Marantz CD72 MK2 SP Modification To DAC

The old CD player is probably sound better than currently "Hi-Res" audio player. Either you music collection has been shift to the streaming provider, or just like me, converting all my CD's collection to WAV files and stored in one media library for more practical use, then unfortunately this CD player might not be use anymore.

In the most cases, those high quality CD player are already broken. Especially for the optical lens which will wear by ages, then replacing those part might be cost very expensive. There is also an issue that the reliability of replacement part might be not as good as the original. It is frustrating to keep using those old CD player for sure.

I took the example of this Marantz CD72 MK2 SP I owned. I loved the sound quality from this CD player and of course the look. It is copper chassis inside out.

More detail, the DAC IC using in this unit is the best high-performance single bit DAC. It is the TDA1547 along with SAA7350, aka DAC7.

So I think, I will modify this CD player to become a stand alone DAC unit. The goal is having the same sound signature from this old CD player but with better input flexibility like the stand alone DAC unit.

Note before doing this mod, I make sure the unit that I want to modify is previously working properly, except for the CD optical and mechanism that I want to rid it off.

I will taken the digital signal before the oversampling IC. The working principal is almost the same for different type of CD player, but I never guarantee the method of this modification will be similar.

As always, please treat this blog and all the posting here as entertainment and educational only. No guarantee or liabilities from me if any loss occurs from following this article.

Marantz CD72 shown the general schematic on the top of it's copper plate. It is clear that the digital input will taken before the oversampling IC, in this case is NPC SM5840FP.

I downloaded the datasheet and the Marantz service manual first to make sure everything correct before I start to modify this unit.

First things first, all CD player has the Mute function. This to make sure no pop noise during CD track changes or during Play or Stop the CD. Since I will modify this CD player to become a stand alone DAC, so this Mute function is not needed anymore and should be disable.

For this Marantz CD72 unit, the main Mute control is located near the SAA7350. I put the circle on the photo below, so you know the transistors and jumper that need to remove.

They are: QN01, QN02, QN05, QN06, U535, and U684.

The other Mute function are located near the NPC SM5840FP oversampling filter IC. It is a resistor marked R519. This resistor need to remove in order to make this oversampling IC works.

Other than that, I find the other Mute function bellow the clock generator IC, PCF2705P need to cut and remove. It is located at U696.

At this point, the Mute function are totally disable. The next step is find the pins for the digital input.

The SM5840FP here is configured to having an I2S input signal. This is a good sign since today digital input more rely on I2S for simple and better digital protocol. I pull out the jumpers that connecting between the CD Player decoder IC to this SM5840FP. They are U678, U679 and U680.

I take the top jumper as the I2S input.

U678 for LRCK

U679 for BCK

U680 for Data

We don't need MCLK, or master clock since I keep using on the board master clock generator from PCF2705P. As long the receiver unit connect to this unit is 256fs, then it will works accordingly.

The +5 volt and ground to powering the receiver unit can be taken on the pins as illustrated in the photo bellow or you can make it separate voltage regulator by using simple 7805 IC.

I test using PCM2706 USB to I2S converter on my previous TDA1543 portable DAC project. I also connect the USB input to my smartphone using USB Audio Pro apps. It is works perfectly since the first time I turn it ON.

I also test the on-board headphone amplifier unit which sound pretty okay. But the sound quality from this headphone out is not better than using the eternal amplifier connected to the fixed output at the back panel of this unit. It is because the analog signal that goes to the fixed output goes though the HDAM modules.

At this point, if you prefer to using the USB as the digital input, then you are all done.

But since my digital player are mostly using SPDIF output, so I will not using this PCM2706 USB to I2S converter. I will assembly a new receiver board using CS8412.

I tried to provide the schematic, but I find Peter Daniel has posting the same schematic on diyaudio.com except it is for dual TDA1543 balanced output. The CS8412 SPDIF to I2S schematic is the same and you can use this as well.

Image courtesy of diyaudio.com

To make this modification sleek, I will using the back panel of this unit previously the RCA digital output to become the input. I determined all the connection surrounding that RCA plug and remove them accordingly so they are not connecting to anything in the board. The new SPDIF cables taken from CS8412 module will connect directly to this RCA plug.

I leave 3 wires from CS8412, they are pin 7, pin 28 and ground to indicate the power ON and Signal Lock LED. Each connection to LED are series with 200 ohm resistor.

It is all done and this is the time to assembly all parts together.

Same with the front LED light. I will using the PLAY button LED as power on indicator and PAUSE button LED as digital lock indicator. All surrounding LED connection need to be cut to avoid any shortcircuit.

At this point, I really love to have this unit working again as a stand alone DAC. It works flawlessly without any sound degradation from the SPDIF input.

Disclaimer: Any statement and photos in this article are not allowed to copy or publish without written permission from the writer. Any injury or loss from following tips in this article is not under writer responsibility.