Author Topic: A+B units: one decoder or two decoders?  (Read 1704 times)

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rva1945

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A+B units: one decoder or two decoders?
« on: October 18, 2016, 11:42:00 AM »
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A+B units: one decoder or two decoders?

Hi:

I have a FT-A already running on DCC and just received the analog FT-B. My plan is to take a couple of wires from the A's motor and connect the B in parallel, so the decoder will drive both, providing they are an electrical and mechanical match.

Now the BEMF topic. Should I turn it off?

And in the long run: will their performances deviate? Will I need a second decoder for the B unit? Can both decoders have the same address so they will respod to the same DCC command?

Thanks


C855B

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Re: A+B units: one decoder or two decoders?
« Reply #1 on: October 18, 2016, 12:03:40 PM »
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Yes, turn off BEMF. It will be confused by the non-synchronized commutator feedback of two motors.

Other than BEMF, I don't see any problem doing this with permanently-coupled units. The Bachmann DDA40X, for instance, has two motors fed by a single decoder.
...mike

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jdcolombo

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Re: A+B units: one decoder or two decoders?
« Reply #2 on: October 18, 2016, 02:15:10 PM »
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The only issue in connecting two permanently-coupled units like this would be the overall current draw on the decoder.

Most modern N-scale locomotives I've tested don't draw more than about a third of an amp at stall; most N-scale motor-only decoders (or sound decoders) are rated for at least .75 amps, so two motors off one decoder should be well within the decoder's current draw, assuming that you are using LED's for lighting (most LED's draw at most 20ma, so even if you were running a headlight, ditch lights, and running lights, that's no more than 80ma, which should still keep you under the 750ma load rating of some N-scale decoders).

To really be sure about this, though, the best thing to do would be to test the stall current of the two units you want to wire together, and add 20ma for each light that will be on at the same time.  If the sum of all that is less than 750ma, you're good to go.  If it's higher, then you can look for a 1-amp N scale decoder (they are available - for example, the TCS M-series is rated at 1 amp; so is the DZ126 from Digitrax).  I suspect you will be fine with pretty much any N-scale decoder.

John C.

rva1945

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Re: A+B units: one decoder or two decoders?
« Reply #3 on: October 18, 2016, 02:37:34 PM »
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My trains are HO and as far as I know my decoders are rated as 1.5A max. I think that as far as the current doesn't exceed 1A, I'm in the comfort zone.

jdcolombo

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Re: A+B units: one decoder or two decoders?
« Reply #4 on: October 18, 2016, 03:50:20 PM »
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Yes, you're fine.  Modern HO scale motors actually don't pull much more current than N scale ones.  The 1.5-amp load rating should be more than enough.

John C.

robert3985

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Re: A+B units: one decoder or two decoders?
« Reply #5 on: October 21, 2016, 08:18:07 PM »
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Although your plan will work just fine functionally to run two HO engines from one decoder here's something to think about.

First considering the low cost of non-sound decoders nowadays... why?  Would seem to me to be advantageous from a speed matching aspect, as well as enabling BEMF in your lashup, to install decoders in BOTH engines.

If you're installing a sound decoder in your A-unit, my experience is that a non-sound decoder (from the same manufacturer) works just fine in the B-unit.  But, for the most realistic sound, sound decoders in both is the way to go, and would be even more necessary in HO scale, since the engines are twice as far apart as they are in N-scale.

Of interest is that also in my experience, an A-B-A lashup sounds very good with sound decoders in only the A-units, which will save quite a bit of cash.  Even an A-B-B-A lashup is okay with sound decoders in only the A-units...but this is in N-scale.  However, I suspect that because of the increase in length, your ears are not going to be entirely happy with sound only in the lead unit in HO scale.

Cheerio!
Bob Gilmore


central.vermont

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Re: A+B units: one decoder or two decoders?
« Reply #6 on: October 21, 2016, 08:46:52 PM »
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Not to muddy up the waters here but I have a similar question. I have an ABA set of IM FP9's and was thinking of using one sound decoder but was wondering if you can put more that one speaker in. Such as one in each unit.

