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Old 4th October 2004, 02:10 PM   #1
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Question +/- supply resistors on op amps?

hi there, i have an old magnavox cbd 492 model cdp which has 10 ohm resistors in series with the+/- supply pins. the original op amps were lm833n dip and i replaced them with opa 2604s. my question is, would it be safe to bypass these resistors with jumper wires or, are they important to maybe reduce the current draw on ps? or maybe there for the lm833 op amps and maybe, not needed now for the opa 2604s? there also seems to be 2700 pf small yellow film type caps at the input pins could they also be bypassed since, i have output blocking caps on the outputs? the main ps has one 6800 uf cap and a couple 1000 uf caps also. i've replaced the 1n4002 100v 1 amp rectifiers with 4 amp 600v stealth diodes from fairchild. not sure about tranny power rating but it has a 300 ma. 250v. fuse. any help would be greatly appreaciated thanks, crippledchicken ps. there are four op amps in question.
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Old 4th October 2004, 04:41 PM   #2
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Default Re: +/- supply resistors on op amps?

Quote:
Originally posted by crippledchicken
hi there, i have an old magnavox cbd 492 model cdp which has 10 ohm resistors in series with the+/- supply pins. the original op amps were lm833n dip and i replaced them with opa 2604s. my question is, would it be safe to bypass these resistors with jumper wires or, are they important to maybe reduce the current draw on ps? or maybe there for the lm833 op amps and maybe, not needed now for the opa 2604s? there also seems to be 2700 pf small yellow film type caps at the input pins could they also be bypassed since, i have output blocking caps on the outputs? the main ps has one 6800 uf cap and a couple 1000 uf caps also. i've replaced the 1n4002 100v 1 amp rectifiers with 4 amp 600v stealth diodes from fairchild. not sure about tranny power rating but it has a 300 ma. 250v. fuse. any help would be greatly appreaciated thanks, crippledchicken ps. there are four op amps in question.
Hi,

The 2700pF's are most probably part of the low-pass filter, NOT blocking caps. leave them in, they are functionally required.

The 10-ohms were often used by Philips at the time. The purpose is to block any ripple generated by an opamp to get to another opamp. In other words, they not only block incoming junk, but also outgoing junk. Most probably there are caps before and/or after the resistors to gnd.

It is difficult to judge the effect after the opamp replacement, but in my opinion it is a sign of inadequate power supply implementation. In other words, if you replace the regulated supply with a high performance regulator, I would take out (jumper) the 10 ohms. In the current set-up, you can always check it out in a listening test.

Jan Didden
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Old 4th October 2004, 07:00 PM   #3
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hi janneman, thanks for the explaination. i've jumpered the 10 ohm resistors and also, the op amp closest to output jacks, had 1 ohm resistors so i jumpered them also. all the op amps except the one closest to the TDA1543 DAC CHIP, have pins 1 and 2 tied together and also, pins 6 and 7 tied together. are these considered followers, buffers ? the op amp without pins tied together is right next to the DAC chip so, i had some j555 crds i used to class a bias this op amp to 2ma. using resistor between output pins and crds as i have heard the resistor helps preserve dynamics. it apears, the output from bridge rectifier feeds direct to op amp supply pins without any regulation other than the resistors that i jumpered out. i have what i need to add another tranny, bridge + ps caps and either lm 78/7915v. regulators or lm317/337 adjustables and all kindsof room to put it in. it sounds pretty good now with no over heating but, another ps just to feed the op amps with regulated +/- 15v. also, might be the best bet? what do you think? or is this old player worth the trouble? thanks again, crippledchicken
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Old 4th October 2004, 07:08 PM   #4
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I have not done this type of mods myself, but most people on the forum report better sound with a well regulated supply for the analog stage, so it seems worthwhile, old player or not.

You know, whatever the drive, those bits come off the disc alright, don't worry. The biggest problems are jitter along the D to A route and the analog output stage. Resolving possible jitter issues involves not only better supply but also better clocks and digital wiring etc. That's a different kettle of fish. Better to tackle it one step at the time.

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Old 4th October 2004, 07:30 PM   #5
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thanks again janneman, i think i will try adding the extra regulated supplys and see what i come up with. you've been a big help! crippledchicken
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Old 4th October 2004, 07:32 PM   #6
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Just out of curiosity, but why 'crippledchicken'??? What wrong with, say, '3-legged dog'
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Old 4th October 2004, 07:40 PM   #7
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i guess i'm just a little to slow to catch on to that reply he he.
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