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Old 12th October 2004, 03:22 PM   #1
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Default Out-stage for Sony CD players.

Resume of Sony CDP players mod's “dead” thread.

Dead thread.

I am working around the output stage of a Sony CDP player XB940 SACD, the XA50/30/20 and XB940/920/720 are similar (VC24+ CXD2562Q or CXD8735N - DAC chip’s). I tested various configurations (OP-Amp’s / discrete) the last one uses a trans former to unbalance the output, but it can be used in balance mode. The sound is really very "natural" and the design is quite simple.

The first and easiest way to improve this player is to bypass the “current pulse chips”, so you get the DSD directly from the dac in a differential voltage mode, both at 2.5V dc, I think you have already seen this in a few sites (VSE, LC-Audio etc...) For the output stage, there are various possibilities : Op-Amp, discrete differential to single ended, transformer…
The goal was also to unbalance the dac outputs, to make them compatible with my preamp/crossover or at least to get the choice between balanced or unbalanced outputs.

About the current pulse converters :

Lars Clausen (In another thread) :

“This is SONY's own explanation of the Current Pulse Converter. ...

Sony’s engineers for the "Extremely high Standard" (ES) series created other decisive factors for excellent sound quality: the Current Pulse D/A Converter and R-Core Transformer. Unlike standard D/A converters, the new Current Pulse D/A Converter does not use a voltage-pulse output. Instead it operates with current-pulse output for constant signal quality. Changes in voltage amplitude (e.g. from mains fluctuations) are eliminated and harmonic distortion is minimised. The R-Core Transformer on the other hand ensures a stable voltage supply independent of load. “

My own tests on them :

The current pulse converters are very sensitive to their supply (+7,-7V) and their reference. I tested in this way, after the current pulse converters, with battery power supplies and batteries for the reference, (battery for the reference was the biggest improvement), this modification is not as effective in the reproduce sound quality as the one I describe here, and is more complicated and difficult to implement. Well… I must say that I didn’t test it with a jocko homo I/V converter...

In order to “try” the signal directly at the output of the dac, and to achieve a smooth filtering, the first and simple way, was to use a “good” op amp. The AD826 or AD828 are among the best ones for this application in my knowledge and the less tricky to implement. I prefer the AD828 (Gv>2) but the AD826 with a current sink of 5mA to V-. will give you about the same results. I didn’t tested the most recent ones from Analog Devices…
This simple mod, bypassing the current-chips and a few Burr Brown op amps improved greatly the sound in all the ways !

Here there is the diagram for picking up the balance bit stream output of the Vc24+ at the input of the “current pulse converters”. Thanks Elso, Ergo, Christer for the thread and VSE for diagram.


About the same subject :

Christer’s thread.

Out-picking.

For the output VC24+ output resistors I choose a rather high value 39 kohms. I tested 4,7k, 10k, 22k, 33k, 39k, It’s seems that high values (> 30 kohms) sound best. Difficult to say more, without the schematic of the VC24+… But now than the global design is defined I will do more accurate listening tests on it.

My first “evaluation designs :

XB-output1.

The DAC output offset between +out and –out, is not 0V but a few mV, and moreover, change according to the playing mode CD (pcm) or SACD (bit stream).
So in this second schematic I implemented a dc-servo.

XB-output2.

Extremely satisfied by these firsts improvements, I went further and carried out a very simple circuit based on fet followers.

Fet evaluation schematic.

The sound was astonishing real, natural, when you reach this degree of transparency, sometimes in some cd’s you wonder whether certain noises come from ! Your room ? Are they recorded ? Until you listen the track once more…

Following these encouraging results I refined the design to make it reliable in the player, now it includes a dc-servo. and a transformer.

Fet with dc-servo.

Fet final schematic.

Fet final schematic xfr-less.

The version I prefer, is the fet one with servo and transformer, the op amp version is not very far in terms of "very very good reproduction", but the last fet version with transformer is simply real, well... much more real.
Actually I use the transformer, to unbalance the output (my crossover input is unbalanced). I tested the schematic below, with just one "leg" the +output, and I increase R101/110 to about 390 Ohms to get 0V dc in the output. It's just a very little better than the transfo version, but unfortunately a little bit noisy, well i can hear it with my ears near the horn of my TAD's (of course, noise is cancelled in the balanced mode).

Noise in my 20 kHz FFT is at -114 dB in CD mode and balanced, about -130 dB in SACD mode.

Fet's are matched by pairs and about 9 mA of Idss.
Transfos are LUNDAHL LL5402, because I have 2 of them from an unused audio distribution amplifier, and their characteristics seems to match my requirements. I will test next days an LL7902.

