ESU 58721 LokSound 5 Micro Direct NMRA DCC Sound Decoder - Decoded

Last Updated 231223

ESU 58721 LokSound 5 Micro Direct NMRA DCC Sound Decoder - N Drop-in Board for Post-2016 Atlas Gold, InterMountain SD40-2 and Others

Originally posted on The Railwire: A Forum for Modelers by user peteski, last updated June 25, 2022, 11:01:03 AM.

This information is shared by and republished here with permission of author.


LokSound V5 58721

This V5 decoder is a slightly reworked 73199 V4 decoder.  Most of the circuitry appears to be identical between those 2 decoders.  The outline of the PC board has been modified slightly. Track pickup pads have been deleted at the outside corners, and the inside corners in the inset areas have been made sharper.  The front and back of the circuit board has been made slightly narrower.  Electrically, the power sections (power supplies, motor driver, audio amplifier, voltage regulator, and other ancillary circuits appear to be identical to 73199.  The changed components are: the microcontroller chip, and some circuitry around it, and more AUX outputs have been added).  There are also more LEDs installed on-board.  Not sure exactly how useful those will be in custom installs.  Nice feature is that all function outputs are high-power, and most are accessible on solder pads on the board.

Here is a comparison of 73199 and 58721 showing just how similar they are.




For those who are interested I also drew a partial schematic diagram of the decoder. This decoder's design is different and more complex from other sound decoders I have dealt with in the past.  It has a power supply circuit with 3 voltage stages where a stay-alive caps or keep-alive module could be hooked up.

Stage 1: Raw rectified track voltage (marked "A" or RED on the diagram). This stage supplies power to the motor driver circuit, and to the next voltage stage (described below).  This is where one of the SuperCap-based keep-alive circuits could be connected to keep both, the decoder's electronics, and the motor powered during power dropouts.  This stage includes what looks like a Zener diode (for over-voltage protection?) and a very small ceramic capacitor (probably  few uF in value, to shunt any voltage spikes coming from the track).

Stage 2: (marked "B" or PURPLE on the diagram). The voltage from stage 1 is passed through a diode (same type of diode as used in the rectifier) to become stage 2. This stage has five multilayer ceramic caps used as a small stay-alive circuit (totaling probably less than few hundred uF, but since they are unmarked I don't know their exact value).  The voltage from this stage is then supplied to stage 3, and probably to few other circuits on the decoder (I didn't do a thorough trace to check what else is powered from this stage).  As shown on the diagram, since it passes through a diode, the voltage in this stage is just few tenths lower than the raw rectified voltage of stage 1.

Stage 3: (marked as "C" or BLUE on the diagram). Voltage from stage 2 is supplied to a 5.3V regulator which produces the stage 3 voltage.  There are two 100uF tantalum caps in this stage to act as a filter/keep-alive. This stage supplies power to most of the decoder's circuitry, including the audio amplifier.  There are also couple more voltages derived from 5.4V. One is 5.1V (not sure where it is used) and also 3V, which powers the "brains" of the decoder (the microcontroller and the Flash memory chip which houses the sound project files).  Voltage from this stage is also used as the common positive for all the decoder's AUX functions (including the V+ solder pad).  The designers of this decoder decided to use the 5.4V as the BLUE common-positive (instead of the usual rectified 12V track voltage used on majority of other decoders).

The tantalum caps in stage 3 (200uF total) do provide minimal protection from short-duration power dropouts, and there is also around 250uF worth of capacitors in stage 2, so we can't really say that the decoder has no stay-alives.  But all those capacitors provide bare minimum of the stay-alive capacitance.

Ground (common) of the decoder is marked on the diagram as "N".

Where to attach stay-alive capacitors, or a keep-alive SuperCap module?

The bottom part of the diagram above shows both sides of the decoder with color-coded locations of where the external caps can be installed.  The green circles indicate that the large copper areas are all connected to ground (common).

A capacitor, or a bank of capacitors, can be installed with its negative lead attached to any of the green marked areas or component pads (as there is no dedicated "ground" pad).  The positive lead can be hooked up to any of the red marked pads (for stage 1), or purple marked pads (for stage 2).  While I also show hookup locations for stage 3 (blue marked pads), I do not think that any additional caps installed in stage 3 will be helpful in keeping the sound uninterrupted and the model running.

A true SuperCap-based keep-alive circuit (hundreds of thousands of micro Farads with its built-in ancillary circuitry to limit the charging current and voltage) should be attached to green and red marked pads of the decoder. If installed there, it will power the decoder's electronics, the motor, and the function outputs. Since the RED pads on the decoder are very small and close to other components, one must be super-careful not to damage any components while adding the keep-alive circuit.  Unfortunately there is no dedicated RED voltage pad and the only attachment points are on small components, so it will require some precision soldering.

If the additional caps will be less than 1000uF in total capacitance then my recommendation is to attach them to the green and purple marked pads of the decoder.  Since the power-hungry motor is not powered from that stage, the keep-alive cap will supply power to the decoder's circuitry for a longer time. Hopefully the flywheels will keep the loco coasting through the intermittent contact spot while the decoder keeps on running and producing sounds.

Of course, any modification to the decoder are done at your own risk - it is highly miniaturized and delicate.

For those interested in more details, here are the locations of some of the decoder's main components.


I found it interesting that the rectifier diodes used in this decoder have a very low forward voltage drop (only 0.2V).  But I only tested it with minimal load so the voltage drop will most likely increase as the current draw increases. Still, they are probably Shottky diodes with the average voltage drop of around 0.5V or less.



The above picture is self explanatory.
In factory-fresh decoders (with the default sound file installed), Aux 1-8 outputs are mapped to F3-F8 functions, so all the on-board LEDs can be turned on and off.

Nice feature of this decoder is that most of the AUX outputs (including headlights) are available as solder pads on the decoder. In the past, the headlight function outputs were not available on solder pads.  Only AUX7 and AUX8 do not have solder pads. Those are only connected to the on-board LEDs.

If someone wants to simply relocate the on-board LEDs while still using the on-board 680 ohm resistors, then unsolder the SMD LED from the PC board, then solder the wire lead extensions to the LED pads.  The LED polarity (anode or positive [A] and cathode or negative [C]) is indicated in the picture.

Instead of calling those outputs "functions" like most DCC manufacturers do, ESU calls them "AUX" outputs.  This is likely due to the fact that all these outputs can easily be mapped to any DCC function.  The output mapping feature on the ESU decoders is much more flexible than on most typical DCC decoders from American manufacturers.  I highly recommend thoroughly reading through the ESU decoder manual to get familiar with the AUX output mappings.  While the mapping can be done by individually programming a bunch of CVs on DCC system's programming track, this task is made *MUCH* easier using the ESU's LokProgrammer interface and software.