8 Ohm speaker reading 6.1 Ohm

[BTRY B 529th FA BN]

I am running a 12 Tube Amplifier (Mesa Boogie 400+) and I recently had to switch out a speaker due to one of the wires going to the coil detaching. This actually blew my amp twice. Solid state amps this won't happen but the impedance has to be correct on the speaker so the tube amp won't blow. I replaced the old speaker with another 8 Ohm 15" EV speaker BX Long-throw 30-4000Hz range, and doing a test, the speaker is measuring 6.1 Ohm. I've heard 6.4 is the lowest.

Thoughts?

 

[brblx]

hence terms like 'nominal impedance'

get the coil hot then measure it.

edit- also a loose wire won't blow an amp or a speaker; if the amp blew, you shorted them together. i've heard of 1/2 ohm stable amps but i don't think they make a 0 ohm.

speaker prolly blew from clipping.

 

[BTRY B 529th FA BN]

When the speaker cab was tested it was dead. I checked it myself and there was nothing.. After I changed out the speaker with the "good" 8 Ohm speaker I got a reading of 6.1 Ohm


EDIT: there was one wire completely off the cone or coil, whatever it's called

 

[Rubycon]

Your meter is reading resistance, a DC measurement. Loudspeakers are measured by impedance, an AC measurement. Most 8Ω speakers will measure considerably lower than 8Ω with an ohmmeter. This is nothing to be concerned with and is about as simplified of an explanation possible.

When the speaker cab was tested it was dead. I checked it myself and there was nothing.. After I changed out the speaker with the "good" 8 Ohm speaker I got a reading of 6.1 Ohm


EDIT: there was one wire completely off the cone or coil, whatever it's called

Those are called "tinsels" and are leads from the backside of the terminals that connect to the voice coil.

 

[BrokenVisage]

Coming up a little short, I shall call this one Ohmbama.

 

[BTRY B 529th FA BN]

Glancing at your name I'm coming up with BroBahma

 

[brblx]

the tinsel lead ripping off is not that uncommon. if something didn't actually hit it, it was probably overexcursion that tore it off, or could just be that the speaker was quite old (tube amp makes me assume as much)

also nice catch ruby, you're right. i've always accepted low readings on a speaker and assumed that it would be high enough once the voice coil warmed and the resistance increased. didn't think about the AC/DC difference.

even so, OP, typically bad speakers will be one of two things- shorted (little to no resistance when measured with a meter) or open (thousands of ohms or just 'OL' usually).

make sure that your amp is powerful enough for your speaker. while you technically can't murder a speaker with 'too little power,' you can do it quite easily with too much volume (forcing a weak amp past its limits, producing the clipped signal that blows speakers).

 

[BTRY B 529th FA BN]

Your meter is reading resistance, a DC measurement. Loudspeakers are measured by impedance, an AC measurement. Most 8Ω speakers will measure considerably lower than 8Ω with an ohmmeter. This is nothing to be concerned with and is about as simplified of an explanation possible.



Those are called "tinsels" and are leads from the backside of the terminals that connect to the voice coil.

Yeah, I was lightweight thinking my meter was weaksauce

I do know they don't measure to the nominal impedance but from what i heard (not sure if accurately credible to my speaker) was 6.4

 

[twinrider1]

hence terms like 'nominal impedance'

get the coil hot then measure it.

edit- also a loose wire won't blow an amp or a speaker; if the amp blew, you shorted them together. i've heard of 1/2 ohm stable amps but i don't think they make a 0 ohm.

speaker prolly blew from clipping.

Running a signal through a tube amp with no load can absolutely kill it. I think a loose wire equals an open which equal no load. Output transformers don't like that. The amp can be on, but if you send a signal through it, you're asking for trouble.

 

[Paul98]

the resistance in speakers change as the freq changes. Just don't over drive the amp and you will be fine.

 

[Paul98]

Are you saying you are replacing a driver in one of your speakers with a different driver? Or is it just a replacement driver?

If it's a different driver you could have a problem with the crossover. You will need specific driver info to get the real expected rating, the "8 ohm", "6 ohm" "4 ohm" are never really those if you just put an ohm meter on it. Usually a good bit lower.

 

[Colt45]

As others said, you're measuring DCR not impedance.

Anyhow, even if it were 6 ohms, it isn't very important to your amp. Distortion and power will both go up a bit, generally.

Dangerous case with tube amps, as you've found out, is no load. A lot of old guitar amps would actually short the output when nothing is plugged in. Can't see that being ideal either, but superior.

I suppose having a little dummy load on there might be the best idea, 100-200 ohms won't steal too much power, or change the load much, but should be enough to prevent meltdown if the speaker disconnects.

 

[BTRY B 529th FA BN]

The old speaker is done. The tinsel wiring broke off right by the cone area and it can't be soldered together as it's too close to the speaker...

The replacement speaker is an 8 Ohm speaker and when it's tested it reads 6.1 I know they don't measure exactly at 8 ohms all the time and when measuring it the signal will produce the lowest reading. My question is 6.1 Ohm too low? I can't find any more documentation on it and the info off the tech specs of the speaker don't say so, which i find annoying.