I am not very qualified in transformers : brands… , low/high “copper”, amorphous… etc… Jensen’s and Lundahl seems to be good devices, as they are rather expensive I can’t make a lot of tests, so some advises are welcome.
The output impedance of the fet followers is about 30 ohms, but it's better not to load them at less than 3 kOhms.

For these first evaluations I used batteries.
I tested rather quickly an emitter and a source follower power supply (capacitor multiplying) after a schottky rectifier and an RCRC network, it seems not to be far from batteries, but I need more listening tests to go further. I will work on it after building the output stage on a pcb, and having chosen the right transformer.

I am trying too, a discrete differential input, differential/single ended output, to solve the noise and in a lesser way the filtering (in the single ended mode), as well as to get rid of the transformer. But it is for in a few weeks, for now I will continue to improve and complete this fet design, and use it later as reference.

Elso :

“Many things could be done to improve the Sony: powersupply, bypassing, other opamps etc. etc. The many SMD parts discourage me from doing it…”

Yes I know… But the modification is worth the sorrow! And with a dedicated pcb, just 8 wires from the dac…

Useful links :

Toshiba FET’s.

LUNDAHL transformers.

JENSEN transformers.

Analog Devices datasheets.

By the way… Have a look to the AD826/828 simplified diagrams VAS, output…
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Old 12th October 2004, 04:20 PM   #2
EUVL is offline EUVL  Europe
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Default Great Effort

Richard,

Great effort, useful information. Many thanks !!

A few pictures would make the thread even more perfect. : )

Keep it up,
Patrick
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Old 12th October 2004, 08:34 PM   #3
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Hi Richard,
in the photo the output tranny of my XB720. They directly couple the dac to RCA, just with 8 68hom resistors and a 1.6n cap.
You can also see the Tent's clock and the discrete regulator for the dac. Dac and clock both has their own PS and regulator.
The sound of this CDP is very good, bur actually I prefer the sound of a Teac VRDS8 with a discrete fet IV and output stage, and Kwak's clock. Much more impressive, expecially in the bass side, but anyway the Sony in his price class is really a very well sounding CDP.
Ciao.
Attached Images
File Type: jpg cd_4.jpg (73.2 KB, 3818 views)
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Old 13th October 2004, 08:31 AM   #4
ergo is offline ergo  Estonia
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To prove that I'm not in hibernation either

My Sony CDP-XB930

Ergo

PS. There are some real breaktrough I have made but it'll be for a week or so and then I'll get my own web iste up and I will explain....
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Old 13th October 2004, 08:40 AM   #5
EUVL is offline EUVL  Europe
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Default Breakthrough

I saw a Tent Clock and 2 Borbely Buffers, right ?
Battery powered ??

Patrick
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Old 13th October 2004, 08:56 AM   #6
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Default Re: Breakthrough

Quote:
Originally posted by EUVL
I saw a Tent Clock and 2 Borbely Buffers, right ?
Battery powered ??

Patrick
No, the power for Borbely buffers comes from the original PS. This is a point that needs addiotional tweaking.

The breakthrough is related to combination of CDP-XB930 and Behringer DCX2496 digital crossover. Part of it can be seen in the upper left corner of the pic.
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Old 13th October 2004, 08:59 AM   #7
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Hi, Michele,

Can you specify the type of transformer, its
wiring, caracteristics-datasheet ?

Wich is the load value you use in the secondary ? Volume pot, input stage ?

Wich ratio is the transformer ?

I recovered an old "Elektor" PCM63 converter, with which I would like to carry out some tests. I will be interested in having a look at your I/V converter, for later, when I will finished this project , so if you can say some more …

Thanks.
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Old 13th October 2004, 09:03 AM   #8
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Good stuff Richard.

Keep the updates coming
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Old 13th October 2004, 10:13 AM   #9
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Hi Richard.
For the transformers give o look at
http://digilander.libero.it/paeng/magnetics_frame.htm
The I/V converter is a D1 rearranged for singol ended output.
Ciao.
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Old 13th October 2004, 08:58 PM   #10
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I am looking for a test disc, CD and SACD layers, with sine waves at various frequencies and levels, if possible from 0 dBFS to –60 –80 dBFS and silence. I need it to measure and tune the circuit as well as I could.
I have just a "CD test disc" with 1kHz tone at -16 db. I recorded a few years ago a "complete CD test with a digital generator but unfortunately the XB940 doesn't play the CD'R !

I try a Google search ... Nothing in SACD, except some papers from Philips and Sony, both without sales mode indication, and going back to a few years.

If someone knows an easy way to get them... or others !

This W.E. I hope! I will work on the pcb with a complete set of outputs :
Xfr-Balanced, Xfr-Unbalanced, and Fet-direct-Out-Balanced at 0V-dc.
For the transformerless unbalanced circuit, I must design a completely new circuit, it will be for later...
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