Jon

robert3985

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Re: A+B units: one decoder or two decoders?
« Reply #7 on: October 21, 2016, 08:59:28 PM »
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Not to muddy up the waters here but I have a similar question. I have an ABA set of IM FP9's and was thinking of using one sound decoder but was wondering if you can put more that one speaker in. Such as one in each unit.

Jon


I'm sure you can.  However you would probably get nearly the same effect by putting the sound decoder in the B-unit, and non-sound decoders in the two A's.  That said, the reason separate sound decoders sound great (rather than merely good) is because of the distinct and separate noises coming from each engine, such as the sound of the prime movers being out-of-sinc with each other or the air-pop-offs (whatever they're called) popping at different times separately in each locomotive.  I notice this a lot in my A-B-A EMD E-9 COLA lashup in N-scale, which has sound decoders in both A-units, but not in the B.

With what you are thinking, the identical sound would be coming from every engine.

If you decide to do this, I'd suggest one speaker in the lead A-unit along with the decoder, another speaker in the trailing A-unit and no speaker in the B.

However, if you want to afford it, sound decoders in both A's is the way to go IMO and is a lot less complicated than all that wire running and making up all those connectors...then hiding them.

Cheerio!
Bob Gilmore

rva1945

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Re: A+B units: one decoder or two decoders?
« Reply #8 on: October 21, 2016, 09:44:07 PM »
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Well here they are working as a team. I had to ada a diode to the A unit to match speed with the B unit:

https://www.youtube.com/watch?v=qErwoi8ehXI&feature=youtu.be

Decoders are not that cheap here in Argentina...nor they are easy to find.

robert3985

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Re: A+B units: one decoder or two decoders?
« Reply #9 on: October 21, 2016, 09:55:08 PM »
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Well here they are working as a team. I had to ada a diode to the A unit to match speed with the B unit:

https://www.youtube.com/watch?v=qErwoi8ehXI&feature=youtu.be

Decoders are not that cheap here in Argentina...nor they are easy to find.

Well, that 'splains that...!   :facepalm:

Cheerio!
Bob Gilmore



central.vermont

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Re: A+B units: one decoder or two decoders?
« Reply #10 on: October 22, 2016, 09:43:03 AM »
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I'm sure you can.  However you would probably get nearly the same effect by putting the sound decoder in the B-unit, and non-sound decoders in the two A's.  That said, the reason separate sound decoders sound great (rather than merely good) is because of the distinct and separate noises coming from each engine, such as the sound of the prime movers being out-of-sinc with each other or the air-pop-offs (whatever they're called) popping at different times separately in each locomotive.  I notice this a lot in my A-B-A EMD E-9 COLA lashup in N-scale, which has sound decoders in both A-units, but not in the B.

With what you are thinking, the identical sound would be coming from every engine.

If you decide to do this, I'd suggest one speaker in the lead A-unit along with the decoder, another speaker in the trailing A-unit and no speaker in the B.

However, if you want to afford it, sound decoders in both A's is the way to go IMO and is a lot less complicated than all that wire running and making up all those connectors...then hiding them.

Cheerio!
Bob Gilmore

Thanks for your input Bob!
That's what I was thinking of doing. Putting a sound decoder in the B unit and a speaker in each A unit. My ABA set is already draw-bared together and common wired together for all wheel pickup so a couple extra wires should not be a problem.

Jon

peteski

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Re: A+B units: one decoder or two decoders?
« Reply #11 on: October 22, 2016, 10:01:19 AM »
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Jon,
if you were to run multiple speakers from a single speaker ouutput in a decoder then you have to consider the following:

If you hook the speakers up in parallel their total impedance will be  lower than a single speaker.   If you hook two 8 ohm speakers in parallel then their impedance will be 4 ohms. Three will have 2.6 ohm impedance.  You might overload the output stage of the audio amplifier on the decoder. If the amp is very robust it might still work but the sound might get distorted. Or you can actually blow out the output transistors once the volume is cranked up (especially with 3 speakers).  But if you find speakers which have higher impedance than what the amp is rated for then you will be ok. For example if the amp is rated for 8 ohms and you find some 16 ohm speakers then two in parallle will have 8 ohm impedance - perfect match for the amp.

