Project Background
 
The Monster Go DJ is one of those cool bits of technology that we'd all like to receive as a gift, but few of us would drop the $500 to buy one.
 

                                                            Monster GO DJ -- Photo credit unknown
 
In most cases like this, one can normally find a far less expensive (and typically far less functional) Chinese clone.  However I haven't really found anything similar to the Monster GO DJ.
 
Of course this is totally understandable.  The Monster Go DJ, cool as it may be, is also totally useless.  One is not going to slide one into a pocket or purse and go jogging -- pausing between each song to seamlessly mix into the next song.  Similarly, no professional DJ is going to show up for a gig, and whip his Go DJ out of his back pocket.
 
It's not cloned because there's no demand for the real thing -- let alone a clone.  Note: Both the Original Monster Go DJ and the Go DJ Plus are now discontinued.
 
The closest I have found is my Numark MixDeck Express, which in my case, was -- in equal parts -- cheaper and less portable.  I found one on eBay advertised as not working for less than $100, and was able to repair it. It works fine, and is a great product for an occasional party or perhaps for a learning or beginning DJ, but I remain obsessed with the elegance and size of the  Go DJ.
 
The other part of the story is my success (and sometimes partial success) in building other projects just by piecing various Chinese modules together.  The NodeMCU clock is basically a display module, a WiFi module, and a small amplifier wired together with a few wires.
 
So why not just buy a pair of cheap MP3 player modules and wire each output to a linear potentiometer?  - Bingo!  A primitive Go DJ for a few $20 bills.
 
 
Other Ways to Realize a Home-made Go DJ Inspired Device
 
Yes, it would certainly be possible to base something on (for example) a Raspberry Pi Zero (or 2), some very nice TFT capacitive touchscreens, and start coding away, but the amount of coding required was far more than I was willing to face.
 
Another alternative is to throw a lightweight Linux distribution on an Intel based tablet, and load up Mixxx DJ software.  But without real analog controls, it would still just be DJ software running on a tablet, it would hardly be an imitation of a Go DJ.
 
 
The Actual Project
 
Something pieced together from Chinese Media Player modules is never going to have the functionality of the custom hardware and software of the real Go DJ.
 
Since this is a zero-coding project, we are limited by whatever functionality the Media Player modules provide.
 
It is likely that these basic and fundamental DJ Deck/mixer functions will not be implementable:

• Beat detection
  
• Speed Control, or Pitch Bend, Brake, etc.
  
• Scratching
  
• While some basic player modules support A-B looping, the interface will be lacking even if the functionality exists.
  
• Waveform Display
  
• While most modules display the time in a given track, customization, such as time remaining may not be supported.
  

With these fundamental features missing, one could argue that the device is unusable as an actual DJ setup.  Of course my argument, again, is that no "real" DJ is going to use a real Go DJ either.  It can still be a fun project to build, and a cool looking device when completed.
 
Here are some features I decided could and should be implemented:

• Physical Fader
  
• Real, analog tone controls even though the module may have preset EQ modes,  as tone mixing is fundamental to some forms of modern mixing.
  
• Battery power for portability
  
• LED VU meters.
  
• Ability to Cue either player to headphones, although the functionality may be limited.
  
• Fingertip digital sample players ("Effects") in addition to the 2 main channels.  
  

In addition, the chosen player modules should allow the following functionality, some of which "Real Devices" such as either the Monster Go DJ or  Numark Mixdeck Express cannot do:

• Bluetooth
  
• FLAC and other formats in addition to MP3
  
• Microphone input, although it will be in place of a player channel
  
• Voice Recording capability. In essence it will also function as a dual digital tape recorder. (Recording of the mix output will not be supported.)
  

Project Status
 
This project is currently in design and parts procurement.
 
A fairly mature front panel layout is shown here:
 

Front Panel Layout - Image is scaled 50%
 
The enclosure will be custom made from Clear Lexan.  It will be painted on the inside, with the areas above the LED VU meter, and the Battery control module left unpainted (masked).  This should look cool while minimizing the number of rectangular cuts needed.
 
The battery (power switch) will completely disconnect the (2) 18650 Lithium Ion batteries, allowing the unit to be stored for months without significant battery drain. The battery board will handle charging the batteries, battery level display, and provide 5V at up to 2A.  The unit will need to be powered during battery charging.
 
The other USB micro connector will allow sample files (up to 8) to be loaded on the effects board.

As we slowly accumulate parts, the design begins to solidify.
 
