Speakers and ohm ratings.

[William]

The question that keeps running through my mind is just how important ohm ratings for speakers are. Whether it is a tube amp, or solid-state amp, can one substitute different ohm speakers and not blow up the amp? If the original speakers are rated at 6-5-ohms will 3.5-ohm speakers work? Can you test the amp to see what ohm's it is putting out? And when there are multiple speakers involved within a unit how does that play with determining ohms. The reason for the question, I have a couple of units that have bad speakers. I also have extra speakers that are the sizes needed so if I can use one of those without hurting anything I like to do that.

Thanks,
Bill

[walyfd]

https://www.bose.com/stories/speaker-im ... PvSApl1UXd

I liken it to running crap gas in my 62... you'll move for awhile but not well and the long term damage can be catastrophic.

Are you going to find a 4 ohm 15" woofer? I doubt it...

[William]

The speakers in question are 10" with a pair of 4" tweeters in each enclosure. Thanks for the link, Walter.

Bill

[wparks]

Hi Bill- There is a lot to unpack here.

Whether an amplifier will work with particular speakers depends upon the particular amplifier. Some are more finicky than others. There is no easy measurement to tell what an amplifier will work with, which is why it is stated.

In the case of stand-alone amplifiers it's generally best to check what is printed on the back, or find out what the manufacturer recommends in the user manual. The amplifier will generally work reasonably well for speakers around that stated range. The higher the ohms, the "lighter" the load is. If the amplifier is designed to run with 4 Ohms, you can use higher resistance 6 Ohm, or 8 Ohm speakers, you will not overtax or damage the amplifier, but you will not get as much power into the 8 Ohm speakers as you would with the 4 Ohm speakers. However, if the amplifier indicates 8 ohms only, or 8 Ohms minimum, trying to run 4 Ohm speakers or lower can cause the amplifier to go unstable, or work too hard, overheat, pop a fuse, and/or be damaged. Takeaway is don't run speakers with lower resistance than the amplifier was designed for, but higher resistance than designed for is OK. There is a workaround- if you only have 4 Ohm speakers and want to run them on an 8 Ohm amplifier, you can connect 4 Ohm resistors in series with the speakers, or use two 4 Ohm speakers in series for each side.

For designs with fixed built-in speakers, the speakers are matched to the design of the amplifier to output maximum power. To find out what that resistance is, you generally have to either check the service literature, or more likely measure the original speaker drivers and calculate the resistance of the combined set which depends upon the individual speaker resistances and how they are wired together

Assuming we are talking about a console, from what I have typically seen console designs like to run speakers in parallel, with the total combination running in the 4 Ohm ballpark. 4 Ohms is about the minimum resistance most amplifiers will work with, and designing a console amp to push this is convenient because most console speaker drivers are in the 6-8 Ohm ballpark, and they want to run both low and high (and sometimes mid) frequency drivers together, so you can put these in parallel and the resistance will be in the 4 Ohm ballpark.

Just to make sure you are solid on the resistance, please forgive if you know this- higher resistance is a lighter load, low resistance a heavy load. Two or more resistors in parallel (plusses all tied together, minuses tied together) decreases combined resistance which is harder to drive. Example- two 8 Ohm drivers in parallel will act like a 4 Ohm speaker. Two or more resistances in series (plus to minus, plus to minus in a string) increases resistance and is an easier to drive load. Example- two 4 Ohm speakers in series adds up to 8 Ohms.

So- as I was saying, consoles like to drive speakers in parallel so overall resistance the amplifier sees per channel is 4 Ohms. The woofer will typically have connections that trace back directly to the amplifier output terminals so they get the full signal. Midranges and tweeters, or horns, typically are connected to the amplifier output through a capacitor, or a crossover network that blocks lower frequencies from overloading these smaller speakers with large bass signals they cannot reproduce. These capacitors or crossovers prevent a direct resistance measurement of the speaker network from the amplifier perspective.

Speaker drivers will each typically be 6-8 Ohms. Measure with a multimeter right at the speaker terminals. If there is a capacitor on the speaker, make sure you are getting the actual speaker terminals, if it reads open (infinite resistance) the capacitor is blocking your measurement. Record the resistance of each of the drivers, and figure out the overall resistance from what is in parallel or series with what. I am betting in your system (with 10" woofer and pair of tweeters) that the amplifier connects directly to the woofer, and that the two tweeters in parallel with each other, then that pair in series with a capacitor, with the tweeters and capacitor are parallel to the woofer. (However, there are many other combinations possible- Each tweeter could each have it's own capacitor, etc). If you need help figuring out the total resistance, sketch a drawing of how they were connected and we can figure it out.

Sure hope my rambling made sense. Please ask any questions you might have. -W

[wparks]

Bottom line is that you can usually find SOME combination of just about any speakers to work with the amplifiers in your systems. We may need to connect speakers in parallel, or in series, or even use some resistors here and there if needed, but you can find an easy solution to use just about any speaker you have on hand and can make fit into the design.

