Speaker impedance.

[richierobins]

I have a pair of speakers which do not have any labels indicating their impedance. Using a multimeter, the DC resistance is twelve ohms for each. Am I right in thinking that the AC impedance will be around sixteen ohms?

 

Cheers.

[Casey4s]

Yes, 16 ohms.

[richierobins]

 

Yes, 16 ohms.

 

 

Thank you.

[WRGKMC]

Not nessasarily 16 ohm. 12 ohms is a common value for speakers designed for a 6 x 12 cab wired for 2 ohms. DC ohms is usually correct for the speaker impediance. 12 ohms is off by 4 for both an 8 ohm or 16. If you have a transistor head that can be set, 16 shouldnt be too bad. If its a tube head that can be set, 8 ohms would be better than a 16 ohm setting.

 

If you're using 2 of them run them in parallel for 6 ohms. Most tube amps without an impediance adjustment can handel between 8~4 ohms but check mfg specs for transistor. If its 8 ohms watch out for the head heating up under high volume.

[amp_surgeon]

 

Not nessasarily 16 ohm. 12 ohms is a common value for speakers designed for a 6 x 12 cab wired for 2 ohms. DC ohms is usually correct for the speaker impediance. 12 ohms is off by 4 for both an 8 ohm or 16.

 

 

That's odd. I always understood that the "rated impedance" was supposed to represent the nominal impedance across the audio spectrum. The DC resistance is the minimum impedance; i.e., when the signal is at or very near zero hertz. The manufacturer's rating is generally 115% of the minimum impedance.

 

Anyway, you would expect the DC resistance of a 16 ohm speaker to be just under 14 ohms, and you would expect the DC resistance of a 12 ohm speaker to be just under 10.5 ohms.

[WRGKMC]

You should start checking speaks yourself. The standard impediance has been measured in DC resistance by most manufacturers for the past 40 years that I been a tech. I've checked hundreds over the years. 99% are very close within an ohm or so if the coil isnt cooked.

Theres no way to have a standard for speaks running with a signal because theres so many variables like frequency, magnet strength, coil diameter, resonant frequency etc.

A typical 8 ohm speaker will vary anywheres from 2~16 ohms under signal load.

Now with really old speakers its a different story. They didnt have any real standerds and each manufacturer did their own thing, plus old speaks were usually tube driven which allowed for some leeway. Its not uncomon to see 3,6,10,20 etc ohms in old setups but the wattage was so low by todays standards and that stuff is pretty useless now anyway.

I have a few 12 ohm speaks left over from an older 6x12 Sun cab. Like I said it was pretty common stuff for getting down to two ohms. They're your typical eminance speakers speakers in most cases with either a square or round black magnet. Maybe 50 watts.

They're pretty useless to me in a 4x12 cab. The only way I could get the impediance to match was to run 2x12 ohms in series with 2x4 ohms. In a series parallel config I can get 8 ohms. The sound isnt super but it wont blow a head.

[richierobins]

Thanks to all for the responses. I'll be wary about what gets plugged into them.

 

Cheers.

[amp_surgeon]

 

Now with really old speakers its a different story. They didnt have any real standerds and each manufacturer did their own thing, plus old speaks were usually tube driven which allowed for some leeway. Its not uncomon to see 3,6,10,20 etc ohms in old setups but the wattage was so low by todays standards and that stuff is pretty useless now anyway.

 

 

It's true that a lot of different impedance values were common in the early days of tube electronics, but speakers were still rated by their nominal impedance, and there definitely WERE standards. The American R.M.A. Standard REC-104, dated January 1947, covered speakers for radio receivers. American R.M.A. Standard SE-103 covered speakers for sound equipment. These standards provided the definition of terms like "speaker impedance", and defined how they were to be measured. Without some standards it would be impossible for an engineer to design a circuit to drive the speakers.