It's finally time for a schematic:
 

 
Schematic notes:
 
The Tone control boards and op amp modules are considered components, and thus details of their circuitry -- including any changes to component values -- are not shown here.
 
Tone Control Boards come from China as unassembled kits. Building will be customized for this purpose.  Note that +/- 12VDC is furnished from a boost converter module.  There is no power transformer. As a result, the power supply portions of the Tone Control Modules will not be installed.  (Diodes, regulators and the larger filter capacitors.)
 
The same is true for the op amp (preamp) modules, although the only component that is currently planned to be removed is the LED.  Feedback resistors R5 and R6 may be replaced to adjust gain as mentioned in the page for the relevant product.
 
At this time it is assumed that all resistors shown separately, including potentiometers, are 10K ohms.
 
 
I am beginning to receive parts.  I received the Adafruit Sound Effects Module and keypads today.  I have already encountered some minor issues.
 
-- The Adafruit Sound Effects Module was not being recognized/mounted as a filesystem on my Ubuntu Computer.  It seems there was filesystem corruption on it's small built-in storage.  Fortunately, I was able to easily format it with the Linux "Disks" utility.  It is now recognized correctly when connected via USB.  Note that the filesystem on the SFX board seems very slow.
 
--  The Adafruit 1x4 membrane keyboards also gave me some problems:  1).  The pins are not in order.  The correct pinout is [GND(Common)] [2][1][4][3].  This is not mentioned anywhere on the page, but it can be gleaned from looking at the picture of the back of the keyboard with the backing removed.
 
-- 2.) One of the Adafruit 1x4 membrane keyboards had a high resistance from GND(Common) to all keys.  (about 500 ohms with a key pressed). I carefully removed the connector shell, and gently squeezed the common connection to the ribbon cable with small pliers.  This reduced the "closed" or pressed resistance to about 20 ohms (the same as the other one I received), and the SFX module now recognizes all key presses.
 
Once the above issues were rectified, I copied over 8 DJ/Party/Event sound effects (in .wav format) to the SFX board's storage, and it works as expected.  The files must be named T00.wav --> T07.wav, corresponding to buttons 1 - 8 respectively.  The included and useful T00.ogg Left-Right stereo test file was unfortunately lost due to the filesystem corruption.
 
I suppose it is possible that I am receiving counterfeit Adafruit products from the eBay vendor.

Update:
 
Unfortunately, I still cannot report any progress on this project.
 
Apparently, Banggood did not address my package correctly, and it is being returned to them.  This is not my 1st time having problems getting Banggood to ship me stuff.
 
In addition to the FX module, I have received a few parts I ordered from eBay.  The potentiometers not associated with the tone boards have arrived, and so have the 1/8" phone jacks.
 
I've ordered replacements for the other parts from AliExpress and eBay vendors, and expect their arrival in a few weeks.  I choose US vendors or US stock whenever possible.
 
Some of the parts are different, and will drive design changes.  These include:
 
1.  The VU Meter is different. It is slightly longer, and has a button to select the mode.  I plan to incorporate this, so I will have to allow for the button.  The VU meter is a kit, and comes with a Lexan case, which will not be used.  The VU meters ship from the US, so putting them together will give me something to do while I wait for the other parts.
2.  The Tone Control boards are different.  They are pre-assembled, and run from a single ended +12VDC source.  The +/- boost converter will be replaced with a single ended boost converter set to ~+12 to 15VDC.

So my comment yesterday about "...evaluate if the whole project is worth it..." should be worrisome to anyone (that is not a 'bot) who is following this.
 
As I continue researching this project and parts, I've come across a number of alternatives in addition to those already mentioned.  The first and most obvious is  actual Monster GO-DJ's for sale on eBay.  There is currently a used one for only $260.  I certainly hope that Banggood is going to refund my money for the order that got shipped back, but if they don't, I've already got more than that in this project.
 
There are also used GO-DJ Plus's available for around US$ 500.
 
Lastly, I've also discovered the Stanton SCS.4DJ.  While not really portable in the sense that a GO-DJ or this project intends to be, it is smaller and lighter than my Numark Mix Deck Express, it has way better features, including true waveform display on an internal 4.3 inch color display.  It also dispenses with the legacy CD drives, which I think is a good thing.
Stanton may not have the name recognition of Denon or Yamaha, but the company has deep roots.  It's not too far off to say that Stanton invented the modern style phono cartridge.
Again, Stanton SCS.4DJ's can be found used on eBay for less than I'll have in this project if I don't get my money back.
 
