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Wiring Multiple Speakers To A Decoder



Sound and volume. 

 

I won't pretend I am an expert on the science of sound.  There is an awful lot that goes on before the digital signal reaches the speaker.  The factors that affect the quality and volume of the sounds the most are, in no particular order, the quality of the recording and the installation of the speaker. 

 

The quality of the sound is more-or-less what it is.  When you buy a sound decoder, that is really what you are buying; a quality sound recording.  The physical cost of the decoder is mostly the same for any manufacturer.  A circuit board, some silica formed into those intriguing little bricks, and a machine and people to put it together.  While it is no small part of the cost, it is about the same across the board.

 

Yes, it's true.  The technology of decoder can impact this.  Some models have built-in equalizers to tweak the recodring.  Some models are 8-bit, some are 16-bit.  Is 16 better than 8?  It's a higher number... must be, right?

 

And yes, if you are a sound engineer, you could make your own recordings, edit them to perfection, and upload them into one of the decoder models that allow you to "roll your own".  I don't have the time, tools, training or motivation to be successful at this and suspect few of us do.  So we are left to contend with the sound file that is provided by the manufacturer.

 

Irrespective of this or any of the thousand other factors that affect the quality of the sound, at the end of the day, all that matters is if it sounds good enough to you, the owner.  Some folks demand 100% accuracy of the sound for their locos, others just want something that sounds good.  Kind of like all those tiny little detail parts.  Some care if the horn is correct for their prototype, some don't.

 

Everyone wants to be able to hear it.  Period.  If you're like me, hours of toil in factories have affected what I hear.  So, even volume is subjective.

 

That said, there are two ways two ways to affect the volume of an installation irregardless of the decoder and sound file selected.

 

The first is speaker selection and wiring.  The second is speaker installation and baffleing.

 

How To #7 will delve into wiring.  How To #8 will look at the installation.

Speaker Selection

I carry a hundred different kinds of speakers.  So which one is the best?  The one that fits, right?

 

Choosing the best speaker is more about physical size and matching the rating to the decoder than the quality of the speaker.  We are not trying to fine tune a $1000 Hi-Fi stereo system.  We are trying to get as much noise as possible from the smallest source we can find.  Don't get me wrong, quality is important.  Junk is junk and broke is broke.  I am simply saying that at some point, size becomes the limiting factor until the technology over comes that.

 

That said, the best speakers for our applications will have a low frequency response range.  A speaker with a low frequency repsonse range is best for model trains since the locomotive sound are heavy with bass.  The lower the frequency response range, the better job the speaker will do to reproduce it.

 

The physical size of the speaker is pretty straight forward.  Bigger is almost always better.  The larger the surface area of the diaphram, the louder the speaker will be. 

 

One way to increase the surface area if you are limited by space is to use multiple speakers

 

Speakers and the amplifier on a decoder are rated by Watts of Power and Ohms of Impedance.  It is very important to try to match this.  A mis-matched configuration will definitely affect the volume and could fatally damage the decoders amplifier.

 

Watts of Power

 

The Watts of Power rating for current production decoders varies by manufacturer and model.  It is best to review the specs for your appliction to be sure what your decoder is rated at.  Most are rated at 1 Watt.  This will be the basis for our discussion.

 

Matching the wattage of the speaker to amp is important because a mismatch can cause poor sound quality and premature failure of the speaker.

 

In general, a higher the wattage will produce a louder the installation.  That's why the rock stars want more power.  What was it Terrible Ted said... "If it's too loud, you're too old"...

 

The actual value of wattage delivered to the speaker is determined by the rating of the amp, the wiring circuit, and the volume setting for playback. 

 

A 1.0 watt amp will deliver 1.0 watts to a single speaker when the volume is set to 100%.

 

A 1.0 watt amp will deliver 0.5 watts to a single speaker when the volume is set to 50%.

 

Multiple speakers affect this calculation even further.  There are a number of sites on the web that discuss the formula for calculating this.  I like to leverage the knowledge of the web and have found an easy way to do the math.  Read on...

 

Ohms of Impedance

 

The Ohms of Impedance rating for current production decoders varies by manufacturer and model.  Again, it is best to review the specs for your appliction to be sure what your decoder is rated at.  Most are rated at 8 Ohms.  This will be the basis for our discussion.

 

Matching the impedance of the speaker to the amp is important because a mismatch can cause poor volume and premature failure of the amplifier.

