Choosing The Right Speaker For Your Sound Decoder

This week I have a ‘how to’ post to share with you about speakers and the importance of choosing the right one.

Sometimes I get locomotives in for repair which have been fitted with a DCC sound decoder and the sound simply doesn’t work.  There are several reasons for this and hopefully it’s something simple like a broken wire.  But sometimes the wrong speaker has been used and it’s damaged the decoder beyond repair.

Most manufactures supply speakers with their decoders, but as they often don’t know what you intend to fit it in, the speaker is a generic size, and in N scale this is never going to fit.  There are all sorts of sizes available as well as shapes as you can see below from this selection I had in my bits box.

Two of these speakers are supplied with ESU V4.0 decoders, the smaller speaker comes with the V4.0 Micro.  However, both are fairly large and it can be struggle to find room for them in many locomotives.  So they are often swapped for smaller speakers.  These then become available to be used with other decoders, after all they’re good speakers, but now there’s a potential problem which could damage the decoder, because they may not be compatible.

Speakers are measured in wattage, this is how much power they can handle, and impedance, which is the property of a speaker that restricts the flow of electrical current through it.  This is measured in Ohms.  If you put too many watts through a speaker, you normally just blow the speaker.  But if the impedance of the speaker is too low then more power is used in the amplifier than sent to the speaker and the amplifier over heats and blows.  The amplifiers on DCC sound decoders are ‘solid state’ which means they are made from electrical components only, no form of valves or vacuum tubes as you used to get in guitar and stereo amplifiers, but given how small the sound decoders are that is not surprising.  But this means there really is no tolerance for getting the impedance wrong.

Some speakers, such as the ones used by ESU with their V4.0 decoders have both values written on the back; 1.5w and 4Ohms.

If this speaker was used with a standard Digitrx, Zimo or Hornby TTS sound decoder it would blow the amplifier right away as these decoders are normally only rated at 8 Ohms.  The lower the Ohm value the more power runs through the amplifier.

All sound decoder manufactures should list, either in the decoder manual or on their website, what the max Ohm value is for their product.  But what if you have a speaker and you don’t know what the Ohm value is?  This can easily be measured with a multi meter which can read Ohms.  Below you can see I have the multi meter set to read up to 200 Ohms and when connected to the ESU speaker it is reading 4.3 Ohms.

So now you can select the right speaker to go with your sound decoder.  But going with the smallest isn’t always the best idea.  Normally the smaller the speaker the quieter it gets and it will have less bass.  One of the best ways to increase the volume and bass, without electric amplification, is to add a chamber to the speaker for the sound to reverberate in.  Putting a speaker inside a locomotive shell will do this naturally as the shell forms a box.  But the shell will not be airtight and as a speaker makes noise by pushing air the increase in sound will be small as the air escapes.  Adding a chamber directly to the speaker is the best way and the ESU speaker I measured earlier has just this.  The speaker clips into the box.  But due to the screw holes in the speaker plate and the wire holes it still isn’t airtight.

Digitrax supply their N Scale speakers with a pull-off strip which leaves a sticky surface around the speaker.  It can then be stuck to the chassis or inside of the shell.  But this doesn’t leave a lot of air for the speaker to push against.

I like to use cell phone speakers for my N scale locomotives as cell phones can be very loud!  Below is a Zimo sound decoder with a 8 Ohm speaker.  When soldering the wires onto your speaker remember that a speaker has a large magnet in it so as the soldering iron gets close make sure to hold the speaker down so it doesn’t jump up and attached itself to the iron.  They tend to get very hold and melt very quickly; don’t ask me how I know this!

In cell phones the speaker normally sits over a cavity and is stuck on to form an airtight box.  This is why some phones sound very loud and appear to have good bass.  I 3D print boxes to go with the speakers in different depths depending on how much room I have to work with.

This particular sound decoder is going into an old Rivarossi Challenger and that has lots of room in the tender so I’ll be using the larger box.

I use superglue to fix the speaker but it’s important not to get any on the actual speaker.  So, using the speaker bag, I put some superglue down and rub the box in it ensuring I get some glue on all sides.

Then I place the box onto the speaker and hold it till the glue sets.  Being superglue this doesn’t take long.

The speaker is now ready to fit into the tender and it will be considerably louder than any of the speakers in the first picture.

The thing to remember is to check the impedance.  Most new decoders now support 8 Ohm speakers, ESU going up to 4 Ohm. But a lot of older decoders, even ESU, may be 32 or even 100 Ohm only.

If anybody is interested in 3D printed speaker enclosures or cell phone speakers please get in touch via the contact page.