Tracking information shows I should be getting the media player modules sometime in the next week or so.  The fate of this project will lie in the arrival and acceptability of those media players.
 
More news once I've powered up the media players on the bench.

More on the specific media player modules that I am using in this project:
There is a thread about these modules on the Parts Express Tech Talk Forums.
 
Specifically, the Original Poster is asking about adding a rotary volume control.  He noticed that there appear to be pads for exactly this purpose.  I'm not sure if he ever was successful or not.  I have a number of rotary encoders left from the repair of a damaged Yamaha 02R96, but since I plan to have analog volume control downstream of the player, I don't really need this.
 
What was interesting however is the other information about this module that was compiled in that thread.
 
The modules are based on an SoC (System on Chip) marked AC19AP1R339-1A8, and a company logo -- "JL" ("JieLi"). According to that thread, an AC19AP1R339-1A8 is a custom programmed AC4602.
 
Not only that, but it seems that most similar players are based on the same or similar SoC's. If it has Bluetooth, and doesn't have a color screen and video capability, it's probably a AC4602 or AC4601.
 
EDIT: More information, including a schematic:  http://sxemy-podnial.net/aiyima-bluetooth-4-2-dekoder-s-zapisyu/ (Also linked from the Parts Express Tech Talk Forums post).
 
Buttons Implemented as a Voltage Divider:
One thing that is interesting is that most of the function buttons are implemented as a voltage divider connected to a single analog (ADC) input.  So basically, 3.3V is "Do Nothing", and ground is "Play/Pause" with all of the other button functions implemented as some voltage in between.  Each button is connected to a resistor that, along with the common pull-up, results in a pre-set voltage.  This ADC pin is even brought out to the "Key" connection on one of the connectors.
 
Fast Forward and Rewind could be implemented by simply adding buttons and the correct value resistors. There is a table from a vendor site that shows the values, but it's not really readable, and I have not been able to find a better version:
 

 
See above link to Russian site, which has full schematic, including resistor values.
 
When I press  [EQ] and [>>|] simultaneously, or  [EQ] and [|<<] simultaneously, I am simply adding resistors in parallel that happen to add up to the correct values to get Fast Forward and Rewind.  This is why the button labeling is incorrect when [>>|] or [|<<] are used this way.
 
If I wanted to add specific keys, it would be fairly easy to measure the values for the resistors associated with [EQ] and [>>|] and calculate resistors in parallel.  I'd then wire a separate button to a resistor of that value, and I should have a dedicated FF button.
 
This also could allow for more "DJ-like" controls. By changing the SMD resistors, buttons could be moved or re-defined.
The provided access to the "Key" line opens the possibility of using the modules and displays, but not the front panel or buttons.
 
Development Tools:
So the intent of this project is to be a zero software effort.  That said, there is development information out there (mostly in Chinese), and also also lots of development boards for systems based on these and similar chips (AC690X -- AC6901AC6901A -- AC6901258A, etc.)
 
For now, I'm sticking with the modules as they are.  The FF/RW situation is a little awkward, but functional.
 
If I were going to do software development, I'd probably start with a ESP32, a couple of audio CODEC chips, and a couple of color OLED displays:
https://docs.espressif.com/projects/esp-adf/en/latest/index.html

Sometimes I Hate the Internet.
 
I'm still waiting on parts for my "Poor Man's version of the Monster GO DJ" project, but I have a few minor updates...
 
As mentioned a couple of posts ago, I just recently learned about the Stanton SCS.4DJ Controller/Mixer/Player while researching devices similar to the Monster GO DJ.  Yeah, I (now) know the Stanton SCS.4DJ been out for several years, but it had never "popped up on my radar" before I began researching this project.
 
What I did know is that you can find anything you want to -- good or bad on the internet, but knowledge is only power if you do, or in this case don't do something based on that knowledge.
 
My failure was a failure to avoid searching for cheap "for parts or repair" Stanton SCS.4DJ's on eBay.  So you already know how this is going to end -- I ended up with an interim project while continuing to wait for parts for my "Poor Man's Monster GO DJ".
 
A couple of clicks, and only a couple of days later, I am the owner of an very used, abused, and smelly Stanton SCS.4DJ:
 

 
This unit was listed as for "parts only" mainly due to a failed player "A" pitch slider.  By the time it got to me, it had not only a bad "A" pitch slider, but it was also missing a headphone mix knob, and it had damaged pots for the Master Volume, and the Headphones Volume. The additional damage being due to poor packaging by the seller, and compromises in the handling of packages by the shipper due to the volume of packages going through the mail in the current epidemic.
 
