Choosing a Proper Speaker for a 4 Amp Decoder

I would be helpful if you could provide the model number of the decoder in question to better determine the specifications of an appropriate speaker to use with it.

at least who is the manufacturer of the decoder.

usually it's ohms you have to watch for. for example earlier LokSound decoders required 4 ohm speakers. I believe that's been changed to 8 ohms.

the amperage rating of a decoder is for the motor driver. not usually the amplifier. that would be something like 2-4 watts.

it could be your decoder is mapped so you've silenced sounds. meaning rather than F8 silencing the motor. the horn button is mapped to that function.

The decoder in this case is a Soundtraxx 884009 TS-2. And I believe I was told the Amplifier is 3 watts. When these are purchased nothing says "caution use with a 8 or more ohm / 3 watt or more speaker. The manuals say something like use with an appropriate speaker.

Frank

As far as mapping goes, I read Some other cv might have gotten mapped onto F8 mute. But if anything did map accidently wrong, that is why I reset the decoder totally twice and I thought that would set it correctly but there was no sound for anything.

Frank

If you were initially using a 8 Ohm 0.2 Watt speaker, the decoder can put out more power than it can handle and likely overloaded and fried the speaker. The audio amp on the decoder is likely still good.

The specifications for this decoder call for a 8 Ohm 3 Watt speaker.  Stick with 8 Ohms.  A speaker with a slightly higher wattage rating is fine.  Speaker wattage is how much power it can handle, not how much it draws.

Here's a link to where I found this information: https://mountain-subdivision-h...4-amp-sound-decoder/

@SteveH posted:

A speaker with a higher wattage rating is fine.  Speaker wattage is how much power it can handle, not how much it draws.

Here's a link to where I found this information: https://mountain-subdivision-h...4-amp-sound-decoder/

Well, yes and no.  You don't want to put a 30W speaker on something that has 3W of power output, it's likely the efficiency of those speakers are such that you'd get very little volume.  I'd be looking for speakers in the 3-5 watt rating range and an efficiency rating of 80db or more.

@gunrunnerjohn  Of course you are correct.  I added the word "slightly" to my previous reply.

Hi Steve,

The speaker was not destroyed. I put back to back capacitors on and tested it on DC and it was fine. I was told the opposite which was the low watt speaker probably destroyed the amplifier which is why I wrote about it as all other decoders are working with low watt speakers and which makes me question if it is true. I am Sgt Schultz on  what kills what. electronic world. Thanks for finding the Specs for this decoder (8 Ohm, 3W+). Those specs seem to tell me also that my .2W speaker should be fried but it is still raw.

Frank

IMO, a low wattage speaker is unlikely to damage the amplifier as a rule.  There are always outlier cases, but that's not generally the case.

Atlas had a history of using low power pancake speakers in their diesels, and I've replaced a bunch of them over the years because the Railsounds board popped them.

Hi Gunrunnerjohn,

When my decoder gets repaired/replaced I have the 8 ohm, 5 watt peak, 90mm x 50mm x 37mm speaker to install in the centipede tender but that will be the only one I ever used of that size. All other decoders have survived with 1990s and up speakers from Radio Shack with sizes for HO guage up to 2" for O guage. That 8 Ohm 5 watt is what Soundtraxx sells PN 810090. Now I have to wonder why I have not fried any other decoders or speakers!

Frank

@trooptrain The 3 Watt speaker specification is a recommendation by Soundtraxx design engineers and would typically be a higher value in Watts than the decoder audio amplifier is capable of delivering.  This is normal design practice to over specify a component to be able to handle a little more power than it would encounter under most situations.

The 3 Watt spec by Soundtreaxx doesn't mean that the audio amplifier will normally put out this much power.  The actual output power depends on two things: Audio signal amplitude and frequency content.  In other words, the louder and more bass the sound has, the more power the amp is sending to the speaker.

If the 0.2 Watt speakers have survived thus far, it's because the sounds haven't been loud enough for long enough to melt the voice coil (wire) in that speaker.

Thanks for that info Steve. My MTH engines are probably not a problem but where I installed low Watt speakers I think I had better go back and reprogram to assure I have a lower  master volume like something below 50%.

Two questions: 1. For a 4 A Soundtraxx decoder that calls for 8 ohm 3+ watt speaker, in order to avoid that large speaker and for smaller tenders, how low in watts do you think is safe using 50% master volume 1 watt? 2 Watts?

