unplugged side. It has paralleled output chips, which I think in D-class they've got up to about 70 watts per chip @ 8 ohms. If you pass signal I think you will at least blow the unplugged side, I believe.

I think your only and safest option, without a single 8-ohm 150 watt speaker to run out of bridge, is to run each side into one of your cabinets. The other thing is we don't know what those Emis are - they might be 50 or 75 watt RMS speakers out of a solidstate 40 or 60 watt amp. So 150 watts into one and it'll signoff sooner or later, too.
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Old June 11th, 2007, 10:03 AM   #16 (permalink)
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Dacious, thanks so much for your help. I think I finally understand my options.
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