[TC Chris]

My take on it is that speaker impedance affects sound more distinctly than it affects amplifier or speaker durability, with a few exceptions. One is my Heathkit AA-22 amp, which likes 8-ohm speakers. 4 ohm, not so much: it has a big resistor in series with the 4-ohm tap so the output transistors always see ca. 8 ohms. Tube amps? Hardly any effect on electronics but some effect on sound, in part because of the effect on damping factor, the extent to which electricity generated by a voice coil moving within a magnetic field gets reflected back to the output transformer and then back farther to the output tubes ad the negative feedback network.

Keep in mind that multiple speakers (woofer/mid/tweeter) may have a lower total impedance because of the voice coils being in parallel, albeit usually with a crossover doing some kind of blocking.

Many radios and mid-grade consoles used speakers with 3.2 ohm voice coils (like my two '56 Maggies). And they used two big woofers in parallel. Of course, the output transformers were wound to match the resulting impedance.

But the real point is that except for amps like my early solid-state Heath, it's not a major concern unless the mismatch causes audio anomalies.

Chris Campbell

[Conelrad]

Keep in mind speakers are rated in impedance, not DC Ohms. For instance, a good 16-Ohm woofer might measure 10-12 Ohms DCR.
In any case, DCR will be lower than the rated impedance.

Impedance might as well be called AC Ohms, and is tricky to deal with in audio circuits. This is because it changes with the frequency being used to measure, as that changes a lot in audio LOL.

If you think of a speaker as a motor, it will make more sense: Run an AC motor on DC, you get flames. The same thing happens when you run a speaker hard on an amp that clips the output waveform into DC. This changes the speaker into a resistor, resistors get hot, and there goes your voice coil.

Far more speakers are ruined by crappy amps (mostly solid state) than by too much power. As long as it can motor the audio input power into cone motion, it will be fine. Well, maybe until it literally shreds the cone apart. Done that, too.

D

[William]

Thanks, guys, I will take a look at how things are wired up and will post what I see for further discussion. What I am looking at is two GE Trimline portables. One is a deluxe SS unit with radio and that one has the 10" woofers with two tweeters each. The other one is a GE Trimline tube unit. That one has 8" woofers with single tweeters. The SS GE was missing one of the 10" woofers when I got it. The tube GE has a bad tweeter in one of its speaker boxes. These are both nice units and I want them to sound there best without burning something up.

Bill

[William]

To continue, we will start with the GE SS model. It is the one with the missing woofer and why anyone would remove a 10" nothing special woofer is beyond me. The remaining woofer measures 6.7 ohms and the tweeters say 16 ohms each but really only read 13 ohms. Everything is in parallel. I now actually have 3 10" woofers but only two will work as one is too deep to fit in the GE speaker box. The original GE speaker has EIA code 137. I have one speaker (I actually have two of these speakers) with code 719 which measures 3.6 ohms, and one with code 145 which measures 3.3 ohms. The two tweeters are paralleled together, they do have a crossover and are located in the center and covered up by the woofer.

Bill

[Conelrad]

Bill,

the tweeters say 16 ohms each but really only read 13 ohms.

Please read my post in this thread...

Best, Dennis

[William]

Hey Dennis,

Yes, they both have 16 ohms printed on them, but when I take a measurement the actually only read 13 ohms.

Does DCR stand for direct current reading? Or do I have that all wrong?

I need to reread everyone's posts. I am still trying to wrap my head around it.

Bill

[Motorola minion]

I greatly appreciate everyone's contributions to this subject and don't want to go far off-track but crossover components may need to be touched upon.

That is, there must be something between a tweeter (or midrange) and the brute power going into that woofer. A series non-polarized capacitor value will depend entirely on the tweeter's impedance. A tweeter is an inductor wound with much lighter wire which cannot tolerate nearly as much current as a woofer does.

This crossover network's series (and sometimes parallel) impedance would limit the low-frequency energy to the tweeter. Sometimes an inductor and resistor are also employed in most Magnavox consoles using horn tweeters. Unless power is over 20 wpc (RMS) this is a common method to protect high-frequency drivers, cone or horn.

Sweeping frequencies between 500 and 1500 hertz into a common 3x10 horn, I was able to confirm that impedance does indeed drop to a minimum at about 1000 hertz, hence the "kilocycle horn" moniker in Magnavox literature. Up to this value, the woofer response drops off as the tweeter increases with frequency, the overlap point is considered the crossover frequency.

Electrostatic tweeters are an entirely different animal to be covered separately, as many Philco, Zenith and European tube radios and consoles used them initially for high-fidelity applications.

[Conelrad]

DCR=direct current resistance.

That is what you get when using a VOM in the Ohms mode.

D

[William]

Thanks, Dennis. I now understand what you are were talking about comparing speaker ohms to meter ohms. I have learned a lot from you all, so thank you one and all.

Bill