 

In general, a lower impedance will produce a louder installation.  But be careful!  This is also the quickest way to smoke the amp.  If the impedance is too low, it will over-drive the amp and cause rapid failure.  The high road here is to be equal to or higher than the actual rating of the amp.

 

The actual value of impedance of the installation is determined by the rating of the speaker and the wiring circuit.

 

A single 8-ohm speaker will create 8-ohms of resistance.

 

Two 8-ohm speakers wired in parallel will create 4-ohms of resistance.

 

Two 8-ohm speakers wired in series will create 16-ohms of resistance.

 

Multiple speakers affect this calculation even further.  There are a number of sites on the web that discuss the formula for calculating this.  As stated previously, I like to leverage the knowledge of the web and have found an easy way to do the math.

 

There is a free excel spreadsheet download available from Duncan Amps that will do the math for you and allow you to experiment with speaker ratings to determine how they will affect the final configuration.

 

While not mathematically perfect, you can also simply measure the impedance of a speaker(s) with a multimeter set to read resistance.  It's close enough for our needs.  This will tell you the ohm's of resistance of the circuit and is a great way to double check your wiring before connecting them to the amp.

 

A Picture Is Worth A Thousand Words

 

They say a picture is worth a thousand words...  Looks like deflation is working hard here 'cause I was a bit windy today.

 

To make things even easier, here are examples of the most common circuits I use... in pictures.

Multiple Speaker Wiring Diagrams

Single Speaker
Amp power 1 watts
Speaker A impedance 8 ohms
Speaker B impedance 0 ohms
Total impedance 8 ohms
Speaker A power 1.00 watts
Speaker B power 0.00 watts
Two Speakers in Series
Amp power 1 watts
Speaker A impedance 8 ohms
Speaker B impedance 8 ohms
Total impedance 16 ohms
Speaker A power 0.50 watts
Speaker B power 0.50 watts
Two Speakers In Parallel
Amp power 1 watts
Speaker A impedance 8 ohms
Speaker B impedance 8 ohms
Total impedance 4 ohms
Speaker A power 0.50 watts
Speaker B power 0.50 watts
Three Speakers In Series/Parallel - A

Amp power 1 watts
Speaker A impedance 8 ohms
Speaker B impedance 8 ohms
Speaker C impedance 8 ohms
Total impedance 12 ohms
Speaker A power 0.17 watts
Speaker B power 0.17 watts
Speaker C power 0.67 watts

Three Speakers In Series/Parallel - B
Amp power 1 watts
Speaker A impedance 8 ohms
Speaker B impedance 8 ohms
Speaker C impedance 8 ohms
Total impedance 5.333333 ohms
Speaker A power 0.17 watts
Speaker B power 0.17 watts
Speaker C power 0.67 watts
Four Speakers In Series/Parallel - B
Amp power 1 watts
Speaker A impedance 8 ohms
Speaker B impedance 8 ohms
Speaker C impedance 8 ohms
Speaker D impedance 8 ohms
Total impedance 8 ohms
Speaker A power 0.25 watts
Speaker B power 0.25 watts
Speaker C power 0.25 watts
Speaker D power 0.25 watts

Multiple Speaker Phasing

The last bit of critical info on this topic is speaker phasing.

 

A speaker is a non-polar device which means it does not have a true positive and negative terminal.  There is no "wrong way" to connect a single speaker.

 

The sound wave generated by multiple speakers is phased however.  The diaphram of the speaker moves up and down to create a sound wave.  For practical purposes of this discussion, when it is up it is in Phase A (+) and when it is down it is in Phase B (-).  They can be discussed as (+) and (-) for simplicity.  The diagrams above simply refer to this as terminal 1 and 2.

 

It is very important to make sure that the speakers are phased properly.  If wired out of phase, the A Phase will cancel the B Phase and you will completely lose the benefit of installing multiple speakers.

 

Generally speaking, all speakers made to function the same way if wired the same way.  If you place two speakers face down on a table, the left terminal can be (+) and the right (-).  If it comes pre-wired, call red (+) and black (-).  It doesn't matter which is (+) or (-) so long as you recognize that they are phased devices and you have observed this phasing when you wired them together.

 

Finally, the wires from the decoder not phased or polarized.  There is no wrong way to connect two speaker wires to two amp output wires... provided you have observed the rules for phase, watts, and impedance.

Using Multiple decoders in 

 

So there it is.  Be sure to download the spreadsheet and pick up a couple extra speakers from SBS4DCC.  Then start experimenting with multiple speaker installs get volume that will blow you away!

 

You can purchase wide variety of SPEAKERS today from my online store

 

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