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Soundtraxx DSX diag ...successful rebuild.

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Soundtraxx DSX diag ...successful rebuild.
Posted by Otis on Wednesday, April 30, 2008 10:37 PM

At the risk of beating this to death, but if anyone else ever has to use this circuit published on the Soundtraxx website....It is wrong!

 

At least, if the section showing "center tap unused" refers to the third wire coming from one side of the transformer, between two others, then the diagram is backwards.  The unused center tap on the recommended Radio Shack transformer is on the input side....not the output side as shown.

When I finally double checked the back of the transformer packaging to see why the darned circuit wouldn't work, I saw the Soundtraxx error.  I should have looked first, rather than connect the transformer up backwards for a week!

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Posted by UpNorth on Thursday, May 1, 2008 3:43 PM
  • Frequency response: 300Hz to 10kHz
  • 1,000-ohm center-tapped primary
  • 8-ohm secondary
  • Low-level impedance matching

I'm confused. The info above is taken from the Radio Shack site.

 

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Posted by Texas Zepher on Thursday, May 1, 2008 6:25 PM

Not a beating dead-horse issue at all.  I find it interesting when something isn't working as it should.

 Otis wrote:
The unused center tap on the recommended Radio Shack transformer is on the input side....not the output side as shown.
The "input" and "output" of a transformer is a relative thing.  The primary widing is the one next to the core and the secondary widing is the one on the "outside".  Either can be used as the input.   I think this transformer is normally used for an amplifier so that the 8 ohm side is "output" to a speaker.  Obviously in this application it is being used AS the speaker so it is the "input".

Or do you just mean that physically the center tap of the 1K side is on the same side as the 8 ohm leads?

Or do you mean that you have now wired it as a step up transformer with the 1K coil connected to the DSX output? 

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Posted by jbinkley60 on Thursday, May 1, 2008 9:27 PM
 Texas Zepher wrote:

Not a beating dead-horse issue at all.  I find it interesting when something isn't working as it should.

 Otis wrote:
The unused center tap on the recommended Radio Shack transformer is on the input side....not the output side as shown.
The "input" and "output" of a transformer is a relative thing.  The primary widing is the one next to the core and the secondary widing is the one on the "outside".  Either can be used as the input.   I think this transformer is normally used for an amplifier so that the 8 ohm side is "output" to a speaker.  Obviously in this application it is being used AS the speaker so it is the "input".

Or do you just mean that physically the center tap of the 1K side is on the same side as the 8 ohm leads?

Or do you mean that you have now wired it as a step up transformer with the 1K coil connected to the DSX output? 

The transformer in question is an output transformer meant to take the output of a high impedance amp and allow it to drive a low impedance (8 ohm) load.  These are typical of older transistor and tube type amplifiers where it was easier to amplify voltage than current.  However, the usage here is to take the low impedance output of the Soundtrax decoder to drive a high impedance input to the amplifier (i.e. an input transformer for the amplifier).  The circuit is correct, so is the transformer.  The transformer was just hooked up backwards.  I think the assumption was that input and output markings on the transformer were correct.  They are correct for their function but this type of application uses the functions in their opposite fashion.

  

Engineer Jeff NS Nut
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Posted by UpNorth on Thursday, May 1, 2008 9:54 PM

Here is the link that I had found regarding wiring the 8ohm/1K output transformer. The recommendation was to use the center tap i just noticed.

http://www.rcsys.com/Downloads/xfmrcpl.txt

 

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Posted by Texas Zepher on Friday, May 2, 2008 1:22 PM
 UpNorth wrote:
Here is the link that I had found regarding wiring the 8ohm/1K output transformer. The recommendation was to use the center tap i just noticed.
Now that is interesting.  Using the center tap to make a push-or-pull output depending if one uses the "left" or "right" side of the stereo jack.   The signal amplitude in this case would be 1/2, but the original circut was cutting the amplitude anyway with the variable resistor.
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Posted by Otis on Friday, May 2, 2008 4:54 PM

Ok, at first I thought I should throw myself on my own sword for being too thick to understand electronics.  Or at least look for a smiley that shows the topic whizzing over my head.  Then I looked again at the info on the back of the transformer pack from Radio Shack and the Soundtraxx diagram.

Notice anything? 

 

 

Red and white wires are shown as the 8 ohm side by Radio Shack.  Soundtraxx shows the 8 ohm side connected to the decoder.  Weeeeelllll. The circuit only works if the red and white wires are connected as output to the stereo.   You tell me what is going on.  I think it means the Soundtraxx circuit is wrong.  If red and white are 8 ohm side, they are on the wrong side of the Soundtraxx diagram.

Vice versa of course.  The circuit only works with the blue and green wires connected to the decoder speaker leads.  Believe me.  It is connected to my stereo line in right now like that with a loco rumbling down the tracks through my subwoofers.

Here is a snap of my test setup just as I described.  If I wire it as shown in the Soundtraxx diag. I get, as one forum member noted in a previous post, a probably reduction of the output by 7.9 ohms leaving no volume whatsoever.

