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Old 24th January 2003, 08:04 AM   #1
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Default Strange decoupling of op-amp power supplies

Hi all,

In my CDP (a heavily modified Rotel RCD955), I have noticed that the op-amp that controls the motor speed (NJM 4560) for the CDP transport has strange power supply decoupling:

The + and - 10V lines have no local decoupling at all (i.e. close to the op-amp) and in fact are connected to each other via a 220nF ceramic capacitor. I fail to see the sense of this ceramic capacitor!

Any idea as to why this configuration (i.e. the linking ceramic) is used and what it does?

I plan to remove the ceramic and install decoupling caps for each +-10V line to the opamp. Any comments? I think it should the supply to the op-amps.

Thanks for any help
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Old 24th January 2003, 08:32 AM   #2
halojoy is offline halojoy  Sweden
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Default Possible

I think you say that the power supply rail
of the both OP amps are connected together by a ceramic cap.

I would guess that the two Opamps work work in anti-phase.
They can drive two opposite windings of the motor-coils.
So one output goes high, while the other goes negative.

In that case a postive spike on one supply rail
comes at same time as a negative on the other.

The ceramic than could a sort of cancel these spikes out.

This is what I can think of, by the little facts you have provided.
It does not have to be the correct reason
for that cap.
But as noone else have found anything better
for the moment - so I post this answer.

Best is if you could draw a simple diagram of the supplies.
Or give some additional information.
------------------------------------------------

Of course I think that you can make a better supply for those
OPamps. The manufacturers make often very simple & cheap
solutions. They take the easy way.
And as long as it work well enough to satisfy the consumer,
they have a better profit margin and an simpler fabrication
of the machines.


/halo - might be right or might be wrong - in this case
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Old 24th January 2003, 09:01 AM   #3
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Default Another idea?

Thanks,


Maybe additional information:

The njm 4560 is a dual op-amp. Pin 4 is the -10V input, pin 8 is the +10V input.

The - and + power supply lines that run into the op-amp (i.e at pin 4 and pin8) are connected via a ceramic cap (220nF).


Someone else suggested that maybe the arrangement allows for common mode noise on the + and -10V lines to be cancelled through the ceramic cap.

I think I will leave the ceramic cap in when I add the additional decoupling for the + and -10V lines.
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Old 24th January 2003, 09:20 AM   #4
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Default strange

Doing that may couple supply (motor) noise to the ground tracks and into other parts of the circuitry. Unless you have a very good reason for this, I would be carefull and as a minimum do before-after listening tests.

Jan Didden
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Old 24th January 2003, 09:38 AM   #5
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Default Reply to jan

Hi Jan and thanks,

In my Marantz CD 52, which shares the same transport electronics (i.e. TDA8808, 8809, TCA0372 and NJM4560x2), they have decoupled the supply lines to the NJM4560, whist not including the strange ceramic cap I refer to above.

I think I will try the decoupling and do the before-after listening.
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Old 25th January 2003, 04:50 PM   #6
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Location: Orleans, France
This is an explanation given by Douglas Self :

"The essential requirement is that the +ve and -ve rails should be decoupled with a 100nF capacitor between them, at a distance of not more than 2 inches. It is NOT necessary, and often NOT desirable to have two capacitors going to ground; every capacitor between a supply rail and ground carries the risk of injecting rail noise into the ground. "

Regards, PIerre Lacombe.
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Old 28th January 2003, 05:58 AM   #7
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Default No discernable difference

I have now removed the "strange ceramics" and used the standard methods of decoupling per line, i.e. 100uF tantalum/electrolytic in combination with a 100nF ceramic cap to ground on each of the +10V and -10V lines.

I cannot really discern a difference in sound after a preliminary 5 minutes of listening, although the bass registers may be ever so slightly more resolved. I've left the beast to run in and if anything does change, I'll report.

Thanks for the posts.
Ryan
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Old 28th January 2003, 07:32 AM   #8
dhaen is offline dhaen  Europe
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Default Don't throw away those ceramics just because they're cheap.

Hi,

Certain types of ceramic caps are actually uniquely good at decoupling.
This is due to the lossiness of the dielectric - exactly the reason why we dont use them for coupling

I would not remove these uniquely good decouplers.
They will be losing some of the unwanted energy as heat (tiny though it may be), rather than (de)coupling it to the nearest ground point, or the other supply rail.



Cheers,
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