Question about speaker Ohms... I don't understand

Model Railroader Layouts and layout building
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I have an under the Table sound system I have built. I use a Lenz set -100 to run my layout and the Sound decoder. All I have is a Sound decoder in an electrical enclosure under the layout. I hook up the power wires to the Right and Laft rail wires on the DCC system and then I have 2 Speakers rated at 6 Ohms Under the layout. Now I have a Soundtrax decoder that has been working fine for about a year.

I just bought a Digitrax Sound bug which is a sound only decoder. I hooked it up to my system just like the Manual said to do. When I turned on the system the decoder Hissed. Now I assume that is the default Steam Idel noise. But that is all it ever did. Then the next night it wouldn’t make a sound. I called Digitrax tech support and the guy told me I probably over drove the speakes. He said the Ohms were to low and it probably ruined the decoder?

Can someone explain what I need for speakers ? How many can I have of a certian Ohm ?? And which Ohm is best. It came with a 32 Ohm and he said it will work with a 16 or 8 Ohm. He acted like I was really bothering him and I guess I was. He probably wishes smarter people would buy there products. I am sorry but I really don’t know what to do. I like the under the table sound system I have. Can anyone help???

Pat …Indiana

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Are you sure your speakers are 6 Ohms? That’s not a common value at all. Most speakers are 4, 8, or 16 Ohm.

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How are the speakers connected? If they are in parallel with one another that means you were only giving the sound processor 3 ohms. If this is the case the processor is probably fried. If they are wired in series then it should be giving the processor 12 ohms, so if what he told you is true should not have been a problem. And Cacole is right. 6 ohms is an uncommon rating.

The manual for the sound processor should specify the required ohms of the speakers. If it says 8 ohms minimum then that is what you should do. Actually that is one reason I don’t like the LocSound. They use 100 ohms speakers.

It depends on how they are hooked up. If you connect speakers in series then the ohms add. If connected in parallel they divide. So two 8 ohm speakers connected in series is 16 ohms, but the same two connected in parallel are only 4 ohms. Speakers ohms are inductance not resistance.

The one specified in the manual. You can go higher and it will not hurt the sound processor, but will have a lower volume. I use higher ohm speakers if I have speakers a long way from the sound processor or need to use smaller wire. This is usually not an issue in model trains.

If it came with a 32 ohm that would be best to use. You could hook four of the 8 ohm speakers in series to get 32 ohms.

Sounds like the Soundtraxx people. They talk down to everyone. They act like they are the only ones who know anything about sound at all. I like their systems, but getting any technical information out of them is like pulling tee

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8 ohm speakers will measure about 6 ohms with an ohm meter. You need the ohm speaker that the amplifier is designed for. Read the specs. Most amps will tolerate a 2:1 or 1:2 mismatch (8 ohm amp will accomadate a 4 ohm to 16 ohm speaker). I said MOST. Two 16 ohm speakers in parallel are 8 ohm. Two 8 ohm speaker in series are 16 ohm. Hope that helps.

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Pilot is correct. An ohmmeter measures only the DC resistance portion of the speaker impedance.

That being said, it is safe to use this resistance number in your calculations since the actual speaker impedance will be at least this much and more is safe unless the mismatch is severe. (Less may not be safe.)

Also when hooking multiple speakers in series or parallel, try to match polarities if there are any markings. This is called “phasing” the speakers. In phase speakers will tend to reinforce each others sound.

Good luck,

Karl

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Did this decoder know when you were applying throttle to commence other sounds? If all it did was idling sounds, it was working properly unless it also new that it was being called upon to produce motion sounds…chuff…turbodiesel…whatever. So, it needed an address and then it needed an input depicting acceleration and speed.

Perhaps I have not understood the question.

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i’m curious how speaker size relates to all this . i’m guessing the speaker under the layout isn’t the typical 1" speaker that is used with a decoder in a model train . would an 8" 8 ohm speaker work fine ? would it just be very quiet ?

you might be right about the soundtraxx people , but if you reread the original message you’ll see that Pat called digitrax tech support [:)]

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Yes It was DIGITRAX I talked to. And I think I see my problem. They do say six Ohm on the back of the speaker. They are cheap stereo speakers I have had for years. I have then wired in parralell. When you put an ohm meter on the total speaker terminal I get about 3 or 4 Ohms. Now I see that is was to low. I think i will just get some better speakers.

Now if I understand right, as long as the total Ohm going into the decoder is more than its smallest rating it is OK ??? Thanks for all your help folks.

Pat…Indiana

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Probably on both counts, but it probably also depends on the exact speaker. I used a 4" 20watt speaker with a monster magnet in one of my G-scale locomotives. It works great.

[:I] reading too fast again. I do mean Sountraxx, though. Now I wonder if it is a common malady among “sound” support people.

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“Ohms” measures electrical resistance, think of a bottleneck on the freeway.

The more ohms, the tighter the bottleneck.

Split the road, so each branch has a bottleneck, you can move twice as much traffic as you can with just one branch, you halved the resistance, and halved the ohms. That road would be wired in parallel.

Put two bottleneck on one road, the traffic has to slow down and merge twice. You double the bottleneck, and double the ohms.

Now you’ve got the general principle, DO NOT use this to calculate effective resistance. This is for ILLUSTRATION ONLY. It’s not even resistance, it’s impedance, which is the same, only different. It’s complicated, ok? Look up Kirchoff’s law if you really want to get a headache.

Now, if traffic is too slow…well everybody gets impatient, but so what? No real problem.

If traffic runs too fast, there are crashes and burning cars, pedestrians getting run over, and worst of all the roads get torn up and the cars overheat and warp the heads and never run right after that.

Not enough ohms is traffic running too fast. Ohms are good. Less ohms are bad.

Now…efficiency…

A fast car will make better time on a good road than on a road construction road with lots of bottlenecks. Less ohms equals louder sound. Until you have too few ohms, and blow up the car’s engine. Or your decoder’s amplifier circuit.

That’s the general rule. Generally, huge speakers require more power to get a certain amount of sound. They move more air, they do better bass, and bass takes power. But not always. Sometimes tiny speakers have more ohms than big speakers, and an amp will sound louder with the big speaker. Sometimes, not.

AFTER you get the ohms right, then you look at efficiency. Efficiency is measured in “loudness” when driven by an amp at 1 watt, measured at 1 meter. 85 decibels at 1 watt, 1 meter is mediocre. 105 decibels at 1 watt, 1 meter is outstanding. If and

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Good analogies and you didn’t even need to cover the differential equations.

Don’t forget Norton if you really want the migrane. Don’t forget Fudd’s first law of opposition either. It’s important.

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Ok, from my experience with the Digitrax SFX decoders, any resistance under 16 ohms will give you said “hiss” or horrible distortion. Digitrax states that an 8ohm speaker will work, but this is not completely true. It will work, but you have to turn down the sound to levels that make the decoder inaudible, expecially with the standard Soundtraxx 8ohm 1w speaker.

I find that if you have 2 or 3 soundtraxx 8 ohm speakers wired in SERIES (hence, adding the ohmage) then you will be withing the ‘good ohmage zone’ for the Digitrax SFX. The best case would be to use the supplied speaker, or if space doesnt allow, a smaller speaker with the same specs (32ohm 2.5watt). The smaller 32 ohm speakers are hard to come by, but take a look at mouser.com to see if they have any instock. I personally use 1/2 inch 32 ohm speakers for my installations.

David B