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Naim CD3.5 mods (not on Acoustica)

Theoretical concerns about removing the anologue output stage....

1)I have removed several orders of filtering with their phase shifts. Naim say the player has linear phase. So with the dac output only being filtered by a 680pf cap (1st order) maybe I no longer have linear phase. Minimum phase maybe ? Phase distortion maybe a factor now.

2)Output impedance of the 3.5 is quoted as being 10 Ohms. Output buffer is now removed and so I am now at the mercy of the dac's output impedance. The TDA1305 data sheet says dynamic output impedance is 10 Ohms (exactly the same as with output stage). Output load resistance is 3K ohms and output load capacitance is 200pf - not sure these two are relevant. I need to learn how the output impedance is affected by my 680pf 1st order filter and 10uf coupling cap with 100k to 0V.

3)Lower voltage output now (down from 2V to 1.5V)

But when it sounds as good as it does to my ears I am not sure why I am worrying about the above ?

One thing I will try is some different coupling caps. Currently using 10uf 50V Wima MKS 2XL. Might try some 1uf paper in oil jobbies as long as the bass is not rolled off too much.

Any comments ?

Stu
 
Try something better for the coupling caps. 2.2 - 4.7uf Wima MKP4 should fit... even down to 1uF if your net load is 100K, will be fine.

As for the rest - if the dac is really 'seeing' a 100Kohm resistive-input load as you have it configured (ignore the 680pf feedback cap, that is intrinsic) I wouldn't''t worry at all: these are limits on what the dac's internal opamp-buffered output can reasonably actually drive (3kohm; 200pF additional load) and meet min datasheet claims. If you are presenting it with a solid 100Kohms, no issue at all (any series coupling cap to such a load is not part of such a limiting thing)
 
Thanks for that Martin and putting my mind at rest.

With limited knowledge it is easy to start worrying about stuff one reads on the internet.

I will in due course try a 1uf coupling cap.

I dont know when to bloody stop with this cdp. Too much reading on diyaudio :rolleyes:

Some guy managed to bypass the internal opamps of a tda1549 (very similar to tda1305) and use something different. I have no idea what he used or how he did it but very interesting non the less.

Stu
 
Still enjoying my sounds from this cdp with only first order passive filter on the end of the dac. Been too busy listening to music to do any more mods. Probably a good sign eh ?

Doesn't stop me thinking about potential things to do

1) 1uf of exotic cap for coupling
2) bypass tda1305 internal opamps somehow and try opa627
3) cathode follower buffer
4) shielding chips, front panel and ribbon cable to front panel (copper foil)
5) coax cable for rf signal from transport to motherboard

Too many things to try that i can be bothered to lst. I started to read Ray's 2000 page Marantz mod thread on diyaudio. Got to page 100, only 1900 to go. Lots of interesting stuff in that thread. Not sure i can stretch this thread out to 2000 pages:eek:
 
Got to page 600 now on the epic marantz 63 mod thread. Some stuff relevant to all cdp's. Even our very own MC pops in now and again to put them straight :D.

It's funny that now i have no anologue stage as such my attention has moved to the digital side. I never paid it much attention as it was always a mystery to me. Anyway, it's amazing what a lot of reading and circuit tracing can do. I now know what is generally going on. Naim like to double up on the decoupling caps - wima 0.1uf right on the load and 10uf electro or tant a little further up stream. These rubycon electro caps are now 21 years old and i had my player continuously powered up for maybe the first ten years of its life. I know better now i must add. I feel the need to change all these old rubycon 10uf 50V parts.

I was thinking that as i have a shed load of 10uf 25V oscon's doing nothing then i might as well use those for the decoupling on the digital side. 100uf 100V NHG caps for lm317 inputs (coz i have them) and 10uf silmic II's on the lm317 output. Dont know what to put on Cadj but i gather it is not critical. First though i am going to change all the Rset resistors that haven't been done already for a couple of green led's. Even if there is no audible improvement i like the christmas tree look :p
 
Some things i have learnt about the digital (dark side). There are bloody resistors everywhere !

Here is my grasp on what goes on in this ere 3.5. Laser reads disc. Sends eye pattern anologue signal to TDA1300 combined rf amplifier and laser supply chip in swing out tray. This chip amplifies the signal and sends it along the ribbon cable to the motherboard. Shame it wasn't coax which is better for this i believe. Signal goes from ribbon cable socket straight to the combined servo/decoder chip (which is a 64 leg square monster). Some decoupling here before the SAA chip but no resistors in line. Decoder deals with this anologue signal and turns it into a digital signal which is basically in the i2s format. 3 data lines from decoder to dac via 330 ohm resistors on each line. Pcb tracks are very short between the 2 chips which is good. Dotb (spdif out) is disabled by the ram chip so no digital out from this player even if you wanted to.

Dac over/up samples the digital signal and converts to anologue (obviously). IV conversion also happens within the tda1305 and there are a couple of opamps ( wish i knew which model) also internally which amplify the signal a little and also act as a buffer. The anologue output stage comes next which filters the signal before a final buffer stage prior to the outside world. As i have bypassed most of the anologue stage i just have a 1st order filter provided by a 680pf polyetyrene cap betweentwo of the dacchip's pins and then 10uf coupling cap and 100k to ground to deal with the nasty 2.5Vdc on the output.
 
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The ribbon cable from the front panel mostly has connections to the pic (ram) chip but also dc power for the micrel display chip on the fascia pcb and of course 0V. There is one trace though that runs off to the transport ribbon cable socket ( from memory). The transport ribbon cable carries the connections to the decoder but also the two tda7073at power driver chips which control the laser focus, sled, motor functions etc. There are also some connections to the ram chip and obviously Vdc and 0V.

It would seem that most of the chips talk to each other to varying degrees. Decoder talks to dac, inverter chip (if you still have one), clock, Ram chip,, tda7073at power drivers, tda1300 chip in transport tray. You name it and the decoder is connected to it :) . Dac talks to clock and decoder. Ram chip talks to front panel, decoder, transport, power drivers. Power drivers talk to the transport, decoder and ram chip.

More insights when i have remembered and learned more ( but then martin has probably forgotten more about this cdp than i have ever known).:(
 
Thought I would update this thread to say that I haven't done anymore to this player in quite a number of years. It's still going strong on the original laser 25 years after production. Might just be luck but who cares, I get so much enjoyment out of this pimped up spinner I am not sure I can be arsed to do anything else to it. Oh hang on ;)

Martin Clark deserves a meddle for his acoustica site and the advice he has given us plebs on here over the years. But go to town (three EI transformer widings) on the dac power supplies and you won't regret it :)
 
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