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Old 14th March 2006, 01:50 AM   #1
robskel is offline robskel  United Kingdom
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Default musical fidelity p170

I am about to replce my op amps from 318's to 637's.
Have noticed that their is a capacitor (i think, it's marked 0.22 k 100 XkT ) soldered across pins 4 & 7 on the 318. Should i replace this on the new 637? I am new to all of this but think that it might be a filter/smoothing component across the op amp power supply
Thanks Rob
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Old 14th March 2006, 06:24 AM   #2
infinia is offline infinia  United States
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Hi
You are correct, it is a PS decoupling for the op amp.
I think the OPA should drop in, if you have already removed the 3 freq compensation parts for the 318 (5pF and the series RC).
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Old 14th March 2006, 11:08 AM   #3
robskel is offline robskel  United Kingdom
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Yes I am going to remove pin no'1 ,5 & 8. but still not sure if you mean i should solder back in the cap across pins 7 & 4.
cheers Rob
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Old 14th March 2006, 12:41 PM   #4
infinia is offline infinia  United States
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Hi,
Normal op amp practice is to use a bypass cap from each supply pin to ground. I see from the hand drawn scheme you provided, there is only one cap across both supplies. Is this correct? Not too sure about this technigue (low CMR), but if that the way it was designed, I would stick with that for now. (better than nothing).
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Old 14th March 2006, 04:07 PM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
some designers specify caps from both pins to ground and from pin to pin.

The pin to pin leads should, as closely as possible, follow the route that the IC uses to transport current through the chip.

This practice is confirmed in a few papers I have found on the web usually written by in house chip designers.
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Old 14th March 2006, 04:50 PM   #6
robskel is offline robskel  United Kingdom
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hi Thanks for the response. Looking at circuit boards of this amp on the net there are no caps across pins 4 & 7 it just seems to be mine. if i replace it with a 627 it might not need it, or does it? what effect would it have if i leave it out. sound quality or amplifier damage , or none. have no idea myself. As i am not an electronics expert. I am a eletrician , trying to learn electronics. Must say ive been bitten by the bug.
Any help in this direction would help both causes
Rob
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Old 14th March 2006, 07:44 PM   #7
infinia is offline infinia  United States
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Add 2 caps 0,1uF from each supply pin at the IC pins to a wide ground trace. This is mainly for op amp stabilty. Also provides better Common Mode Rejection Ratio performance at HF (CMRR).
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Old 14th March 2006, 10:35 PM   #8
robskel is offline robskel  United Kingdom
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thanks for your help. but i'm not quite sure what you mean. are you saying put two 0.1 uf caps in parrallel to the positive rail from pins 4. Or put a 0.1 uf cap between pin 4 to Negative and a 0.1uf cap between pin 7 positive.


sorry to sound so dim but i am new to this.


trying hard Rob
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Old 14th March 2006, 10:54 PM   #9
infinia is offline infinia  United States
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Sorry chap my fault.
2 caps total, one for each supply pin at the op-amp.
hope this clears up the confusion.
regards
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