Op-Amp Bypassing Problems In Marantz CD63 - diyAudio
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Old 15th March 2003, 08:32 PM   #1
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Default Op-Amp Bypassing Problems In Marantz CD63

Hello,

I am in the processing of changing op-amps in my modified Marantz CD63.

I have in the past replaced the originals with AD827 and OPA2134 and have had no problems.

I recently tried LM6172 and bypassed the op-amp power inputs to ground with 2.2uf tantalum and .1uf tantalum. When I did this, the sound became very distorted and I lost my display. I removed the bypass caps and the LM6172 and put back the AD827 with no bypass caps and everything was good again including my display.

Just today I replaced op-amps with AD826 and I knew that I would have problems bypassing the power inputs to ground. So what I did was place the same two caps as above between V+ and V- on the op-amp. Upon testing the unit I had distorted sound and lost my display again.

Has anyone experienced this problem in this cd player or other cd players?.

What should I do? I know the AD826 has a very good PSSR so maybe I do not need to use the bypass caps.

Any and all input would be appreciated.

Thanks
KevinLee

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Old 15th March 2003, 08:46 PM   #2
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If you have an oscilloscope I would check for oscillation because of the pretty heavy effects you describe.

Are the used caps new ? It could be that they are faulty and shorted the supplies. Try some normal new electrolytics to see if the faults occur again.

BTW The use of tantalum caps in audio is a curse. Nothing screws sound more up than they do. Use electrolytics or the fancier but more expensive BG or whatever as long as it isn't tantalum.


Quote:
I know the AD826 has a very good PSSR so maybe I do not need to use the bypass caps.
Always use bypass caps ( check datasheets of the opamps ) with opamps and the PSRR of the AD826 maybe good but that is not so relevant here. It is a high frequency device and has to be bypassed to prevent it from oscillation.
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Old 15th March 2003, 08:47 PM   #3
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Default Tantalum capacitors...playing games...

Hi kevinLee

Are you sure that the tantalum capacitors are not shorted???

Tantalum capacitors conected directely to a voltage surce(as the regulated power rails) have a tendency to short!

So cheek them!

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Old 15th March 2003, 08:51 PM   #4
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Hello Jean-Paul,

The bypass caps are new. They are not the same ones I used with the LM6172.

It was my understanding that tantalum were ok for bypassing as they are not directly in the signal path.

I am confused as when I replace the op-amps with AD827 or OPA2134/2604 and no bypassing, everthing seems ok.

I do not have a scope. What would you suggest as a next step knowing that I do not have a scope?.


Thanks
KevinLee
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Old 15th March 2003, 08:55 PM   #5
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Hello Tube_Dude,

Thanks for the advice.

Do you mean to check and see if the cap has changed value?

The caps only have a 35volt rating, but the circuit is only 12-13 volts.

Any more suggestions would be appreciated.

KevinLee
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Old 15th March 2003, 08:57 PM   #6
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What more suggestions do you need ? Just grab your multimeter and check if they have resistance !

As Tube_Dude explained these inferior ( for audio ) caps have a tendency of shorting so there is a risk your tantalums are shorted...

If the short is intermittent connect the cap to a power supply with a Ampere meter in between and turn up the voltage. If it is shorted sometimes you will see it on you meter. Certainly if you reach the higher voltages ( say 30 V max. ).

For safety reasons: wear glasses/goggles. If your PSU has current limiting adjust it to a low Amperage or use a series resistor although the use of too big resistor can mask the fault.
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Old 15th March 2003, 09:02 PM   #7
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Default A Short track...

Quote:
Do you mean to check and see if the cap has changed value?
No...with your multimeter in low Ohms range see if they are not shorted.(near 0 ohms)...

Quote:
The caps only have a 35volt rating, but the circuit is only 12-13 volts.
Ok but if you conect them with the DC voltage on...the surge will break them!

This is your problem!!
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Old 15th March 2003, 09:09 PM   #8
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If electrolytic caps are old ( you never know how long were they stocked in the store ) it is good practice to format them by putting a low voltage on it with a 1 M resistor in series for 24 hours. It is better if they are flaky to buy others from another brand or from another store.

BTW I don't know your skills so don't take this as an insult please: do you know they are polarized ? Is there a possibility that you reversed polarity ?
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Old 15th March 2003, 09:15 PM   #9
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Default The solid true...

Quote:
If electrolytic caps / tantalums are old it is very good practice to format them again by putting a low voltage on it with a 1 M resistor in series for 24 hours.
This is not true in the case of tantalum capacitors...they are solid capacitors...no dielectric...no chance to format them!!!

But it can help in the case of electrolitics...!!!
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Old 15th March 2003, 09:18 PM   #10
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You got me there Jorge ! I edited the text straight away. But they have a dielectric otherwise they wouldn't be caps and there are wet tantalums as well....

But ... I wonder if dry tantalums don't suffer from oxides internally ( when not used for a long time ) but if you say so I believe you.

Had my share with the unreliability of tantalums a long time ago so my knowledge of them is a bit rusty...
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