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.