 

[Modelworks]

6.1 ohms is fine if you are using a digital meter on ohms. Digital meters are not the best thing to have for reading speaker impedance, the methods they use are not reliable for anything but resistors, and coils in a speaker are also inductors which will react to the nature of the digital meter when it takes a reading. Analog meters are much better at taking a reading because they supply a consistent voltage that directly drives the meter.

 

[disappoint]

lol tube amp. Is it getting warm in here?

 

[disappoint]

As others said, you're measuring DCR not impedance.

Anyhow, even if it were 6 ohms, it isn't very important to your amp. Distortion and power will both go up a bit, generally.

Dangerous case with tube amps, as you've found out, is no load. A lot of old guitar amps would actually short the output when nothing is plugged in. Can't see that being ideal either, but superior.

I suppose having a little dummy load on there might be the best idea, 100-200 ohms won't steal too much power, or change the load much, but should be enough to prevent meltdown if the speaker disconnects.

Why would your speaker disconnect? What are you playing in traffic? BA BA NA BA BA BANANANANA BA BA BA NANA TEQUILA! O LOOKOUT HERE COMES A BUS!

 

[Rubycon]

Why would your speaker disconnect? What are you playing in traffic? BA BA NA BA BA BANANANANA BA BA BA NANA TEQUILA! O LOOKOUT HERE COMES A BUS!

A sudden driver failure happens. Anything can happen. With 12dB/octave crossovers the loss of a tweeter diaphragm (open voice coil) will cause severe impedance dips to the amplifier on high passages. If a protection means is not in place either passively (poly switch) or active (current limiting) all sorts of nasties can occur.

Additionally some folks are still using TRS (as opposed to Speakon) connectors and something as simple as a tug on the line can unplug the speaker/monitor while people are singing or your lead is slaying away, etc. Any of these can spell disaster to the head end if safeguards aren't in place.

Not just tube amps will get unstable with no load. Transistor amps can also get unstable and have their finals "blow" (short out) if driven into no load at very high levels approaching power supply voltage clipping.

RE: Analog meters...most transistor meters have a pretty high input sensitivity and will still read similar DC resistance as a DMM.

 

[Modelworks]

RE: Analog meters...most transistor meters have a pretty high input sensitivity and will still read similar DC resistance as a DMM.

Some models of DMM use voltage pulses rather than a constant voltage for measuring resistance. The pulses , depending on the sampling rate, alter the inductor readings because of the voltage generated when power is switched off between readings.

 

[Rubycon]

Some models of DMM use voltage pulses rather than a constant voltage for measuring resistance. The pulses , depending on the sampling rate, alter the inductor readings because of the voltage generated when power is switched off between readings.

That's important and the equivalent sinus switching rate is probably not going to make much difference to the comparatively large inductance of a large low frequency driver's motor.

Have you scoped the bias on one? The waveform as well as frequency would be interesting to note.

 

[alkemyst]

Your meter is reading resistance, a DC measurement. Loudspeakers are measured by impedance, an AC measurement. Most 8Ω speakers will measure considerably lower than 8Ω with an ohmmeter. This is nothing to be concerned with and is about as simplified of an explanation possible.



Those are called "tinsels" and are leads from the backside of the terminals that connect to the voice coil.

This.

 

[Modelworks]

That's important and the equivalent sinus switching rate is probably not going to make much difference to the comparatively large inductance of a large low frequency driver's motor.

Have you scoped the bias on one? The waveform as well as frequency would be interesting to note.

I hadn't scoped one in a long time so I gave it a go today. Here is what I found the meter using, model agilent u3401A

Ohms measurement setting - triangle wave @ 50hz
diode measurement - triangle wave @ 1Khz

Why the different frequencies for ohms vs diodes I have no idea.
The output wave forms were definitely effected by the coil of thespeaker. The output had a very clear peak after about the 3rd peak in the input wave. It didn't effect the reading on the meter much, about .1 - .5 ohms. I guess it is going to depend on the size of the coil and the design. I did notice that even blowing against the speaker cone would drastically alter the meter reading , double or tripling the ohms reading. People taking readings need to remember that and make sure they are not in a noisy environment or touching the cone.

 

[Rubycon]

50 Hz definitely is going to matter and ditto on speaker microphonics. Of course the ohmagically challenged musician is mainly concerned whether or not the voice coil is open and can move freely.

 

[jtvang125]

Go buy a 1.9 ohms resistor. Wire it inline. Problem solved.

 

[So]

50 Hz definitely is going to matter and ditto on speaker microphonics. Of course the ohmagically challenged musician is mainly concerned whether or not the voice coil is open and can move freely.

ATOT is full of certified geniuses. Everyone has a PhD in everything. But when it comes to remembering their complex numbers from high school algebra, we find that speakers are a scientific mystery. Absolutely nobody has any clue what makes them go. It may be the blue smoke, or the Holy Ghost, or hispanic laborers pedaling furiously.

 

[AlienCraft]

Don;t let the smoke out!