If you hook the speaker up in series the impedance will be a sum of all the speaker impedances.  If an am is rated for 8 orms and you hook up two 8 ohm speakers in series then they will present a 16 ohm load to the amp. That won't damage anythign but the amp will only be able to push half of its rated wattage to the speakers, so they will not be as loud.  As in above example, you can choose the speaker impedance to match the amp. If you hook up two 4 ohm speakers in series then their total impedance will be 8 ohm - perfect match for the ampl.

The problem with the multi-speaker setup is finding speakers with the impedance needed for them to have a matched impedance with the amp.

But just for fun, if you have an amp rated for 8 ohms, you can get four 8 ohm speakers and hook 2 pairs in series (so each is 16 ohms), then those 2 pair sets in parallel. That results in a total impedance of 8 ohms, so you're good. Just make sure to hook them up so their polarity is not reversed, so the cones all move in the same direction.  Polarity is important in any multi-speaker setup.

Another alternative is to get the QSI Titan sound decoder. It is a stereo sound decoder (it has 2 audio amps and stereo sound recordings).  It is designed for having 2 speakers.  It also has a mixer so you can balance each type of sound between those speakers.  It is a rather large (H0) decoder but you might be able to shoehorn it in a full width cab diesel.
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GhengisKong

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Re: A+B units: one decoder or two decoders?
« Reply #12 on: April 11, 2018, 04:43:07 PM »
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Well here they are working as a team. I had to ada a diode to the A unit to match speed with the B unit:

https://www.youtube.com/watch?v=qErwoi8ehXI&feature=youtu.be

Decoders are not that cheap here in Argentina...nor they are easy to find.

What diode did you use and how did you calculate to find the correct one?

Lemosteam

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Re: A+B units: one decoder or two decoders?
« Reply #13 on: April 12, 2018, 08:36:28 AM »
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Jon,
if you were to run multiple speakers from a single speaker ouutput in a decoder then you have to consider the following:

If you hook the speakers up in parallel their total impedance will be  lower than a single speaker.   If you hook two 8 ohm speakers in parallel then their impedance will be 4 ohms. Three will have 2.6 ohm impedance.  You might overload the output stage of the audio amplifier on the decoder. If the amp is very robust it might still work but the sound might get distorted. Or you can actually blow out the output transistors once the volume is cranked up (especially with 3 speakers).  But if you find speakers which have higher impedance than what the amp is rated for then you will be ok. For example if the amp is rated for 8 ohms and you find some 16 ohm speakers then two in parallle will have 8 ohm impedance - perfect match for the amp.

If you hook the speaker up in series the impedance will be a sum of all the speaker impedances.  If an am is rated for 8 orms and you hook up two 8 ohm speakers in series then they will present a 16 ohm load to the amp. That won't damage anythign but the amp will only be able to push half of its rated wattage to the speakers, so they will not be as loud.  As in above example, you can choose the speaker impedance to match the amp. If you hook up two 4 ohm speakers in series then their total impedance will be 8 ohm - perfect match for the ampl.

The problem with the multi-speaker setup is finding speakers with the impedance needed for them to have a matched impedance with the amp.

But just for fun, if you have an amp rated for 8 ohms, you can get four 8 ohm speakers and hook 2 pairs in series (so each is 16 ohms), then those 2 pair sets in parallel. That results in a total impedance of 8 ohms, so you're good. Just make sure to hook them up so their polarity is not reversed, so the cones all move in the same direction.  Polarity is important in any multi-speaker setup.

Another alternative is to get the QSI Titan sound decoder. It is a stereo sound decoder (it has 2 audio amps and stereo sound recordings).  It is designed for having 2 speakers.  It also has a mixer so you can balance each type of sound between those speakers.  It is a rather large (H0) decoder but you might be able to shoehorn it in a full width cab diesel.

So back to the question of having duplicate sound in each unit- would there be a way to delay each speaker to appear as though each unit sounds as though it has its own sound decoder?  Almost like an echo.

Jamesn320

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Re: A+B units: one decoder or two decoders?
« Reply #14 on: April 12, 2018, 10:06:07 AM »
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What diode did you use and how did you calculate to find the correct one?

This is a post I did for my blog a few years ago, it may help with the diode question for running two locos from one decoder.  I've done it many times now.

https://jamestrainparts.com/2014/01/20/cost-effective-dcc-installs-for-consisted-locomotives/

-James