Because the circuit works in reverse to what is shown by Soundtraxx, one of the two diagrams above....the Soundtraxx one or the Radio Shack one....must be wrong.  But I am no electronics expert.  I only know what is actually happening.

 

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Posted by Vail and Southwestern RR on Friday, May 2, 2008 7:04 PM
The thing that puzzles me is that the Soundtraxx documention, in more that one place, is clear that the decoder wants to drive an 8 Ohm load.  And I would have thought that the line input on the stereo would have been a high impedence input.  So, I grow confused.  I'm interested in this, as I have often thought that this would be how I get sound on my N scale layout.

Jeff But it's a dry heat!

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Posted by Texas Zepher on Saturday, May 3, 2008 12:56 AM

Curiouser, and curiouser.  

 Otis wrote:
I think it means the Soundtraxx circuit is wrong.  If red and white are 8 ohm side, they are on the wrong side of the Soundtraxx diagram.
How it is drawn left/right on the diagram is irrelevant, just turn the Radio Shack package upside down and then it matches the soundtrax diagram.  Electrical theory says the red and white wires (8 ohm side) should be connected to the output of the sound processor (in this case the DSX).  

The circuit only works with the blue and green wires connected to the decoder speaker leads.
working as a step up transformer.   Nothing says that shouldn't work that way.  It certianly is less work for the DSX to have the higher impedance on its output.   Now I'm going to have to go to Radio Shake and get one for myself to figure out why it isn't working the other way.

It is connected to my stereo line in right now like that with a loco rumbling down the tracks through my subwoofers.
If it is sounding through a sub-woofer then something else is wrong, because the DSX doesn't produce any audio-output signals in the sub-woofer range.

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Posted by Otis on Saturday, May 3, 2008 1:40 AM
 Texas Zepher wrote:

Curiouser, and curiouser.  

 Otis wrote:
I think it means the Soundtraxx circuit is wrong.  If red and white are 8 ohm side, they are on the wrong side of the Soundtraxx diagram.
How it is drawn left/right on the diagram is irrelevant, just turn the Radio Shack package upside down and then it matches the soundtrax diagram.  Electrical theory says the red and white wires (8 ohm side) should be connected to the output of the sound processor (in this case the DSX).  

The circuit only works with the blue and green wires connected to the decoder speaker leads.
working as a step up transformer.   Nothing says that shouldn't work that way.  It certianly is less work for the DSX to have the higher impedance on its output.   Now I'm going to have to go to Radio Shake and get one for myself to figure out why it isn't working the other way.

It is connected to my stereo line in right now like that with a loco rumbling down the tracks through my subwoofers.
If it is sounding through a sub-woofer then something else is wrong, because the DSX doesn't produce any audio-output signals in the sub-woofer range.

Well, I certainly bow to greater wisdom in this.  I am just glad I got it to work.

I do see your point...kind of like there is no up and down/left and right in outer space.

Texas, the reason it is playing through my subwoofers is that I use a Paradigm active crossover.  I can dial in a big frequency range to bring them over to the subwoofers.  The frequencies are not subwoofer range, but pushing the sound through twinned 15" drivers really gives the room a "trackside" feeling.

A question for you:  another poster on another occasion said that the purpose of the capacitor is merely to take out the low frequencies so that the DSX will play normally through "smallish" speakers.  What do you think?  Should I take it out and give it a try?

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Posted by jbinkley60 on Saturday, May 3, 2008 6:10 AM
 Otis wrote:
 Texas Zepher wrote:

Curiouser, and curiouser.  

 Otis wrote:
I think it means the Soundtraxx circuit is wrong.  If red and white are 8 ohm side, they are on the wrong side of the Soundtraxx diagram.
How it is drawn left/right on the diagram is irrelevant, just turn the Radio Shack package upside down and then it matches the soundtrax diagram.  Electrical theory says the red and white wires (8 ohm side) should be connected to the output of the sound processor (in this case the DSX).  

The circuit only works with the blue and green wires connected to the decoder speaker leads.
working as a step up transformer.   Nothing says that shouldn't work that way.  It certianly is less work for the DSX to have the higher impedance on its output.   Now I'm going to have to go to Radio Shake and get one for myself to figure out why it isn't working the other way.

It is connected to my stereo line in right now like that with a loco rumbling down the tracks through my subwoofers.
If it is sounding through a sub-woofer then something else is wrong, because the DSX doesn't produce any audio-output signals in the sub-woofer range.

Well, I certainly bow to greater wisdom in this.  I am just glad I got it to work.

I do see your point...kind of like there is no up and down/left and right in outer space.

Texas, the reason it is playing through my subwoofers is that I use a Paradigm active crossover.  I can dial in a big frequency range to bring them over to the subwoofers.  The frequencies are not subwoofer range, but pushing the sound through twinned 15" drivers really gives the room a "trackside" feeling.

A question for you:  another poster on another occasion said that the purpose of the capacitor is merely to take out the low frequencies so that the DSX will play normally through "smallish" speakers.  What do you think?  Should I take it out and give it a try?