2. If I set master volume at CV128 =128 instead of 255 and I choose to set a sound CV at 255 , did I over come the master volume or do I get the full 255 of the master volume setting which would still only be 128?

Thanks Steve and others who have been replying to this topic!

Frank

@trooptrain posted:

Thanks for that info Steve. My MTH engines are probably not a problem but where I installed low Watt speakers I think I had better go back and reprogram to assure I have a lower  master volume like something below 50%.

That seems like a good idea.

@trooptrain posted:

Two questions: 1. For a 4 A Soundtraxx decoder that calls for 8 ohm 3+ watt speaker, in order to avoid that large speaker and for smaller tenders, how low in watts do you think is safe using 50% master volume 1 watt? 2 Watts?

2. If I set master volume at CV128 =128 instead of 255 and I choose to set a sound CV at 255 , did I over come the master volume or do I get the full 255 of the master volume setting which would still only be 128?

1. A 1 watt speaker rating may or may not be ok.  Can't say for certain without doing electrical measurements.  At 50% volume, 2 watts should be fine with this decoder.

2.  I'm not familiar enough with the Soundtraxx decoders to say definitively, but in general, audio gain structure attenuators (volume controls) are configured in series, such that they serve to reduce the maximum signal gain at each stage.  Meaning in most audio devices, if you turn down the master gain control to 50% and controls that come before it are maxed, the final output is still only 50% of it's rated output.

In my opinion, if you typically run your trains' sounds at a low enough volume that there hasn't been an issue so far with blowing speakers, then unless that changes, they should be fine going forward.  Really the worst thing that can happen is that you may blow a speaker.  Using lower wattage speakers is unlikely to damage the decoder audio amp.  The only way I can possibly conceive that the amp would be damaged is for the speaker to somehow present a direct short to the amp, which is highly unlikely at these power levels.  In my 30+ years as an audio engineer, I've only ever seen blown speakers present as an open circuit, which cause no harm to the amp.

Individual decoder sound settings set at max 255 will not exceed the master volume setting. Whatever that may be.

Say you want the whistle to be louder than the chuff. You’d set the volume for each accordingly.

Thanks again Steve for your reply- that 's a printed keeper! I have a 2 " Dia. speaker that lost it's  makings on the end due to mounting in a previous engine with contact cement. I have a test meter. What do I need to do to determine it's Ohms and Watts. W= IV. Do I put back to back capacitors on and power up with max AC  voltage out of my NEC  power, then measure amps across the speaker terminals and multiply times my track voltage?

Frank

PRR Man,

Thanks - Had not thought of that until today but understand it now.

Frank

@trooptrain posted:

Thanks again Steve for your reply- that 's a printed keeper! I have a 2 " Dia. speaker that lost it's  makings on the end due to mounting in a previous engine with contact cement. I have a test meter. What do I need to do to determine it's Ohms and Watts. W= IV. Do I put back to back capacitors on and power up with max AC  voltage out of my NEC  power, then measure amps across the speaker terminals and multiply times my track voltage?

Frank

Determining a speakers impedance is fairly straight forward using a multi-meter, set to measure DC resistance in Ohms.  With the speaker disconnected from any other circuits, set the meter to the lowest setting above 50 Ohms and connect its leads to the two speaker terminals.  This measured DC resistance will be around 90% of the speaker's AC impedance.  As you may know, speaker impedance is the combination of resistance and inductance in the voice coil.

While is it possible to measure the Watts in an amplifier circuit connected to a speaker, this would only tell you how much power is passing through the circuit for a given sound source at a given volume.  That's what I was referring to earlier.  This won't determine the power rating of the speaker.

In your case, it's not practical to electrically measure the power handling capability of a single speaker without blowing the speaker.  Power handling (wattage rating) of a speaker is the maximum power it can handle before it blows.  To do that you'd need to take continuous power measurements as the power is stepped up until the speaker finally blows, defeating the purpose for a one-off speaker test.

It's not very practical, but the only way I know to non-destructively determine the approximate the power handling capability of a speaker would be to measure the uninsulated gauge of the wire in the voice coil (remember that the coil wire has a coating on it that would need to be partially removed before measuring) and compare that to an ampacity chart to find its current handling ability.  Combining Ohm's and Watt's laws gives the Power handling capability of the speaker (P) as: P = I²R, where I is the current in Amps from the (Ampacity chart) squared; and R = the speaker impedance.  Again, this is not very practical and would only be an approximation.

My recommendation would be to, if possible, find a 3 Watt 8 Ohm speaker that will fit inside the tender.