[richierobins]

The amp I am planning to use with these speakers is an old Marshall 100watt "Lead & Bass", Model 2195. I have just dragged it down from my attic and found a label on the back warning that cabinets lower than 4 ohms should not be used, and quoting power outputs for 4, 8, and 16 ohms, so my two speakers should be fine in parallel as a two by twelve cabinet.

 

The speakers are "Forte Series", made by McKenzie Acoustics Ltd. of Birdwell, Barnsley, Yorkshire. A Google search suggests that they are no longer in business, but were part of the Wharfedale group.

 

Again, thank you to all who posted replies.

[WRGKMC]

Rich had a speaker that was most likely 12 ohms and not a 16

Speaker ohms measured with a meter as dc are usually close by an ohm or two. DC ohms is the only way for a laymen to check speaker ohms. DC resistance does vary between manufacturers but its rarely off by 4 ohms unless the magnet strength involved dictates that that varience is needed to get a nominal impediance impediance.

DC resistance is just a layman rule of thumb with no technical BS because most readers are not formally trained in electronics and dont have a clue to most anything else involved.

As far as the ohms IMPEDIANCE which i may not have been specific about varies with frequency, these were the first three articles I googled up to express what I was getting at and as to the difficulty in measuring impediance vs dc resistance. Theres hundreds more out there. They pretty much state very simply what occurs with frequency vs impediance. All in all thay react like any other coil does in a circuit accept for the magnet thats involved.

 

http://www.hometoys.com/htinews/feb04/articles/polk/impedence.htm

 

"The blue line in the graph above shows the actual impedance of a speaker that is rated as "8-Ohms." As you can plainly see, at 75Hz the speaker's impedance is 40 Ohms. At around 100Hz the impedance drops to just over 5-Ohms. Simply averaging the plot doesn't begin to describe the impedance of the speaker! There is one last thing technical thing you should know, as we pull right into the station at "Difficult."

 

http://news.cnet.com/8301-13645_3-9790255-47.html

 

"Back to the question at hand: speaker impedance. It's never a fixed number, a 6 ohm speaker may be, on average 6 ohms, but its actual impedance varies with the frequency the speaker is reproducing at any given instant. For example, it may be 4 ohms at 50 Hertz, shooting up to 21 ohms at 100 Hz, dropping back to 7 ohms at 1,000 Hz, and up again to 9 ohms at 10,000 Hz. So in other words, it's impossible to "match" a speaker to a receiver. Impedance is a moving target".

 

 

http://www.tape.com/resource/impedance.html

 

"HOW ABOUT SPEAKER IMPEDANCE?

Most loudspeakers are 4 or 8 ohms nominal. Actually the impedance varies with frequency. A speaker rated at 8 ohms impedance might range between 4 and 50 ohms, depending on the frequency of the signal".

 

 

I stated it would go from 2 to 16, I appoligize, my memory was way too conservative as to how much varience there is.(At least I remember it occured)

 

The peak will usually be in the Resonant frequency range which is a whole other subject. Its been a long time since I actually tuned a cabinet and is more a Hi Fi/bass cab/pa cab thing vs a guitar cab. (Guitar cabs are rarely tuned to resonant frequencies). A speaker in an improperly tuned cabinet can actually impede coil movement which affects resonant frequency impedance.

[richierobins]

Just out of interest, I measured the DC resistance of my four by ten Marshall 1965A cabinet - an 8 ohm cabinet. I plugged a short lead into the cabinet and measured between the tip and sleeve of the jack plug. The reading was 8.1 ohms.

[isaac42]

 

Just out of interest, I measured the DC resistance of my four by ten Marshall 1965A cabinet - an 8 ohm cabinet. I plugged a short lead into the cabinet and measured between the tip and sleeve of the jack plug. The reading was 8.1 ohms.

 

 

That's very unusual. My experience, as well as all of the information I've seen form driver specifications, tells me that amp_surgeon is correct in this, and that WRGKMC is not.