Add to that a strange smell that seems to be a combination of old electronics, and whatever other odors may be floating around the previous owner's abode.
 
In addition to several failed pots the "Playlists" button must be pushed very hard in order to work.
 
I de-soldered the "A" pitch linear slide pot, opened the part itself, and was actually able to make it work, although as one might expect after attempting such an "improvised" repair, It feels absolutely terrible now.  The volume controls were made temporarily functional by basically just squeezing them back together with a pair of pliers, but again, they will never work as they should, and will be replaced.
 
Apart from the physical damage, I am impressed with the Stanton SCS.4DJ -- although like knowledge itself, "impressed" can be either good or bad.  Here are a number of observations:

• I'm very happy that I did not pay more for this device.  It's build quality is just not up to my expectations for even "Party grade" DJ equipment.  It certainly is no where near "Pro Audio" in quality or construction.  I don't consider my Numark Mixdeck Express to be "Pro Audio", but the SCS.4DJ is a big step down from that in terms of weight, durability, or ruggedness.
  
• Even the "good" (non broken) sliders and other controls (especially the buttons) feel very cheap.  I wouldn't consider them adequate for even home entertainment equipment.
  
• They've really made the most out of a wholly inadequate processor.  It took several hours to build a database of BPM, and waveforms for a thumb drive with about 250 songs on it.  The processor is a tiny chip with no heat sink. IMHO, they should have just put some more CPU power in it -- especially for the retail price.
  
• Based on GPL and LGPL references on in the manual, and other similar products, I'm pretty sure it's running Linux which is appropriate since we are discussing it on a Linux website.
  

The parts for the Stanton will probably be here before the rest of the parts for the main project of this thread.
 
Speaking of parts arriving, the small VU Meter board has arrived.  I connected it directly to one of the Media Player modules, and it seems to work well.  It is nice and bright just like the other one.  The only potential problem is that it seems to be set for a pretty high line level even with the trimmers all the way up.  The 1st orange LED (or "0") is set for 0.775Vrms signal.  I suppose this is "accurate", but that means that when I go directly into a straight (tube) power amp, only a few green LEDs light for normal listening levels.  We'll have to see what kinda of overall gain the tone boards have.
 
I noticed that the Stanton takes the VU levels before the Master Volume.  Most professional mixers give you a choice of pre-fader, or post fader for the VU meters.  I plan to set up the Numark to refresh my memory as to what it does.  I thought it displayed the actual output levels on the VU meters (i.e. after the Master Gain).
 
UPDATE:  The Numark works as I remembered.  The 2 units are different in how they implement the VU meters.
 
I think the Mixxx DJ software displays the VU graph pre-Master.
 
If I go with Pre-master, that will result in a minor schematic change.

Finally -- Major Update!
 
All of the major components of my "Poor Man's GO DJ" have finally arrived!
 

 
I had no idea that it would take me over 4 months before I was able to post that picture.
 
If you have been following this project, you should be able to find most of the parts and components mentioned in previous posts, or shown on the drawings. (It's kinda like one of those books for children)
 
A few of the major components/highlights:

• Obviously, the 2 media players detailed previously.
  
• The Tone Boards (To the upper left of the remote controls)
  
• The Fader Board (Left of the remote controls)
  
• The Op amps (Near center, still in packaging)
  
• Bags of both types of pots, although only 1 type is planned to be used.
  
• Obviously I will not use all of those JST connectors (Right)
  
• 2 Sheets of Lexan (Underneath other components)
  

What is NOT in the picture:

• Hardware (Screws, Standoffs, Feet, etc.)
  
• Wire
  
• Perfboard (if/as needed)
  
• Additional Metal Knobs (More Knobs are on the way and can be changed at a later date.)
  

There are a few things I still need to do before I can actually begin cutting/drilling Lexan:

• Measure the spacing of the holes for the tone boards, and change drawings if needed.
  
• Address the issue of a Main/Headphones Mix (Circuit Design point of view)
  
• Address the issue of a Main/Headphones Mix (Panel Layout point of view)
  
• Determine Mounting of Op Amp boards and misc discrete parts.
  
• Determine gains of tone and buffer stages, and change components if necessary.
  

 

As promised... A picture that looks much more like it will look when finished...
 

