Using Philips CD940 as a transport

[philpoole]

Hi All,

I thought I'd finally start a thread showing what I've been up to, and what I'm working on.
I started all this with a discrete headphone amp, then by modding a Marantz CD63, then I decided to move on from there (only because of incompetently breaking the '63 ).

So, I found an old Philips CD624 and sniffed off I2S and built a TDA1541A unfiltered NOS DAC.

I was shocked. This very basic DAC (poor regulation {often shared PSU with headphone amp}, built on stripboard, probably very poor grounding schema etc) sounded different, but on a par with my old modded Marantz. Obviously, I couldn't do blind testing, but they were definitely in the same ball park.

I really liked the CD624, with its CDM4, but I wanted to keep it unchanged (so I have an I2S source for messing about with), and I wanted to have a transport of sorts.

I did consider building a CDPro2, and I will probably have a go in the future, but in the end settled on CDM9.
I eventually found a Philips CD940 on Ebay. As a player, what with bitstream and all that, it sounded alright, but somehow lacked in dynamics or something (I gather this was quite a high end player in 1994, I've no idea how much it should of cost brand new {I threw out all my old What Hifi's of that era when I went away to uni}).

Anyway, it worked, and I wanted to make it my transport.

Firstly, I built a variant of a Kwak clock (its single regulated, amongst other minor changes, and probably no improvement on the original v7), which I powered with a seperate PSU, and mounted all this on the back panel.

The new clock signal is fed directly to the decoder chip, rather than to the DAC and then fed through, and the DAC and digital filter circuits, along with the IV conversion and headphone amplifier etc have had the power completely removed.
So, the only things alive on the board are the CPU, the display, the CDM and all its servo circuitry and the decoder.

Beyond that, I haven't actually done any mods as such to the board. I hope to improve the regulation, but its not a priority. I'm sure it would be better if this was done (based on my experiences with modding the '63) but thankfully, the power rails aren't being shared with too much.

To complete the 'design', I added a differential output for the I2S signals. I did this using two NatSemi DS9638 (Dual differential Line Driver) to convert the TTL signals into basically RS-423. I built a plug in board that plugs into a DIL socket where the I2S-SPDIF converter chip normally sits (I can't remember its name at the moment). For the pins of the plug in board I wasted a few TO220 packaged transistors (nothing like DIY LOL) and managed to make a piggy back board, that sniffed off I2S, powered the RS423 chips, and allowed the SPDIF encoder to remain in circuit (well, would of if it wanted to continue working! - nevermind, I don't need it any more).
Anyway, this daughter board drove 8 wires, in 4 CAT5 twisted pairs to a 9 pin D type socket on the rear of the transport. Feeding BCLK, DATA, WS and also the clock signal (which I was hoping to use for reclocking at the DAC, but haven't, and might well disconnect it).

At the other end, I had the complimentary DS9637 receivers (make sure the two inputs per receiver have a 100 ohm resisitor across them or it doesn't work {I learnt that the hard way } - its not too clear in the datasheet), feeding my first, still experimental DAC.

Using a metre of cable, I works fine, and I can tell no difference between a metre and 3 inches of cable. I think I have about 5 metres of CAT5 in the house, and might try and see what its like with a cable run of that length, it should be fine according to the datasheet, and I don't need it that long, so it might happen maybe one day.

I'm really happy with the transport, and if I ever get back to doing stuff to it, I'll be modding its regulator stages, sorting out grounding, mechanical stiffening and dampening of the chassis etc. At the moment though, I need a new DAC...

Pictures to follow.

Cheers,
Phil

[philpoole]

Firstly, here's an overview of the modded CD940...

[philpoole]

Here is the Kwak-esque clock on the rear panel...

[philpoole]

Here is the piggy back board, used to output the diff I2S signal...

[philpoole]

This is how it connects to the outside world...

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Using a 9 pin socket..

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Only a few pictures left (I promise..)
This is at the DAC end, the RS423 receiver, top left, and the DAC itself...

[philpoole]

And (finally), how it all connects together...

[kaulagalvis]

cdm9 is a good transport,I have cd930 ,I replace all caps for panasonic fc and sanyo oscon,replace opamps for ad826,replace sockets,power cable.its has a good sound.sound is light,detailed.its good for light music.other my cd (cd204,cd303) have a other sound.its "difficult" simillar to "analog",but it very intresting too.

[philpoole]

Hi Kaulagalvis,

I chose the 940 because of the CDM9. I really like it too.
I'm really pleased with how its turned out so far.
I think there are a few who have commented on the 930/940's poor grounding scheme, and poor decoupling (there is one servo chip that has no local PS decoupling cap near it at all).

Thankfully, if there are earthing issues (which I think I saw), on mine its been limited by disabling 50% of the circuitry, and using a differential output, the transport's earth is isolated from other parts of the system.

As a player, I think you're absolutely right, this player is quite modifiable.

Have you considered fitting a new clock module to the DAC?

Cheers,
Phil