Soundtraxx DSX Digital Sound Decoder Tips

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The Soundtraxx DSX

dsxThe Soundtraxx DSX is a sound only decoder intended to add sound to a loco that already has a DCC decoder to run the motor and other functions. The decoder is a subset of the DSD sound decoder family. The manual that comes with it is for the DSD family. You are left to sort out what applies to this decoder and what does not (which is most of it). The DSX only does sound. There are no motor or function controls.

There are a couple of cautions that must be observed when using the DSX.

First, the speaker leads must be DC blocked. The instruction book indicates that a 100 uF 16 volt capacitor must be inserted in series with the speaker leads. On the DSX, both leads are purple and there is no indication as to which lead the capacitor needs to be inserted in. However, the DSX is delivered with a BI-POLAR capacitor. You MUST use this part but it is not polarity sensitive and can be inserted in either lead and in either direction.

Second, the DSX is limited in the amount of input voltage that it can accept without damage. A 39 ohm resistor is provided for use where the DCC track voltage is greater than 18 volts. For large scale systems, this is ALL of them. Wire the resistor in series with either power input lead. The resistor limits the power that the DSX can receive and therefore limits it temperature. It also limits the audio power but there is a solution for that too.

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DSX Performance

As far as sound quality goes, the DSX is quite good. However, where the DSX hurts most is in sound QUANTITY. The audio output power is very weak compared to other large scale sound systems. The system is rated at 1 watt. The RMS output voltage is only 2 volts or so which means that the actual audio power is about a half watt. This might even be generous as it doesn't sound that loud. Fortunately, this can be fixed with an external audio power amplifier.

I bought the DSX to install in a USA Trains GP9 but I didn't put in for over 3 years because I couldn't find a DCC decoder that would drive the GP9 without overloading until the NCE D808 came out. In the meantime, I put it into an LGB 2060. The installation was unsuccessful for two reasons. First, the speaker was buried too deeply in the loco and I could hardly hear it. Second, I removed the DCC decoder from that loco in favor of a battery R/C installation because the loco just didn't have reliable enough power pickup (not enough wheels) to work well with DCC. The DSX resided in a box for several years until I finally put it into the originally intended GP9. In this case, I mounted the speaker much better and the loco has adequate power pickup. The sound is almost loud enough and the DSX runs steadily without pops or glitches in this loco as long as the tracks and wheels are reasonably clean.

If you use the DSX without an "afterburner", be prepared to mount the speaker FACING UP so that you have the best chance of actually hearing it. Fuel tank mounted speakers don't work very well as the sound is directed into the track and damped before you get a chance to hear it.

The horn affect is better than any other sound system that I have heard, especially as it relates to manually controlled horn control. You can play the horn quite effectively with the F2 button on most DCC throttles. The horn DOES NOT have an automatic mode like the Soundtraxx Sierra so that the system won't automatically play start and stop horn signals.

Since there aren't inputs for external sound functions, the coupler clank effect is also manual. It normally plays on function F3. Dynamic brakes play on F4. The bell is also manual and plays on F1.

The DSX has little internal charge storage so that it is sensitive to power drop outs. Upon loss and recovery of power, it'll pop, reset and go through its startup cycle before hearing more packets from the command station an resuming where it left off. On reasonably clean track, it works fine. On dirty track, you'll want to shut it off.

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Programming The DSX

The DSX has a fair amount of programmability, but there are some strange responses to programming some of the CVs with a Digitrax Chief and a DT-400 throttle.

The relative loudness (or lack thereof) can be set for the horn, bell, dynamic brakes and engine sound. The bell ring rate is adjustable as is the throttle notching response. All of this works in OPS mode on the main so that it is fairly quick to set these up.

Some of the CV's don't respond well to OPS mode or PG mode on the programming track. I haven't figured it all out, but on some of the CV's, you must use DIR mode and other you have to use PG mode. I had a particular problem using PG mode while trying to program CV33. When I did, it would reset the ADDRESS as well. When I reset the address, that value would show up in CV33. When this kind of weirdness happens, try DIR mode for that CV.

Sometimes the DSX will not readback CV on the programming track in ANY mode. It may accept programming, but you may not be able to read back it's contents. In OPS mode, the DSX will usually beep when it gets and accepts a CV change.

Note that if you use the same address for the DSX as the main motor decoder (like I do), then you MUST provide a method to physically turn off the other decoder while doing programming on the programming track. Otherwise, both will try to respond to the programming. If you set the decoders to different addresses and set the DSX decoder as a consist member, then you can program on the main by using OPS mode for the DSX address and then set it back to the old address when you are done.

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Boosting the Audio Power Output Of The DSX

The DSX just doesn't quite do enough audio power to suit me. Since the audio output voltage is so low (5 volts p-p or so) with respect to the available voltage (about 18 volts including the drop in the external series resistor), taking the risk of reducing the series resistor is not worth it. It probably won't help much and there is a risk of toasting the decoder.

Instead, I've added an integrated circuit audio power amplifier (aka an "afterburner") to the DSX. I could have bought a whole new sound system but the cost would have been too high and the DSX worked fine except for it's weak audio power. The afterburner approach seems to be the most reasonable considering that I've already bought the DSX.