The biploar cap will act as a high pass filter and will reduce the amplitude of lower frequency signals.  Another reason it is there is to block DC offset from the out put of the DSX amplifier because a continuous DC offset can cause the speaker voice coils to burn out.  Your transformer will take care of any DC offset issues.  I'd remove the capacitor.  You will get back some low frequency response.  I am not sure how much of it you will notice but it won't hurt anything to remove it.  If the DSX output has no audio in the low frequecy range then you will likely hear no difference.

 As an FYI, the cutoff frequency for a first order high pass filter like this (-3db power point) for a 330uf capacitor with an 8 ohm load is 379hz. 

 

Engineer Jeff NS Nut
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Posted by Texas Zepher on Saturday, May 3, 2008 10:28 AM
 Otis wrote:
A question for you:  another poster on another occasion said that the purpose of the capacitor is merely to take out the low frequencies so that the DSX will play normally through "smallish" speakers.  What do you think?  Should I take it out and give it a try?
I think I was trusting what Soundtraxx told me in an e-mail years ago when I asked the same question.  Their response was something like the capacitor was necessary to prevent damage to the decoder.   Never really made sense to me as a capacitor used as such is a low frequency filter (normal crossover theory).  I now am wondering if the person sending the e-mail just had one of those brain says x hands type y moments and meant to say to prevent damage to the SPEAKER....???   Anyway as I write this I had just stopped to check messages before I was off to the RS to get a transformer to see if I can figure all this out and more importantly WHY it is working the way it is.  I'll include "capacitor" experiments now too.
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Posted by jbinkley60 on Saturday, May 3, 2008 2:31 PM

 Texas Zepher wrote:
 Otis wrote:
A question for you:  another poster on another occasion said that the purpose of the capacitor is merely to take out the low frequencies so that the DSX will play normally through "smallish" speakers.  What do you think?  Should I take it out and give it a try?
I think I was trusting what Soundtraxx told me in an e-mail years ago when I asked the same question.  Their response was something like the capacitor was necessary to prevent damage to the decoder.   Never really made sense to me as a capacitor used as such is a low frequency filter (normal crossover theory).  I now am wondering if the person sending the e-mail just had one of those brain says x hands type y moments and meant to say to prevent damage to the SPEAKER....???   Anyway as I write this I had just stopped to check messages before I was off to the RS to get a transformer to see if I can figure all this out and more importantly WHY it is working the way it is.  I'll include "capacitor" experiments now too.

I think you are quite correct.  I see no way a speakker could damage the sound decoder.  Now perhaps they were worried someone could connect the output to another input that also had a DC offset and that the offset could be fed back into the sound decoder and do it harm.  I am pretty confident that the capacitor is to block DC getting to the speaker.  With this type of power input we don't have a differential voltage supply powering the amplifier output stage (i.e. +V and -V) with a defined neutral so the amplifier output stage will need to compensate for this condition. 

Older tube and transistor amplifiers compensated for this with a large output transformera (in order to pass the low freqnencies and block the DC).  The problem was that they had to be large enough to pass the pwoer to the speaker and thus tended to be large and bulky.  They also had the problem of high freqency cutoff due to transformer losses at higher frequencies.  As bipolar capacitors became more popular they were used as output coupling devices, where extreme low frequency was not of great importance. 

 

Engineer Jeff NS Nut
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Posted by Otis on Tuesday, May 6, 2008 4:32 PM

When all else fails, start over.

LAST NIGHT'S RETEST..SAME OLD SAME OLD 

I retried the connections as illustrated by Soundtraxx last night.  The decoder heated up too hot and the sound was not present.  Just as before.

I switched the connections back to the "backwards" setup shown in my earlier photo and the sound plays very nicely through my stereo card and thence to my stereo. As before.  However, I do not want to have to play the DSX through my sound card.

TODAY'S REBUILD...SUCCESS! 

So, I broke open a fresh pack of speaker wires. Opened a second new Radio Shack transformer.  Took our another DSX. Connected, cleaned and tested the RCA plugs on route to the stereo AUX line in. Stripped every non-essential out of the circuit....the pot, the capacitor.  (In the photo below, the test speaker and the center black wire of the transformer are present, but not connected to anything.)

You guessed it, Texas Zephyr and Vail, it runs as designed now, thundering through the stereo and two sets of speakers, with the transformer wired as it should be.

Some component was haywire.  One day I may try to find which one, but as I changed several at once, it would take me some testing.

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Posted by UpNorth on Tuesday, May 6, 2008 9:12 PM
It was fun working with you on this...
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Posted by Vail and Southwestern RR on Tuesday, May 6, 2008 9:25 PM
My guess would be that the first transformer was mislabelled, or just wrong.  That's the only thing that makes sense to me.

Jeff But it's a dry heat!

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Posted by Otis on Wednesday, May 7, 2008 1:45 AM

 UpNorth wrote:
It was fun working with you on this...

My thanks to everyone who took an interest in this---From Hosers to Ohioans and a few others who chipped/chirped in on earlier threads. 

I've got all the parts to make this "in the cab" synchronized sound system really do a job.  Picked up and old Aiwa stereo amp from the junk shop on Saturday for $15 too.  Sounds great with the DSX sound files --- beauty eh?

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