I recently bought an ADCOM GDD-1 for multiple reasons, but mainly the unit just made me curious since it is one I haven't worked on before.

After getting the unit and performing some preliminary tests, I wanted to see if it could be modified to output all of its inputs through a single coaxial or optical out connection (just like the other processors).

There is no publicly available service manual for this unit, but it has some distant cousins (GTP-740 & GTP-750) that have service manuals published. These have a lot of similar ICs used on the various boards.

I've done as much research as I can but am kind of stuck at this point. I wanted to publish my findings here and see if anyone can help me further with this project.

The main board that seems to have the audio signals going through it is the CS4226. It handles all of the inputs (AC-3, Digital, AUX 1 & AUX 2). The same board also handles outputting to the 6 Analog RCA Jacks. Where I'm trying to intercept the signal and output to a switched optical connection is Pin 35 or 36. This is SDOUT2 and SDOUT1 respectively. I connected a test optical jack last night and used both 35 and 36 for digital signal but didn't get anything with an AC-3 signal locked and going through the processor.

Obviously, I could tap right into PD4046 and call it a day like the other processors but that's boring.

So, question to you all is do I keep trying to connect to CS4226 on a different pin or look for a different IC to connect to. I'm linking to my Google Drive here https://drive.google.com/drive/folders/1ROgxgZbMbR6Ux9LdRTgWzaul9KEkLbs6?usp=sharing but will come back later and publish photos in the thread so you don't have to click through. The Documents folder is the main thing to look at.

My opinion is that the CS4226 does appear to have a I2S digital output capability and it might work with an I2S to S/PDIF converter but the chip might not be programmed to output data in its arrangement in the Adcom.

If this is indeed true, it would be the same issue I found w/ the Marantz DP870 and I tried to use an I2S to S/PDIF adapter to try and get the digital data but I soon realized that the Sanyo LC-8904Q (used for switching in 870) was really designed w/ EIAJ instead of I2S. Hence, I decided to go about it another way and I have a board design ready to go if only I can get some time away from work and my kids to get it done .

Thanks for the reply, ldfan.

If you want any help with testing the board or the theory of this, I still have the ADCOM and plan to keep it for a little bit.

It's a long shot, but you might try writing to the current iteration of Adcom to see if they have a service manual for the GDD-1. They aren't the same company that made the GDD-1, but they did send me a document with a bunch of IR codes for various old components which saved me lots of time and trouble (I have a GTP-450 that I wanted to test, but not the original remote control - that info is posted here https://www.hifi-remote.com/forums/app. ... e_id=27260 ) so maybe they would have an archived copy of the service manual they'd be willing to share.

I remember noticing the GDD-1 on the list of remote codes, and thought about getting one until I realized, as you have, that it didn't have a digital output which I'd want to use my LDP with a modern AVR.

Anyway, here's their web site

https://adcom.com/contact/

Well... finally have someone that is interested in this after all .

I wouldn't expect you to spend time on this as there are a couple of specialized parts that need to be acquired and there is no guarantee that it will even work (although I am pretty confident it will but might need more tweeking as I work toward a finished product). I probably will get to building my circuit in possibly a month but hard to say w/ my work and family schedule.


So here is the design I have at this point (my third revision).....



The concept I am proposing is to not even try to find a digital output from the Marantz DP-870's audio switching circuit but instead simply build a new switching circuit that allows me to basically "Y-cable" all the inputs into it and then create a digital output from there to get access to all three inputs in the unit. The switching will be done by utilizing the LED indicators on the front panel (I have tested them and they should provide adequate switching voltage) and build a truth table based on those combinations of high and low switches.

The circuit design I have here utilizes an 8 to 1 Mux switching chip (SN74HC151) of which I am only utilizing three inputs of the eight (I could have used a 4 to 1 Mux but I preferred having a third select line). The other chip (SN74HCU04N) is simply my digital buffer that is strictly for the coaxial digital input (not 100% necessary but I wanted to include it as that is how many coax digital inputs are built in other devices). Also, the DP870 doesn't have a typical buffer for its coaxial input as I think it's built into the LC8904Q switching chip which can't be accessed and I still wanted some protection for the Mux chip.



So in looking @ the Adcom, I think a modified version of this circuit could work to obtain a digital output to access all the inputs. The unit utilizes a similar LED input display as the DP-870 so that makes switching hopefully easy (you'll need to get some readings on a voltmeter to see what comes up; the Marantz gave me all inverted readings). The Adcom's coaxial digital inputs appear to have buffer circuity (I know the schematic you provided was from another Adcom device so you'll have to take a closer look) so that would be good because you can tap that digital signal between the buffer and the CS4226 and just build a board w/ just the Mux chip only.

Let me know what you think and if you go ahead with it I would love feedback to see how it works out.


Addendum…..


Well… the Marantz mod is basically done and works amazingly well .

https://forum.lddb.com/viewtopic.php?f=24&t=12221

I also just acquired a GDD-1 so that is now on the drawing board to create a board for it (or find an I2S output that can work; studying the options on the CS4226 now). First things first; the Onkyo ED-901 is the next mod I’ll be doing.

So after thinking about this more, I'm having a change of heart about seeing what I can do w/ the GDD-1.

I think I have a better chance of using an I2S > S/PDIF board such as this to tap the CS4226....





I tried to use this board on the Marantz DP870 but in the end I should have known better that it wasn't going to work as the LC8904Q IC wasn't I2S but EIAJ (the GDD-1 is another story as it definitely is I2S).

Right now, this is what I am going to try and tackle on the CS4226 w/ the I2S > S/PDIF board.





The only question mark is the data output as I'm not sure if SDOUT1 or SDOUT2 or neither is the data line that can deliver the data to make a mod work. If it turns out that it doesn't work, then I will shift to the same concept I used w/ the DP870 which is to create a Mux circuit for it once it’s verified to have good switching voltage.