 
Note that in this picture, the plastic covering is still on most of the plexiglass.  I am trying to prevent scratches during the handling that is necessary during construction.  Also, and again for obvious reasons, the knobs are not yet installed.
 
Inside, it is still mostly empty.  I have a *lot* of wiring ahead of me.  While the modules are, of course, printed circuit boards, the rest will be built with old fashion style point-to-point wiring.  Soldering small coax to very small PC mount potentiometers is not something that I am looking forward to.
 
So far, the only wiring done is the Headphone jack, and the harnesses to connect the keyboards to the sound effects module.
 
I am not rushing to get this thing working.  I'll work on it at my leisure, and stop when I get frustrated with the soldering.  Until it gets to the point were I can power it up, it won't look much different, so don't expect more pictures for a while.
 

Here's a pretty much final picture:
 

 
 
Current Status/Notes:
 

• It runs from the internal LiPo battery.  I went with the separate boost converter to go from battery voltage to the +5V.  The charger/display board handles charging and battery level only. Battery life seems to be pretty good.  As mentioned, it draws less than 0.25A.  With a 5000mah battery, it will last longer than any party I will ever get invited to.
  
• The issues with the cue switches was due to a short from soldering in very tight spaces.  2 soldering/wiring issues have been corrected.
  
• I have not fixed the issue of the fader not completely silencing the opposite channel. Since the fader (and the headphone cue/live mixer) are just wired as resistors in the line-level audio instead of actual stage gain controls, they are not going to fully cut off the audio.   More engineering is needed.  I am not sure how much more effort I want to put into this project.
  
• I turns out that the fader (linear potentiometer) board was not wired like I though it was.  Basically, both sides are wired in parallel on that board. The result of this was that it was not actually playing in stereo. I had to de-solder and re-work the board to make it 2 electrically independent pots.
  
• If you look closely, you can see that I re-taped one of the membrane keyboards, and that the other is sill coming up near the center.  I'll re-glue this when I have the time and patience to do so.
  
• Labels are not installed.  I have made some labels with Libre Office.  Again, I'll print, cut, and apply them when I have the time and patience to do so.
  

Finally -- As promised -- The demonstration video:
 
 

 

How do you fix software bugs?  Change the hardware!
 
As mentioned previously, I’ve been using my “Poor Man’s Go DJ” to actually DJ public (although very casual) events.  I like using it and for some smaller local gigs, this home made player/mixer is the only player I take with me.  The features I like about the “Poor Man’s Go DJ” are outlined in the previous 2 posts, and I stand by those comments.
 
That said, I have also pointed out some of the deficiencies of the “Poor Man’s Go DJ”, and that most of those deficiencies are due to the lack of functionality of the very basic JeiLi 690X based Media Players used in this project.
 
I have actually looked into what it would take to do software development for the JeiLi based players in order to fix bugs and add functionality.  It turns out that there is a development board available (Search “JeiLi Development” on sites like AliExpress), but buying the development board would put the cost of this project over what it would cost to buy a real Monster Go DJ and it looks like the tools are Windows only.  While the source code is provided, I’m not going to jump into something like that for a media player with such basic functionality.
 

JeiLi (JL) 690X Development board.  Picture from vendor site on AliExpress.  Linuxslate.com is not affiliated with, and does not recommend any particular AliExpress vendor.
 
 
Enter to the story the “CD-028 Wireless Bluetooth 5.0 Decoder Board Stereo Audio HD Video Player MP3 FLAC WAV APE Decoding FM Radio USB TF Music player”.
 
These are similar player modules to the JL modules, but with a color screen, and more functionality.
 

The "CD-028 Wireless Bluetooth 5.0 Decoder Board Stereo Audio HD Video Player MP3 FLAC WAV APE Decoding FM Radio USB TF Music player"
is similar to the JeiLi Media Players, but with a color screen and more functionality.  
Picture from vendor site on AliExpress.  Linuxslate.com is not affiliated with, and does not recommend any particular AliExpress vendor.
 
While these are improvements, these players are still fairly primitive, and of course they still don’t have features that would be expected on a “real” DJ deck. They don’t have anything like jog, scratching, pitch, etc. In fact, there does not seem to be any way to position playback within a track at all without using the remote control.  Obviously, they also don’t have features such as beat detection, or beat synchronization. There is also no A-B looping.
 
I have purchased 2 of these players, and I would like to describe them here, with particular attention to features relevant to the “Poor Man’s Go DJ”.
 
