CDP output stage from scratch

[coredump]

G'day,

after looking at the schematics of my NAD540 CDP I came to the conclusion it might be a good thing to build a whole new analogue output stage for it. I'm planning to go for a Naim-like approach (external PSU, connected via XLR though).
There are commercial solutions out there (Zapfilter, CD-OUT), but I haven't listened to either so I figured I could DIY something, as I won't know the outcome of this either.
Any recommendations on the way to go? Would it be worthwhile to buy the CDX schematics and try cloning its output stage? Or are there sort of 'proven' designs that I should go for? Are there any 'obstacles' concerning the DAC (Burr Brown PCM1716) I should keep in mind?

Thanks for your comments,

Oliver

 

[ron]

A pcm1716 dac already has a built in analogue output stage inside the chip as well as using another circuit in the output stage in the cdp, some people just bypass the cdp's analogue out circuit and add a suitable coupling cap on the output pins and run it straight to the phono outs, unfortunately you can't bypass the internal op-amps in this type of chip so you can only improve the output stage in the cdp.
There is also 2 electrolytic capacitors on a couple of the pins thats used for de-coupling the analogue stage, the quality of these 2 caps are critical to get the best out of this chip

 

[ron]

pin 13 is right out
pin16 is left out

You will probably loose some gain if you just use a coupling cap on the output of the pcm1716 depending on what type of pre-amp you use.
Those electrolytic de-coupling caps are on pins 11 & 18

 

[coredump]

ron,

thanks for this excellent information; I think I will check out the chip's de-coupling caps first. The service manual tells me they're 10uF, 16V types (20%). I guess it would be wrong to go for a "bigger=better"-approach here?

If that doesn't help I'll try to wire up a reversible passive output.

After that, it's probably a new DAC or a new player...

Best regards,

Oliver

 

[ron]

No probs

I modded a Arcam Alpha 7 for a friend, this uses a pcm1716, The decoupling caps (11&18) was changed to 100uf Elna Silmics, 2.2uf film coupling caps straight from dac pins to phono sockets, diodes changed to 11DQ10 (does not sound good in all cdp's) all caps that decoupled the +5 & -/+15v changed to a mixture of Cerafines and Silmics.
After the mods this cdp sounded a hundred times better than the stock model, it wasn't a giant killer but for the price it cost was well worth it, my friend was well happy
Give the Nad a try, for about 30 pounds you should get good results