The circuit I've selected is the National Semiconductor LM384. This is a 5 watt audio power amp that will work up to 26 VDC and drive an 8 ohm speaker. The circuit is short circuit and thermally protected. The gain is quite high (34 dB) but this can be taken care of with a resistive pad at the input so that the DSX volume controls can still be used over their full 4 bit range. The part is also cheap, listed at $0.66. The NTE862 is a replacement part for this circuit.

5 watts is plenty for a large scale loco, especially considering that I've used a 6 watt speaker. There is also a smaller version of this circuit, the LM380 (NTE740a) which is rated at 2.5 watts. These two circuits appear to be fundamentally identical with the LM384 just being an uprated version.

The LM384 is a pretty normal complementary-symmetry output operational amplifier with internally defined feedback to set the gain at 34 dB (50x voltage gain). It will operate from 12 to 26 volts. At the lowest voltage, it will only do a little over a watt of output power. At the 21 volts that I use (rectified from my 22.5 v p-p DCC track power), it does a little more than 5 watts.

lm384 with Staver V7 heatsinkI had some difficulty locating the LM384. This is an older circuit and it is not carried by many retailers, nor are the NTE equivalents. I eventually did find a few on the net. I also located a few pieces of the Staver V7 heatsink that was recommended for this part. There are other integrated amplifiers out there, you may want to select a newer design. I paid $1.89 (Debco) for the circuit and another $1.11 for the heatsink (Metco). The rest of the parts came from my parts stock.

This heatsink is designed to be placed over the circuit and make intimate contact to the three center leads on each side so that the heat can be extracted. If you use this part, be sure that the two tabs on the heatsink are well soldered to the associated leads.

lm384 afterburnerI built up the amplifier per a diagram in the LM384 data sheet and I initially had some trouble. The amp was putting out a fair amount of voltage at 1.07 MHz. This just happens to be the frequency of KNX, a high power AM radio station located about a mile away. The amp also became unstable and oscillated at about 867 kHz after it warmed up. A 0.01 uF capacitor across the input lead fixed both problems. When driven from a sine wave source, the amp would generate 7 VRMS just at clipping from a 22 volt power supply. This works out to 6 watts. The clipping was symmetrical and clean.

The power to the audio amp and the DSX aren't actually wired as shown. I powered the audio amp from the remaining lighting circuit (cab light only) and the DSX from the sound circuit. This is done to aid programming of the DSX. If the audio amp couldn't be disconnected, then it would be difficult to program the DSX on the programming track. As it is, when the DSX needs programming, the Motor, Smoke and Lights switches under the loco are turned off and the only thing getting power is the DSX. When the D808 needs programming, the Sound, Smoke and Lights switches are turned off so that the D808 is the only thing that gets powered.

When I connected the amp to the DSX through a 22:1 voltage divider and powered the amp from a bench supply, it worked fine. The output signal with the DSX running at full volume would just barely clip on some peaks. I anticipated some difficulty when I powered the amp from a bridge rectifier connected to the DCC track power. I was not disappointed. There was a large amount of a high pitched hiss. This is the DCC signal itself.

The LM384 has a differential input and I tried capacitively coupling the DSX output to the LM384 input. This worked better, but there was still some DCC hiss. This is due to the 150K internal resistors that connect to the LM384's internal ground and disturb its ultimate common mode rejection capability.

The DSX has about 9 VDC on each of the purple leads (referenced to a virtual "ground" formed by the external bridge rectifier powering the amplifier) indicating that it's output stage might be an H-bridge configuration. This means that neither speaker lead is "ground." It is designed to drive an isolated load. The solution was to provide it with the isolated load that it desired in the form of a small audio transformer. This was a part that I pulled from one of my older junk boxes. Judging from the vintage of the box, this part had been in there for at least 35 years. It had 500 ohm/8 ohm handwritten on it. This kind of transformer was quite common in the early days of transistor radio design, but newer circuits have made them obsolete. A Radio Shack p/n 273-1380 ($3) appears to be similar and would probably work fine. This part is 1000 ohm center-tapped to 8 ohm so if only half of the primary winding was used, it would be a 500 to 8 ohm transformer.

Since the LM384 has excess gain, I wired the transformer in a step down configuration and the output voltage was just about right. A 2:1 voltage divider provided just enough input to the LM384 such that it was again just barely clipping on the peaks of the DSX output. The 100 ohm resistor on the output of the DSX provides it a primarily resistive load. It might not be required, but I figured that it couldn't hurt.

afterburner installedThe LM384 and its direct support circuitry was built on a small piece of perforated board and installed below the rear fans where it wouldn't interfere with the regulator for the smoke unit. The board is simply stuck to the hood wall with double back foam tape.

dsx support circuits installedThe DSX, the power rectifier for the afterburner and the audio coupling transformer were installed, also with foam tape, on the hood walls near the speaker.

Now this loco has PLENTY of audio volume, so much so that I've had to decrease the DSX audio output to avoid bothering my neighbors.

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© 2002-2009 George Schreyer
Created Nov 10, 2002
Last Updated April 14, 2009