Here is a list of the most significant improvements over the JL-based player modules:

• While there is still no way to display track remaining time, they do display track total time in addition to time in track, and they provide a very basic display of position in track.  For doing simple mixing between 2 players, this is a huge improvement.
  
• They have several options for repeat, including “once”. If this option is selected, it causes the players to stop at the end of each track, which is the expected behavior for a DJ deck.
  

Note the right curving arrow symbol at the top center of the display.
This shows that the unit is in "once" play mode.  In this mode, playback will stop at the end of each track
which is the expected operation of a DJ player deck.
One can also select the preset equalizer functionality from this screen. (Small white icon, center right of screen)
NOTE: The light color of the right side of the screen is an artifact of the camera only.
This is not a deficiency of the screen.

• They have a “Folder” function, which preserves the Directory Structure of the USB or SD media.  It also displays several files at a time, and allows the user to navigate the structure while a track is playing in the “Music” function.
  

In the next post, I’ll continue with some other information about the functionality of these players -- both from the point of view of using them in the “Poor Man’s Go DJ”, as well as information applicable to other uses of the “CD-028” Media Player modules.

New Player Modules Installed.
 

 
I have installed the new player modules in the "Poor Man's Go DJ".
 
Here are a few notes on other functions of these players:

• Bluetooth does not work at all.  Both Players have the same Bluetooth Name (CD-028), and they respond to a Bluetooth Pairing request even when not in Bluetooth mode, and even when not powered on.  There is no way to disable Bluetooth, and no way to change the name.  So if there are 2 of these players any where near each other (not just in the same box) they continuously attempt to pair and then disconnect.  See below for possible work arounds.
  
• The Microphone jacks on the PM Go DJ are no longer functional.  These modules do not support recording or an external microphone.  The internal mics (seen at the top of each player) are for a Bluetooth phone call only.
  
• Line-In does not seem to work either.  I connected the RCA Input jacks to the line input of player 1, but all I get is that same Bluetooth connecting sound.  More research into this issue is needed, but the result of this is that I currently have no way to play my phone through the PM Go DJ.  Connecting the phone to a separate mixer is the only option.
  
• As mentioned, the high power consumption of these modules is a significant problem.  In addition to the problem of reducing battery life, this causes two other issues:  1.) The 5VDC power supply (Boost module) currently installed in the PM Go DJ is only rated for 1A continuous, 2A surge.  2 modules, plus the other hardware in the PM Go DJ now add up to nearly an amp @5VDC, and thus well over 1A draw from the battery.  In addition, the built-in charger only charges at 1A, in other words, I am am likely discharging the battery even when powered from external USB power -- no matter the rating of the USB power source.  With both modules "off",  draw is still over 600mA, so I am charging the battery only very slowly.
  

A Little Good News:

• As long as the batteries and the boost module hold out, it works very well as a basic, improvised DJ player/mixer.  As I mention in both the YouTube video and post #20, Having at least some indication of time remaining, as well as stopping at the end of the track improve the usability of the PM Go DJ enormously.
  
• The Color Screens make the PM Go DJ look more like a contemporary product.
  
• Battery life should still be somewhere around 4 hours (assuming the battery can be fully charged).  Battery life would be significantly extended beyond that by using an external USB power source.
  
• It's even more fun to use and play music on, although due to the issues mentioned above, I would not rely on it to actually DJ an event right now.
  
• It still sounds great, and seems to have a little stronger output level if the media player volumes are turned up.
  

Solutions:

• The obvious solution to the additional load on the 5VDC boost module is to use a boost module rated for the observed current.  This is not a problem from a cost or space point of view.  I will order a 4A/5A rated module in the next few days and install it when it arrives.
  
• The charging problem can be solved by simply rearranging the power as it is in "normal" devices.  In other words connecting the power switch downstream of the charger circuit.  I like having a true battery disconnect from the point of view of both safety and shelf life.
  
• Another solution would be to install a separate power switch for one or both of the modules.  This would have several benefits:
     1.) It would likely solve the Bluetooth issue since one module could be completely shut down during Bluetooth pairing and/or operation.
     2.) It would allow faster charging by eliminating draw during charging.
     3.) A single player could be used when not actively mixing tracks, allowing much longer battery life.
  
• It may be possible to change the Bluetooth name of one (or both) of the player modules. Note that the Bluetooth chip is made by JieLi -- the same company that makes the main chip in the previous (monochrome) players. While it seems this is at least plausible, it is unlikely it is worth the effort.
  
