Dual Speakers

[rbdamon]

I was trying to see if I could squeeze out just a little more prime mover sound out of one of my engines by installing dual speakers. I added a second iphone 4 speaker and it seemed to crackle a little bit and not give much of any more sound per say. Turning down the volume did not seem to correct it. I tried wires both ways just to make sure and the results were the same. Am I missing something or will these speakers not work in pairs?

[KPack]

It sounds like the second speaker may be bad.  It's not common but I have seen it happen a few times, particularly with these cheaper speakers.  My guess is that the speakers you have are not Apple OEM product, but rather a third party brand that fits Apple products, and may or may not be up to spec or quality control.  I've had a couple sugar cube speakers that were obviously of lower quality than the others that came in the same batch.

Those (or any identical) speakers should work just fine in pairs.  I tend to run mine in series rather than parallel because I'm not trying to blow eardrums.  Plus it is gentler on the speaker.

-Kevin

[rbdamon]

Thanks Kevin

The speakers came from Bill at PDC. I was wiring them in parallel and I had tried a different second speaker with the same result. A single speaker sounds fine. I will try messing with it again over the weekend. Perhaps I'll wire them in series and see what happens as well.

Ben

[Josephunh]

If you are trying to install two speakers in an engine you should really be using a crossover circuit to drive them both.

https://en.wikipedia.org/wiki/Audio_crossover

[CPRail]

Ever since Rapido put 2 speakers in the DASH 8-40CM, I've been debating how to do this, especially in my Big Alcos. One iPhone4 speaker just doesn't give enough bark!

Are there audio crossover circuits that are small enough to install in a Hood diesel and where does one acquire these?

Ian

[Josephunh]

There in lies the problem as not sure anyone makes one that small.

[Alan]

Crossovers are for dividing the frequencies of a signal into various ranges, typically low (woofer), mid (midrange or squawker), and high (tweeter). You are not trying to separate the frequencies with your dual speaker installation. You want both speakers to reproduce the entire frequency range.

Two speakers can be wired to one output with either series or parallel hookup. Each wiring arrangement presents a different load on the amplifier output. Series wiring sums the impedance of both speakers and so reduces the output of the amplifier. Parallel wiring cuts the impedance in half so it doubles the output of the amplifier. Unfortunately, the amplifier has only so much to give. If the impedance of the load is too low then excessive current flow will damage the amplifier. Therefore, parallel wiring of speakers can be dangerous for the amp.

Series wiring speakers will decrease the overall volume even though there are more speakers because of the higher net impedance. Parallel wiring speakers will increase the volume providing the resulting impedance is above the amplifier minimum. I suspect this is where you are running into trouble. The two iPhone speakers in parallel are quite likely presenting insufficient impedance to the amplifier resulting in excessive current in the output transistors which causes distortion which is the crackling you hear.

[KPack]

Thanks Alan for the explanation.  The Railpro amp is capable of outputting 2 watts of power (if I remember Tim Ring correctly), which is possible if you hook up two 8 ohm speakers in parallel.  The speakers I use are normally only rated for 1 watt max power, so when using multiple speakers (which I always do) I wire in series so the amp is only putting out 0.5 watts of power.  With this arrangement I am minimizing damage to both the amplifier and speakers.  Plus it is PLENTY loud.  Model railroad sound should be pleasant to listen to and add to the experience, not give you a migraine.

-Kevin