So here are the details of the Marantz DP870 digital switcher mod that hopefully some of you might find useful. I realize it’s not the easiest mod to perform but it’s something to think about for possibly those of you who already own this unit (or the clone Harmon Kardon ADP-303) and are adventurous enough to want to tackle it.
So first things first, let’s take a look at the DP870 for those of you unfamiliar with it.
Front View Of The Processor
Back Panel "Before Mod"
So in taking this unit for what it is in today’s world and in relation to LD, it’s a pretty barebones component.
Integrating it with today’s modern AVR’s and Pre Pro’s would be impossible without the inclusion of a 5.1 or 7.1 analog input which have mostly disappeared around 2016. And even if you have one of those 2016 or older AVR’s, you are stuck using the DP870’s built in processing which is very archaic in comparison to what we have today in respect to auto room correction, variable x-over settings (on more higher end models), DSP decoding algorithms, and backward compatibility w/ matrix surround codecs (the DP870 can’t even decode a two channel audio source to Dolby Pro-Logic). And that is why in just about all cases, a stand alone AC3-RF Demodulator (or Processor w/ a digital output) is way more useful than this unit and is the reason it is not worth much on the open market (I acquired mine for about $70 including shipping and I know of others getting it for less than that).
Now let’s take a look at the back panel in the following pic.
Back Panel "After Mod"
As you can see, there is now an optical digital out jack as noted w/ a white label. And this output is special because unlike any DP870 (or Harmon Kardon ADP-303) that has ever been modded,
this one will output all three of the inputs of this unit out of that jack. That means this DP870 is now a digital audio switching device on top of being an AC-3 RF Demodulator. In addition, because it’s now acting as a “pass through” for all the inputs, it can deliver AC-3, PCM, and DTS downstream to any modern AVR or Pre Pro one desires to set it up with (not to mention that it simplifies one’s LD digital audio connections by allowing two digital connections to the DP-870 so just one digital input needs to be assigned on a modern AVR).
So now I'll break this mod down into five sections; 1) Board Design, 2) Board Construction, 3) Tap Points Within The Processor, 4) Punch Out For New Jack, and 5) Completed Job.
Board Design:Here is the schematic of the design I created.
So one might be asking, “Why does it have to be done this way?”
Well, the simple answer is because there is no existing interface within the design of the processor that would allow a switchable digital output to work (you can simply tap an output from the AC-3 Demodulator to get a digital extraction for the AC-3 RF input but that is all you’ll get).
And let me add that I have tried other means to get a digital output to work on all the inputs. One such method involved the use of an I2S extractor that I was hoping could work with the Sanyo LC8904Q digital interface IC that routes the inputs. However, after spending some eight months on it, I realized it wouldn’t work because the chip was formatted for EIAJ instead and that is a whole can of worms in trying to figure out an interface that could read either EIAJ to S/PDIF or EIAJ to I2S.
Moving on, I did eventually come up with the idea of a mux switching IC which is the basis of the schematic. The concept here is that instead of trying to find a universal digital output to tap, why not create a new digital input switch and then I can simply tap an output from there. Thus, now we have a simple idea of just “y-cabling” the three digital inputs into this IC and getting that universal digital out from it. The only minor obstacle was how to switch the inputs but I figured that out pretty quickly by using the front panel LED input indicators as voltage switch lines. And that is essentially it.
I also added an additional IC on the board for just the coaxial digital input to provide a buffer circuit to ensure proper signal integrity (the DP870 appears to have a buffer built into the LC8904Q which made it inaccessible for my mux IC).
Lastly, I will not give myself full credit for this board design as it’s roughly based on how the Sony SDP-E800 / EP9ES switching and buffer circuit is laid out.
Board Construction:Not going to say much about this as the pictures speak for themselves.
However, I will add that my choice of IC used for the mux didn’t have to be an “8 input to 1 output” design as I easily could have used a “4 to 1” to cover all the inputs. In my journey at this point, I simply wanted an IC with three switch lines just in case two switch lines might not work. The same goes for the buffer IC as the one I chose could accommodate up to three coaxial digital inputs (in this case I wanted a through hole IC and anything smaller would have been a surface mount variant). In any case, any unused ports (as you may have noticed in the schematic) goes to GND to prevent floating and any other functional issues.
One other thing to add, I use quite a number of 0.1uf capacitors between power & GND and that is by design to ensure clean stable power (I do that with the AC-3 RF boards I build as well).
Tap Points Within The Processor:Let’s start with the voltage switch lines. Based on the schematic, I designated Pins 8, 9, 10 of J605 as the place to tap into the LED display indicators on the front panel. Unfortunately, because the solder points at J605 were smaller than I liked, I diverted the taps over to the front display board as shown here (same points that make continuity back to J605)....
And then connected them over to the new board….
Next up is the power and AC-3 Demodulator points (blue wire is on pin 44 of LC8904Q connected via the opposite end of RR01; same as pin 65 of PD-4606A)…
Moving on to the Coax and Optical Digital taps on the input jack board (this is actually connector JD03 which connects to J601 w/ the same pin numbers) ….
Now the wiring construction of the optical output jack and its connection to the new board (orange wire is data)…
…And the board now completely wired up and ready to go.
Punch Out For New Jack:It was a no-brainer to me that I wanted to place the jack in a location close to the digital inputs simply to be consistent with grouping similar connections to each other. Also, there really wasn’t any other place I could fit it in.
I went about the jack alignment with electrical tape to establish the horizontal borders in relation to the optical input cut out....
….. and eyeballed the vertical borders with the help of a spare optical jack…..
……next up was to double stick my template over the proposed cut out area on the electrical tape….
…..and then punching out a hole w/ my Roper Whitney #5 Junior Hand Punch using the 3/32" punch size (note that it’s not perfectly centered but it doesn’t matter since I just needed access for a file)....
…… and we file away…..
…… until it’s a perfect square
Completed Job:The final completed job with the board now mounted and jack installed,….
…. a view of the voltage switch line wiring on the back of the display board,…
..... and another view of the back panel from a slight angle.
So in conclusion, I’m happy to say that I didn’t waste the last 20 months getting this done and now I have another fully capable switcher / demodulator @ my disposal for hook up to another system whenever that may come along. In addition, it is still fully functional as a processor if I decide to integrate it w/ an older Pro-Logic AVR. I’ll update this posting one last time w/ an updated video further explaining the advantages of the switcher in relation to the numerous audio options of LD (Analog, PCM, AC-3, and DTS).
My next project may be the Adcom GDD-1 (running a test soon for an I2S board on that unit to see if that works). If the board fails, then it means the GDD-1 will get a similar mod as the DP870. However, I’ll probably wait on doing that mod as I am more anxious to tackle an Onkyo ED-901 which is built in a very similar manner to the DP870 but with a few interesting differences that will make it a fun project.
Thanks list
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