[richierobins]

 

That's very unusual. My experience, as well as all of the information I've seen form driver specifications, tells me that amp_surgeon is correct in this, and that WRGKMC is not.

 

 

The common factor in all my measurements is the multi-meter. I wonder how accurate it is.

[teemuk]

What does it say when you short the probes together?

[richierobins]

 

What does it say when you short the probes together?

 

 

On the higher ranges: 0 as expected. On the 200 ohm range the reading changes constantly around 35 ohms before eventually dropping to zero. The low-battery symbol is showing. I'll swap the battery before trying again.

[richierobins]

Curiouser and curiouser...

 

With a freshly charged battery in my meter I get about 13 ohms from the McKenzie speakers; pretty much the same reading as before. The reading between the tip and sleeve of the jack plug on a 1.2 metre long cable with the other end plugged into an 8 ohm cabinet, is around 45 ohms. It won't settle on a reading, it keeps changing. Tip to tip, and sleeve to sleeve on the unplugged cable, both read close to zero.

[WRGKMC]

Issac, maybe its because you cant distinguish the difference between where we're coming from. Neither of us HAS to be wrong.

Ampsurgeon is a well educated tech and appears to have great info.

Its nothing I havent read in the past 40 years of being an electronic tech

but I do give him a thumbs up for being able to pull it up and reference it like that.

I dont usually have to go dig up proof of my statements except for the cases where someone needs it to help them out.

 

What I was trying to do is give a "Simple Method" any musician can work from. Great part about it is it does work for general troubleshooting.

 

Otherwise it would take a good 6 months formal education to get a beginner to understand impediance which isnt my goal nor am i being paid to do so as I have been in the past.

 

If I needed to plug an amp head into an cab with unknown impediance

I would use a meter to test it to read the DC ohms.

ITS NOT PERFECT BUT ITS GOOD ENOUGH TO WORK WITH.

I should be able to get something close to 4/8/16 ohms.

I dont need to know any more than that.

I dont need to do use trigonometry at different frequencies to validate my findings because I know I will be safe enough to power up at that point.

 

Thats a realistic enough check every musician should know to be safe.

 

What I would question is some really wierd reading and follow up with the MFGs site and see if their voice coil DC resistance was wacky in comparison to others. Havent found any like that but I wouldnt allow the possibility.

Theres always new magnet alloys and high wattage coil designs coming out.

I just havent found any that affects guitar speakers.

 

Might make a good thread to get a consensus as to what guys measure off their speakers with a meter.

 

Richie did find out through his own curiosity though.

He read 8 ohms on a 4x8 cab with a meter which should is good enough for most to work with.

If he had a bad speaker or mis matched impediance or improperly wired cab it should have been very apparent with that test.

 

 

Speaker "Impediance" is dependant on "AC" frequency

"Z" (Total Impediance) is a combination of Impediance resistance and DC Resistance.

Total Impediance is measured over the entire speaker frequency spectrum averaged out and incorperates DC resistance as well.

 

Now thats simple stuff off the top of my head and I

dont think its changed those laws have changed since I learned them.

If someone doesnt theres not much else I can do.

They definately wouldnt have a clue with a text book explanation.

[WRGKMC]

Curiouser and curiouser...

 

With a freshly charged battery in my meter I get about 13 ohms from the McKenzie speakers; pretty much the same reading as before. The reading between the tip and sleeve of the jack plug on a 1.2 metre long cable with the other end plugged into an 8 ohm cabinet, is around 45 ohms. It won't settle on a reading, it keeps changing. Tip to tip, and sleeve to sleeve on the unplugged cable, both read close to zero.

 

 

Are these Stereo speaker cabs?

If so they may have a passive crossover inside to split the Highs and Lows.

You need to take the measurement off the speaker itself disconnected from the circuit.