• Another solution to the Bluetooth issue would be to disable Bluetooth on one module by simply destroying the JieLi Bluetooth chip.  Of course if this affects other functionality, it would probably be necessary to replace the entire player module.  
  

 
Sadly, this is a classic case of an engineering trade off -- trading improved functionality in one area for less in another.
 
The "Poor Man's Go DJ" is still a very cool and nice looking (IMHO) project.  It is now more usable as a fun personal media player with analog controls, and a useable backup media player/mixer for casual events.
 
The ultimate project would be to combine 2 Raspberry Pi's with quality DAC's, and quality touchscreens with the analog hardware from this project. It would also need some good artwork, and lots of programming.  This would result in a consumer -- or possibly even professional -- grade product that could represent not just a replacement for the Monster Go DJ, but a significant improvement over it.
 

Previously in this thread, I have mentioned other products similar to the "Poor Man's Go DJ".
 
I have since come across 2 additional products that are even more similar -- Both bearing the "Technical Pro" brand name.  These units are particularly pertinent to this thread, because these units seem to share the idea of using simple, generic music player modules.
 

Technical Pro DJ2USB.  Photo from Manufacture's website
 
 
 
An even more basic unit seems to exist, or have existed:
 

Technical Pro DJ4PB.  Photo from eBay vendor
 
Given how basic the players look in both of these units, I'm going to say that they probably have similar or more limitations than either the original players used in the "Poor Man's Go DJ", or the new, color screen media players currently installed.
 
I want to be clear that I have never seen either of these Technical Pro units in person, and of course never used them, but I am confident that as with the project described in this thread, the Technical Pro offerings also lack features such as:

• Beat detection
  
• Speed Control, or Pitch Bend, Brake, etc.
  
• Scratching
  
• A-B looping
  
• Waveform Display
  

 
Note also that they do not seem to share the USB or SD card slots.  Like the "Poor Man's Go DJ", separate media for each player appears to be required.
 
I would hope that at the vary least, the media players display time remaining, but unless Technical Pro developed or modified the players, these decks (and it's quite a compliment to call them "Decks") may suffer from the same awkwardness as the players originally used in the "Poor Man's Go DJ".
 
I'd like to point out the fact that the DJ4PB has 2 microphone inputs with separate level controls, and Bluetooth on one of the players.  The DJ2USB has analog tone controls (although "High" and "Low" only, no Mid Range).
 
They appear to support only MP3 and WAV file formats.
 
There is no indication that either of these units will run from Batteries.
 
Am I publicly bragging or dissing Technical Pro here?  Well -- Yeah -- Maybe a little.  The "Poor Man's Go DJ" has been a very successful project.  It has more features, and likely better sound (when playing a better quality FLAC file) than the Technical Pro offerings.  It also has the 8 sound effect/beat/sample player.
 
The Technical Pro units win in terms of a mountable metal chassis, and microphone inputs.  I should also mention that Technical Pro also make players other than the ones shown here, and some of those do have more elaborate player mixers, and some support scratching/etc.
 
However, while Technical Pro has some very compelling and desirable Karaoke mixers/amplifiers, for casual events, they are not really known for living up to the "Pro" in their name.
 
 

So this project is finished (or is it?), but I still have a few updates:
 
1.  I continue to use the Poor Man's GO DJ to DJ local car shows and other events.  I prefer it over the Real Go DJ for the reasons mentioned.  Most importantly it stays on the table.  The Real Go DJ has no rubber feet, and very little weight.  It tends to move around on the table when used, or pulled by the cables.
2.  I use the Real Go DJ as a bedroom audio player, and a compact back up for events.
3.  I have made a simple modification to the power system in my Poor Man's  GO DJ so that the batteries remain connected to the charger module.  The main power switch isolates the rest of the circuit.  Some may say this is how it should have been in the first place -- and I'll take that.  The load of the 2 media players was never allowing the battery to charge.  I ran the unit on batteries last evening for almost an hour.  I have a major event coming up in a few hours, (supposedly televised on 3 local TV stations), so perhaps making a change like this wasn't smart either, but we'll see how it works.  It sure wasn't working acceptably the other way.
 
More updates if/as needed.
 

The Poor Man's GO DJ let me down during an all - day Car Show earlier this month.  For part of the show, I had to switch over to the real Monster GO DJ, which also presented a problem -- I was set up in direct sunlight, and I couldn't see the displays of the real GO DJ.
 
