October 18, 2010

Thinking about sound decoders

Having extensively dissected my Haupt locomotive this weekend I am now much more familiar with the internal wiring. It is really quite simple. To install a decoder will be a simple matter of swapping out the existing harness but leaving the chassis connections intact.

I think the optimal solution for sound is to install the decoder in the boiler with one speaker. Potential speaker locations include the stack or the fire box. The info sheet at this web site suggests not venting the speaker through the stack, as upward sound propagation of front wave  does work as well.  So the firebox location might be better. It is also a large internal volume allowing a bigger speaker. This tech note is quite interesting and worth reading.

The internal wiring in the boiler  is easy to reroute.  Currently there is one 2-pin plug from the tender to the engine. I would replace this with a 4-pin plug to carry track power to the decoder and then motor power back to the tender for the motor. But I have some questions that perhaps a reader can help answer.

1. What decoder to use? The boiler tube on the Haupt measures 0.96 inches ID. The QSI decoder Rev-U decoder measures 2.13 by 0.69, so it should fit. The current rating for the QSI Rev-U is 2+ amps stall and 1.3 Amp continuous. What is not clear to me from the QSI website is how the sound will be. They don't seem to have as extensive a variety as Soundtraxx.

2.  Dave Schneider at SMR advises against the Soundtrax decoders because they might not handle the current rating, but Mitch Oldham did a test on my McCallum and it showed  stall current less than that. Will current draw be an issue if I go with Soundtraxx? The light steam Soundtraxx decoder would be a good fit.

In reading the Easy DCC manual. I learned that you can control the various sounds from the decoder. So having inappropriate sounds such as an air pump or steam blower is not a big concern, as you can turn them off with the appropriate Control Variable (CV).

3. Connector plugs?  Miniatronics  makes a nice 4 pin plug,  (Photo at the right) but their website says the current limit is only 1 Amp. These appear to be the same type of plugs that SMR uses on their locos, so I wonder if that will be a problem.

4. A 0.75 inch speaker will fit in the boiler, while a small oval speaker might work in the firebox, which is essentially a empty box about 0.75 inches wide and 1.7 inches tall. The round speaker would make using the boiler as a resonant cavity easier. According to the tech report mentioned above, a bigger resonant cavity is better and brass boiler tubes make could cavities as they have still walls, but don't absorb sound energy like die cast metal. Plastic resonant cavities are less effective as they vibrate too much.On the other hand, folks are reporting good results with QSI High Bass  aluminum cone speakers, but the smallest is 1.06 inches in diameter and too big for the boiler, but it might fit in the fire box if it fired sideways.

I have been ignoring the  DCC   technology for the past couple years while I focused on the technology of the 19th century, so I am a bit out of date. There sure are a lot of new products to consider. I don't have a lot of experience in doing this, so I welcome your comments. 

Whew, all this electronics talk has me exhausted. I think I'll go build a structure or something.

1 comment:

  1. I recently put a Soundtraxx Tsunami decoder in an HO scale PFM 2-8-4. Use as big a speaker as you can to capture the bass...the firebox is your friend!

    When it comes to the decoder, why not use two? I put a motor decoder in the boiler of my steamer and just used the sound functions of the Tsunami. It's true that there are some load-compensated sounds that you'll lose with this method, but it mitigates the concern of frying a $200 sound decoder by using a $25 motor decoder to handle the big current. I had to do some work to isolate the decoders, but in the end it was worth it.

    Since you're working in O scale, this may not be a problem, but I had difficulty with the Miniatronics 2-pin connector binding on curves and derailing the tender. I wound up using the Soundtraxx micro connectors and was very pleased. Litchfield Station sells some VERY flexible 51-strand 29 AWG wire that helps with these connections.

    Hope this helps!!