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Old 16th October 2011, 06:10 PM   #1
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Default Sony CDP-491

Hello everyone,

As the title says I've just bought a Sony CDP-491 (got it very cheap, as it's going to be my first modding experience).

I'm after some info on mods worth doing, I've looked around the site and can't find anyone who has done one.

Already I have put some rubber between the chassis and the CD mech to reduce vibrations. I'm wanting to remove the muting transistors next (but I can't find a complete service manual with the circuit layout, If anyone has one I'd appreciate it). Is there anyway to identify them on the board, ie, are they always near the RCA outputs?

Also would the caps be worth upgrading, it has rubycon caps in at the mo, but was thinking about the Nichicon muse caps. And the last question, is it worth doing anything with the crystal?

Thanks in advance
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Old 17th October 2011, 06:36 AM   #2
Mooly is offline Mooly  United Kingdom
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Muting transistors usually are near the output sockets. Often there are two per channel and identified as shorting the output line to ground. They all follow a similar pattern so just look at any other players circuit to see how they work.

You could replace them with FET's which would maintain the mute function and be essentially transparent to the audio.

Crystal... leave as is.

Biggest gains might be looking at and replacing opamps used in the final audio stages.
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Old 17th October 2011, 03:29 PM   #3
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Thanks Mooly,

I had a look at the circuit last night and removed 2 transistors next to the output stage, didn't realise that there are 2 per channel so will have a look tonight.

Will also check to see what opamps are in use.

Would moving the power supply transformer of the main board make any difference?

I bought this player as my first step in to moddying so it will be a fun learning curve.

So far I've mounted some rubber under the CD mech, removed the headphone socket and removed the muting transistors. Slight difference in sound, abit more clearer but nothing major.

I wonder if I'm starting with the right player lol
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Old 17th October 2011, 07:59 PM   #4
pchw is offline pchw  United States
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Hi Mark,

What a coincidence. I was doing the same thing after I found I had an el cheapo Yamaha CDX-393 in my parent's garage over the weekend. I couldn't find any schematic but opened it anyway. Hopefully, I will learn something and can improve it along the way.

In the CDX-393, there is one opamp (JRC2068DD) after the Dac's shaping network.
After the opamp, there is a output cap and then a series of 2 sets of resistors and transsistors (2C2878) which seems to match what Mooly suggested.

For a little thread jack, if you don't mind, Mark. A couple questions for Mooly,

Can I bypass the muting circuit and wire the output cap directly to the output RCA which has a 22K to the ground?

if so, the output cap is a Elna 22uF El Cap. Can I replace it with a small film cap and increase the 22K?


I also planned to replace the 2068DD, but the PS supply is only +/- 5V, so the choices are relatively limited. I have a couple opa2134 in hand, which should work with this voltage.

Thanks!!
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Old 18th October 2011, 07:29 AM   #5
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by mark31385 View Post
Thanks Mooly,

I had a look at the circuit last night and removed 2 transistors next to the output stage, didn't realise that there are 2 per channel so will have a look tonight.

Will also check to see what opamps are in use.

Would moving the power supply transformer of the main board make any difference?

I bought this player as my first step in to moddying so it will be a fun learning curve.

So far I've mounted some rubber under the CD mech, removed the headphone socket and removed the muting transistors. Slight difference in sound, abit more clearer but nothing major.

I wonder if I'm starting with the right player lol

tbh the only real gains you will get (and the biggest by far) will be to look at replacing the opamps. That can make a massive difference.

Moving the transformer won't really do anything.

As a first project it's fun to do though. Maybe fit sockets so you can try some different opamps.
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Old 18th October 2011, 07:47 AM   #6
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by pchw View Post
Hi Mark,

What a coincidence. I was doing the same thing after I found I had an el cheapo Yamaha CDX-393 in my parent's garage over the weekend. I couldn't find any schematic but opened it anyway. Hopefully, I will learn something and can improve it along the way.

In the CDX-393, there is one opamp (JRC2068DD) after the Dac's shaping network.
After the opamp, there is a output cap and then a series of 2 sets of resistors and transsistors (2C2878) which seems to match what Mooly suggested.

For a little thread jack, if you don't mind, Mark. A couple questions for Mooly,

Can I bypass the muting circuit and wire the output cap directly to the output RCA which has a 22K to the ground?

if so, the output cap is a Elna 22uF El Cap. Can I replace it with a small film cap and increase the 22K?


I also planned to replace the 2068DD, but the PS supply is only +/- 5V, so the choices are relatively limited. I have a couple opa2134 in hand, which should work with this voltage.

Thanks!!
As above, I would perhaps fit a socket and then you can try different opamps.
As well as the OPA2134 you could try the OPA2604 which is genuinely different in the way it sounds. That works on -/+4.5 volts and above.
Try a TL072 as well... don't discount it until you have tried it

The problem with muting transistors is in the way they are used and so just bypassing them isn't enough as they are still in circuit and presenting a non linear load to the opamp. So they have to be removed if you want to dispense with them. The series resistors will be low in value (around 100 to 470 ohm ?) and should be left in place as they aid stability when capacitive loads are connected (interconnects). This is of more importance when using better opamps.

If you replace the cap remember it forms a HP filter with whatever impedance is hung on the end of it... such as your amp. If the DC offset is zero from the opamp you could try linking the cap out as long as you are aware of the consequencies.
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Old 18th October 2011, 07:52 AM   #7
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To both of you...

Although in 98% of cases there is no problem whatsoever in replacing the opamp/s (and certainly no damage will occur) I always advise checking for stability and so for completeness I always add this,

Swapping Op-Amps... you have checked to see it's stable haven't you ?

The type of opamps and players you have and the way the opamps will be used and I don't think there will be any problems whatsoever.
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Old 19th October 2011, 02:20 AM   #8
pchw is offline pchw  United States
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Hi Mooly,

I did read your thread some time ago. Makes sense.
My initial goal is to see how far I can go with my existing parts. I already put a
socket in and plan to try opa627, LM4562, LM6172, opa2134 that I already
have.

I meant to say to remove the mute circuit but use the wrong term :-) There
is a 380R in series to the output. I will leave it there to avoid unnecessary trouble
and remove everything in between. Since the integrated amp that I intend to use has
47K input impedance, Can I replace the 22uF electrolytic with something like 4.7uF film cap without losing much low frequency?

Thanks!!
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Old 19th October 2011, 06:57 AM   #9
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4.7uf and 47k has a -3db point of 0.7 hz so no problem with losing low frequencies.

The OPA627 is a single not a dual opamp.
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Old 19th October 2011, 07:20 AM   #10
pchw is offline pchw  United States
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Good morning!!
I have a brown dog adapter to use 2 opa627; however, I think it is a total waste in this cd player. But like you said, it won't hurt to try :-)

Cheers,
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