If theyre raw woofers that are rated for 16 ohms it may be because McKenzy has a device built in to prevent them from blowing. I have no idea what it is or weather it affects the DC resistance reading. We used to use high voltage caps over the speakers to protect them from heads that have gone DC when troubleshooting. If they do have caps across them it can affect how some older meters read. Newer digital meters with low internal resistance shouldnt have any problem though. Otherwise I have no clue what the Britts consider a good reading without finding their spec charts

 

Like I said I'm pretty sure theyre 12 ohm speaks.

I highly doubt they are 16 ohms.

They will be OK in most applications.

2 wired in parallel will be about 6 ohms

which will be fine with most 4 or 8 ohm amps.

The ones I have read 11.5 on one, 12.4 on another

and 13 on the other.

They were from a 6x12 Sunn Bass cab from the 70s wired for 2 ohms.

The cab is great but it makes for hell trying to get a regular 4 or 8 ohms

without oddball speaker impediances using 6 speakers so I just have it in storage for now.

[richierobins]

 

Are these Stereo speaker cabs? If so they may have a passive crossover inside to split the Highs and Lows. You need to take the measurement off the speaker itself disconnected from the circuit. If theyre raw woofers that are rated for 16 ohms it may be because McKenzy has a device built in to prevent them from blowing. I have no idea what it is or weather it affects the DC resistance reading. We used to use high voltage caps over the speakers to protect them from heads that have gone DC when troubleshooting. If they do have caps across them it can affect how some older meters read. Newer digital meters with low internal resistance shouldnt have any problem though. Otherwise I have no clue what the Britts consider a good reading without finding their spec charts

 

 

The McKenzies are just unmounted speakers: no cabinet, no capacitors or any other components attached. They give a steady reading of about 12 ohms.

 

The cabinet that I measured is a standard Marshall 4 x 10",model type 1965A.

 

While the weird measurement from the 1965A is puzzling, it works fine so I won't worry about it.

 

Putting the McKenzies to good use is just for fun. If I get a decent sound from them it'll be a bonus.

 

Thanks again for everybody's help.

[WRGKMC]

If they are made by Fane as some posts eluded to they may very well kick butt.

[isaac42]

 

Issac, maybe its because you cant distinguish the difference between where we're coming from. Neither of us HAS to be wrong. Ampsurgeon is a well educated tech and appears to have great info. Its nothing I havent read in the past 40 years of being an electronic tech but I do give him a thumbs up for being able to pull it up and reference it like that. I dont usually have to go dig up proof of my statements except for the cases where someone needs it to help them out. What I was trying to do is give a "Simple Method" any musician can work from. Great part about it is it does work for general troubleshooting. Otherwise it would take a good 6 months formal education to get a beginner to understand impediance which isnt my goal nor am i being paid to do so as I have been in the past. If I needed to plug an amp head into an cab with unknown impediance I would use a meter to test it to read the DC ohms. ITS NOT PERFECT BUT ITS GOOD ENOUGH TO WORK WITH. I should be able to get something close to 4/8/16 ohms. I dont need to know any more than that. I dont need to do use trigonometry at different frequencies to validate my findings because I know I will be safe enough to power up at that point. Thats a realistic enough check every musician should know to be safe. What I would question is some really wierd reading and follow up with the MFGs site and see if their voice coil DC resistance was wacky in comparison to others. Havent found any like that but I wouldnt allow the possibility. Theres always new magnet alloys and high wattage coil designs coming out. I just havent found any that affects guitar speakers. Might make a good thread to get a consensus as to what guys measure off their speakers with a meter. Richie did find out through his own curiosity though. He read 8 ohms on a 4x8 cab with a meter which should is good enough for most to work with. If he had a bad speaker or mis matched impediance or improperly wired cab it should have been very apparent with that test. Speaker "Impediance" is dependant on "AC" frequency "Z" (Total Impediance) is a combination of Impediance resistance and DC Resistance. Total Impediance is measured over the entire speaker frequency spectrum averaged out and incorperates DC resistance as well. Now thats simple stuff off the top of my head and I dont think its changed those laws have changed since I learned them. If someone doesnt theres not much else I can do. They definately wouldnt have a clue with a text book explanation.