I finally had to go sound off, unplug the real GO DJ, take it over to some shade, find the backlight brightness setting burried in the various settings tabs and pages, and turn it all of the way up.  I'll note that there is no contrast setting. Even with this done, I still struggled to see the screen of the real GO DJ.  I was not having any problem seeing the screens of the Poor Man's GO DJ under the same conditions.
 
As it turned out, once the Poor Man's GO DJ charged for a while, I was able to go back to using it to finish out the day.  It ended up only being 2 breif periods without music.  I still got paid, and I got several compliments on the song choices and sound quality.
 
So why did my Poor Man's GO DJ let me down?  As mentioned previously, the new, color LCD player modules draw a lot more power than the B&W screeen modules used in the original design.
 
While the charge/discharge module mentioned in the pevious post seems to power the PM GO DJ fine from the internal battery, it does not -- unfortunately -- maintain 5VDC output during charging.
 
When charging, the output voltage (to the load, not to the battery) drops to several tenths of a volt lower than the input voltage.
 
Given the length of wire from a 5 V USB charger, and the load of both charging the batteries and running the device, I end up with about 4.7 V at the USB connector, and less than 4.6V for what is suppose to be a 5V supply.
 
Typically, devices designed to run from a regulated 5 VDC power source have a tolerance of 5VDC +/- .25V.  So a voltage down around 4.6 won't cut it.  The player modules glitch or spontaneously re-boot.
 
I should also mention that the +/- boost converter that supplies the dual rail voltage for the analog sections seemed perfectly happy with this input voltage.  The failure is silent -- the music simply stops -- there are no pops, clicks, etc. when this happens.
 
So I still have the original problem -- I need a steady source of +5VDC at about 1 amp under all conditions.  This should be easy, but I have not found a module that will do this reliably.
 
Solution space:
 
1.  Forgo battery power --- After all, I am never going to do a DJ job on battery power.  All of the other equipment (except wireless mics) need external power available, so why shouldn't the player have the same requirement?  I designed the player to run from a battery only because the real Monster GO DJ has a built-in battery.
 
1a.  Power the PM GO DJ from an internal 5VDC SMPS.  SMPS's with more than enough current capability are redily available, inexpensive, and would easily fit in the current player.
 
1b.  Simply remove the battery, and rely on the power directly from an external USB source.  A quality USB charger and cable should provide the needed voltage without a problem.
 
2.  Down - then up again solution -- use a buck converter to drop the output of the charger module to about 4 V then boost it again to a stable 5.1 V.  The chager module would always provide enough voltage to operate the buck, and I have bucks on hand that are rated for the full load current even at 4V.
 
3. Up - then down again solution -- take the output of the charger module and boost it to ~8 V, then use a buck to make a regulated 5V.  Again, the charger module should provide sufficient voltage for the boost converter under all circumstances.  With a minimum of 4.6 V input, and ~8V output, the boost converter is working at lower currents than it was in the original design (when it was boosting the battery voltage), so it should be running in a more efficient range, and thus it should remain cool.
 
4.  One additonal Conversion -- Find a buck/boost converter that will reliably provide 5VDC output from inputs very close to that same value.  So far, I have not found this.  Most boost converters want the output to be at least 1V more than the input, and similarly, most buck converters need  the imput to be at least 1V more than the desired output.
 
5.  By using a very short USB cable, the charger module may provide enough voltage as-is.  I would then power the GO DJ from a USB power source very close to the unit.  If I can eliminate voltage drop in a long USB cable, I may be able to tolerate the drop through the charger chip.  As long as I can reliably stay over 4.8V, I should be good.
 

QUICK UPDATE:
 
I DJ'ed for a 5 hour gig yesterday (Saturday).  The Poor Man's GO DJ worked almost perfectly for the whole gig, and using the new digital voltmeter, I could tell that the batteries were actually charging throughout the day.  This was the first time I was confident that power would not be an issue.
 
I should also mention that I was using a normal, unmodified USB mini cable, although it was a qood quality one.
 
I did have a problem with several MP3 files ending abruptly, but this is not the fault of the player.  There is a known issue with lame that causes wrong header information to be written to Variable Bit Rate (VBR) MP3 files, so if I miss running ripped music through vbrfix, this issue can result.  FLAC files are not effected.
 
Other than that, the day was pretty much flawless.  Nice weather, lots of very nice cars, and -- Food Trucks !!, One of which donated a fried shrimp meal to me.
 
The real GO DJ?  It stayed in the truck.