While most, if not all, new DCC sound decoders come shipped with a speaker in the box, they can sometimes be lacklustre and many people opt to change them out for something a little better.
Deciding which speaker or combination of speakers to pair with your DCC sound decoder can, however, be daunting. Not only are there so many on the market to choose from, but it’s vital that the impedance (measured in ohms) matches the specifications of the decoder in question.
We’ve produced this short guide to the impedance requirements of a range of the most popular DCC sound decoders available to buy at this time. Whilst the rules are relatively straightforward, mismatching impedance with a decoder can result in it failing beyond repair.
It is therefore vital that you take care to calculate overall impedance to match speakers and decoders accurately; if there is any doubt or discrepancy, please always refer to the manufacturer’s instructions.
Measuring the impedance of a speaker
Occasionally, you may find that a speaker doesn’t have its impedance marked on it; this is particularly common if the speaker has been rattling around in a box for some time and the markings have been rubbed off. A straightforward way of testing a speaker’s impedance is to get a digital multimeter and measure the resistance across its terminals.
While resistance and impedance are both measured in ohms, they are not precisely the same thing. For that reason, the reading on the meter will generally be slightly less than the impedance rating of the speaker.
For example, an 8 ohm speaker is likely to have a resistance reading of slightly over 7 ohms, while a 4 ohm speaker will have a resistance reading slightly over 3 ohms.
Combining speakers and calculating impedance
The most straightforward way of ensuring that the impedance of the speaker load is suitable for the decoder is to match the decoder with a single speaker of the appropriate rating.
However, there are a range of reasons why you might not wish to do this, including the limited speaker options available to you at a given point in time or the desire to get as much volume and dynamic range out of a digital sound project as is possible.
It is possible, therefore, to wire more than one speaker – in series or parallel – to build up or reduce impedance so that a number of speakers can work together while remaining within the specifications of the decoder.
As most people look to wire no more than two speakers together, we’ll stick to looking at two, however a good article delving into impedance in more detail, including calculations for three or more speakers, can be found here.
Wiring in series: when wired in series (sometimes also called daisy chaining), you add the impedance values of each speaker to give you the total impedance of the speaker circuit. So, two 4 ohm speakers wired in series would produce 8 ohms, while two 8 ohm speakers wired in series would produce 16 ohms.
Wiring in parallel: when wired in parallel, each additional speaker added to the circuit results in the total impedance dropping. So, two 8 ohm speakers wired in parallel would produce 4 ohms, while two 16 ohm speakers wired in parallel would produce 8 ohms.
NB. To keep things simple and straightforward, we suggest that you use speakers with matching impedance. If speakers wired in parallel do, however, have different individual impedance values, it’s vital that you refer to the parallel wiring equation below to calculate the cumulative impedance (where R1, R2, R3 etc. equals the impedance of each speaker).
The correct type of speaker/speaker combination for use with LokSound decoders depends on the model of decoder. Older LokSound decoders require speakers with an impedance of 100 ohm or greater, while newer ones can handle anything above 4 ohms (although the greater the impedance, the quieter the sound level will be).
While most LokSound V4 decoders are shipped with a 4 ohm speaker, and when combined with a sealed sound box these can often produce reasonable sound, if there is room within the locomotive, we recommend that you consider upgrading to something better, such as our maxi bass reflex speaker.
- LokSound 3.5 or older: 100 ohm (or greater)
- LokSound Select (all variants): 4 ohm (or greater)
- LokSound V4.0 (all variants): 4 ohm (or greater)
- LokSound XL V3.5 and LokSound XL V4.0: 8 ohm to 32 ohms
There can be a number of sub-models within each LokSound range, meaning, for example, that not all LokSound V4 decoders look alike. If there is any doubt as to what model of decoder you have, please contact ESU direct for guidance.
All Zimo DCC sound decoders are designed to work with 8 ohm speakers. This means that single 8 ohm speaker, such as our responsive mega bass speaker, will work just fine with their decoders, however, many people choose to wire two 4 ohm speakers in series to produce 8 ohms and provide extra volume and dynamic range.
NB. Zimo’s documentation suggests that notwithstanding the ‘8 ohm rule’ for Zimo decoders, the MX645 decoder can also power 4 ohm speakers.
Hornby Twin Track Sound (TTS)
Hornby TTS decoders must be used in conjunction with an 8 ohm speaker load. This can be achieved using a single 8 ohm speaker, such as our popular Optimax bass reflex speaker, or by wiring two 4 ohm speakers in series, such as our 23mm square mega bass mini, which gives a combined impedance of 8 ohms.
You can download a PDF version of this How-to guide here:
Whether your sound box is injection moulded, made of resin or 3D printed, sound box installation is basically the same…and it’s very simple!
You’ll need some glue (cyanoacrylate/superglue) or solvent cement, a few toothpicks, a hobby knife and some tweezers.
The sound box should be fitted on the diaphragm side of the speaker. Depending on the make/model of sugar cube speaker, this may be open to the touch or it may be covered by a thin layer of protective black material and/or surrounded by a thin white gasket.
We recommend that, where fitted, the white gasket surrounding the diaphragm, along with any black covering material, is carefully removed prior to fixing the sound box. This will allow for a better seal between the sound box and the speaker.
When you have the speaker prepared, apply a small but continuous bead of glue or solvent cement around the narrow lip of the sound box. It’s best using a cocktail stick to do this at it enables accurate application. Be careful not to get any glue or cement onto the diaphragm or surround.
Offer up the speaker with diaphragm facing into the sound box and press the two parts together, holding them firmly until the glue has dried or the cement cured. Follow up by applying a thin bead of glue or cement (again, using a cocktail stick) around the outside of the join between the gasket frame and the basket.
You’re now ready to solder the speaker to the sound decoder and install it in the locomotive. We always recommend that a tiny amount of liquid flux is used on solder tabs and pre-tinned wires to ensure that good solder joints are achieved.
NB. Due to the nature of manufacturing process, when using a 3D printed sound box, occasionally printed parts may have small air gaps between some layers. Brushing a thin coat of solvent cement across the inner surfaces of the sound box prior to installation should seal any such gaps. You may also be able to use PVA glue to the same effect, but it may take longer to dry.