Sony CDP-950 Mods - NOS mod produces a lot of noise when CD not playing

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Hello All,

I have been working on a SONY CDP-950 (Aug. 1988 with TDA1541A) over the weekend and have hit an issue with spurious sounds when the CD is not playing. The original sound of the 950 was very muffled and veiled. Knowing that the TDA1541A is capable of so much more, as I also have a tweaked (opamps and non-oversampling) Rotel 955AX, I read some earlier posts (eg Mod CDP SONY 950) and broke out the soldering iron.

Step 1 was to remove the 5532 opamps (IC400 and IC500) and the 4 x muting transistors (Q401, Q402, Q501 and Q502).

950-partialpcb.jpg


I then dropped in a couple of LM4562 opamps and after a couple of hours of use the sound opened up with a better soundstage and clarity, but still a bit veiled in presentation. The lack of muting transistors caused a thud at power on/off, but that is something I live with on the 955AX and is not a problem for me.

Step 2 was the the non-oversampling mod, described on the lampizator web pages. I did all the work on top of the PCB by snipping jumpers and resistors. The basic mod steps are as below. In the circuit diagram IC521 is the TDA1541 and IC302 is the CXD1125.

In Sony CDP-333ESD with one TDA and CXD1125 demodulator and typical digital filter CXD1088
we go like this: the whole job is done on the bottom of the PCB.

  • data goes straight from CCD1125 leg 78 (lift R311) to TDA pin 3 (floated)
  • BCLK goes from pin 76 of the 1125 (trace it to the first breaking point - a jumper or resistor and then float it by lifting that part) to the pin 8 of the 74HC and from there straight to TDA pin 2 (joined with 4 and together - floated)
  • LRCK goes from pin 80 of the 1125 (trace it to the first breaking point - a jumper or resistor and then float it by lifting that part) to the pin 1 of the 74HC and from pin 12 (output of 74HC) to TDA pin 1 floated

950-NOS.jpg


Now the sound really opened up and sounded way better, but there was a downside. The expected garbage music when skipping tracks was present, but there was also a lot of quite loud electrical noise all the time when the CD was not playing. Really not good at all.

I then put the muting transistors back in, which has cleared up the noise when the CD is not playing, the expected garbage when skipping tracks remains.

I'm wondering what to do next as I prefer the sound without the muting transistors but cannot live with the noise when the CD is not playing. I've double-checked the wiring on the NOS mod and everything seems to be OK, so I am wondering if anyone else has experienced this or has any suggestions.

Thanks for any help
 
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The muting transistors should be transparent in operation... that's another debate though.

If you are convinced they impair the sound then try using JFET's in place of the transistors. You want something with a low on resistance (low Rds) and you need to confirm the logic levels are suitable to fully pinch off the FET. The muting wont be as deep as the transistors but could be adequate. You could also increase the series resistance through the chain to increase the effectiveness of the FET's
 
I think a lot of Sony's have separate Analogue and Digital Mute Lines.

The Analogue Mute will still be working but the Digital one (if you've bypassed the Digital Filter) won't.
You will need to make the Digital Mute line also activate the Analogue Mute Transistors/Relay. I am just working out how to do this on a Sony CDP-227ES.

P.
 
I think a lot of Sony's have separate Analogue and Digital Mute Lines.

The Analogue Mute will still be working but the Digital one (if you've bypassed the Digital Filter) won't.
You will need to make the Digital Mute line also activate the Analogue Mute Transistors/Relay. I am just working out how to do this on a Sony CDP-227ES.

P.

That's jogged something in my brain. I *think* I recall reading something when I was researching which cd players to go for - the Rotel and the Sony models were on the short list. I do recall something about having to tap something in the CXD1088. Now, this could be a figment of my imagination but it does ring a very small bell. Time for some googling.

Edit - I did a quick google on this and couldn't find anything obvious, so it may well be my brain misfiring.
 
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The Analogue Mute from the System Control IC301 Drives IC940 then Q521 and then the Mute Transistors themselves.

The Digital Mute comes from the System Control IC301 and feeds the Digtal Filter on Pin 9 but won't have any effect if you've bypassed the Filter.
You should be able to use the DMUTE feed though to also drive the Muting Transistors using another Transistor, it may need inverting but should be possible.

It'll be the DMUTE that mutes during Skip etc.

P.
 
I found the below in the link on the lampizator website I referenced in my original post. This was for a Sony CDP 227ESD. I wonder if we can glean anything from the schematics for that model and how it implements things ?

I was getting some noise when the CD player was not playing any CD's but this goes away if you switch the Digital Out at the back of the CD player off.
 
Interestingly, I tried some opamp rolling tonight. I took a well used pair of LM4562 from my bugle2 and they sound a whole lot different than the initial pair of 44562 I had used. Better clarity, less rolled off at the top-end and a lot more punchy and musical.

I wonder if I had a duff pair in the spares box - as I am not sure I believe that opamps need much/any break in time ?
 
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The Mouser order of capacitors arrived today. I replaced 4 x duorex caps in the audio output, more as a precaution against leaks than anything else, and not much to report about change in audio.

Not sure if it replacing the decoupling caps on the TDA as the sound is pretty damn good at the moment. Very spacious and some good reach down into the bass levels.

Oh, and the duff pair of LM4562 were cast into the bin. I tried them in the Bugle and they were subpar after more than 48 hours of burn in time. I'm guessing I bought these off ebay in a moment of weakness
 
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Yeah, like I wasn't going to replace the decoupling caps .....

All 14 replaced with 4.7uF 63V WIMA MK2 caps, which all fitted neatly together on top of the board. I'll give them some more time to settle in and my ears to get used to them. but initial thoughts are improved bass and highs.
 
Hello everybody.

I have been working in avoiding the noise when skip forward or back a track in a NOS sony 227ESD using the method on the lampizator webpage. The key point is, as Percival007 said before, the DMUTE line. I have solved the problem using a 74LS32N (OR GATES). You have to find the Analog mute line and D mute line, make the OR operation with the 74LS32N and then connect the output of OR opertion to the input of the 74HC02P that control de activation of the mute relay. I have done the MOD removing two bridges in the board. If anyone is interested I can post photos of the MOD.
 
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Thank you...

This is the 74LS32N (Or gates), I have just used one gate (pin 13 A Mute, 12 D Mute, 11 output to 74HC02).

Blue wire A Mute
White wire D Mute
Red wire, AMute OR DMUTE, to 74HC02 that drive the mute relay

Pin 14 5V

Pin 7 GND

I have removed two bridges, where the white wire is connected and when the red and blue wires are connected.


From the first bridge, I take the D mute signal (white) that goes to filter (that is unused with oversampling).

From the second bridge I take the A mute signal (blue), and the second point goes to 74HC02 that drives the mute relay.

It is easiest to do that explain it!
 

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These other photos are related to NOS mod based on lampizator webpage (but removing the resistors that are unused...) in my 227ESD.
 

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