 

 

I understand what you're saying. The only thing I have an issue with is your statement that manufacturers use Re as the nominal impedance. I have never known this to be the case. In every instance I am aware of, the nominal impedance has been higher than the DC resistance. I have seen specs on 8 ohm speakers with DC resistances as low as 5.x ohms and as high as 7.x, but never 8 ohms. You have said that this is common practice, but I've never seen it. Given that, it seems unlikely in the extreme that a driver which measures 12 ohms would in fact, be a 12 ohm driver. Rather, that is a reasonable resistance for a 16 ohm driver. I have no problem with anything else you've said here.

 

As it stands, it seems that the point is moot, anyway, as he's using a solid state amp. As long as the load is 4 ohms or better, he'll be fine, whether it's 16 or 12, 8 or 6.

[amp_surgeon]

First, thanks for the links. Great articles!

 

I stated it would go from 2 to 16, I appoligize, my memory was way too conservative as to how much varience there is.(At least I remember it occured)

 

No worries. I wasn't questioning that the impedance would vary with frequency. There's no doubt this is true. I just thought the value of 2 ohms for the minimum was on the low side for an 8 ohm speaker. The minimum shouldn't go below the DCR, since resistance doesn't change with frequency. However, it's possible to get an indication that impedance is falling below DCR, depending on how the measurement is made. Remember that the coil is moving through a magnetic field, which will induce currents in the coil that either aid or oppose the much larger current driving the coil. It might appear that the resistance of the coil has dropped because the current is higher, but it hasn't - the induced currents act as a spoiler.

 

As far as how high the impedance will go, well, that's a whole 'nother story. The reactance is a function of the inductance vs. frequency. The inductance is a function of many factors, including the number of turns, the radius of the coil, the turns density, the number of layers (if it's a multilayer coil), and the core material. The last factor isn't even a constant with speakers, since the coil is moving around the core. The fact that the core is also magnetized further complicates this, since the magnetic field from the core will sometimes present a strong opposition to the magnetic field from the coil (increasing the reactance), and other times it will present little or no opposition, depending on the position of the coil and the phase angle of the signal going through it. This causes the "spoiler" currents I mentioned above. So, not only does the impedance change with frequency, the instantaneous impedance changes with coil position and phase angle. The best you're going to get at any frequency is an average for a complete cycle.

 

The whole mess can make your head spin.

 

Anyway, all of this is only part of the reason that DCR varies between speakers with the same rated impedance. Another reason is because different manufacturers use different methods to calculate impedance. Some will measure the impedance at a specific frequency - 400Hz and 1000Hz are common. Some measure impedance at the lowest point immediately after the resonant spike. Ted Weber's advice is simple and to the point - round the measured DCR up to the next higher standard value, and you've got the impedance for the speaker.

[richierobins]

 

It seems as if Richie may have an accuracy problem with his meter. At one point, after your post above, he measured 13 ohms on the speakers. Assuming his meter is in the ballpark somewhere, I'd feel comfortable calling them 16 ohm speakers. Of course, if you can't depend on the test equipment then the tests aren't of much use, especially when the difference between one rating and the next is only a few ohms. Since he talks about changing the range on the meter, I'm presuming this is an inexpensive meter. Richie , it might be time to invest in a better quality instrument. The meter must be accurate within no more than an ohm to get a reliable indication of the speaker's impedance.

 

 

It is indeed a "cheap 'n cheerful" meter: a Rapitest mas830l. I bought it for simple jobs around the house; I'm just a musician, not an electronics engineer. Armed with the info. that you all chaps have given, I feel safe knocking up a cabinet with the McKenzies. If anything does go wrong it won't be too much of a disaster; the amp is an old ebay bargain that I kept as a back-up, and the speakers were free. Cheers.