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Old 23rd November 2003, 09:29 AM   #1
Raj1 is offline Raj1  United Kingdom
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Default opamp replacement & dc offset trouble

Hi,

I had a couple of spare opa 627's, and also a spare pioneer pd-s 801, I haven't really used this machine cause I already have a modified pd-s model. In any case I thought I'd just replace the opamps in the 801 with the op627's. Trouble is I'm getting a massive dc offset of 1200-1400 mv per channel. Either the opamps are blown or perhaps something else is amiss (checked orientation of opamps, 'dot' is to pin one). The player worked fine with the 5532's in place. Normally the 627's have a low dc offset, which means that the problem lies elsewhere. Whilst I was working on the machine, I did not spot any dc coupling caps, I think it may have a dc servo instead, it must have had some kind of dc protection for the 5532's. Any ideas?

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Raja
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Old 23rd November 2003, 09:41 AM   #2
PMA is offline PMA  Europe
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1) NE5532 have high input bias current, this results in DC voltage offset across input resistors, was not there a dc compensation for NE5532, this would make troubles with OPA627.

2) Do you have a chance to measure oscillations, if not, try to approach an AM/FM radio near to the circuit and listen if there are any whistles.
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Old 23rd November 2003, 10:05 AM   #3
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isn't 5532 a dual opamp....?
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Old 23rd November 2003, 10:17 AM   #4
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Of course, tbla, you got it
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Old 23rd November 2003, 11:51 AM   #5
Raj1 is offline Raj1  United Kingdom
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hi,

It would sppear that you are correct, I wrongly assumed these were single opamps, as there is one per channel in the player........

don't have schematics, but do know it uses dual diff. dacs....



oh well!!


Thanks
Raja
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Old 23rd November 2003, 12:27 PM   #6
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So try OPA2134.
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Old 23rd November 2003, 12:51 PM   #7
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.......or OPA2604..............
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Old 4th December 2003, 10:44 PM   #8
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Hi,

I'm also in the state of changing my opamp to something different.
There seems to be two choices.
- Go with the opamps (mine is now an lm318)
-Go Discrete like this

What is the best option te choose?
Dicrete is more work-effort so it seems.

Regards,
Remco Blankesteijn.
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Old 5th December 2003, 05:09 PM   #9
Raj1 is offline Raj1  United Kingdom
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Hi,

replacing opamps is easier, and will give some kind of results. I've built the cd enhancer II, believe me, Remco is very talented, even though he sings the praise of each circuit, they are far better than he mentions, if he says they're better than opamps then they are!!

Thanks
Raja
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Old 5th December 2003, 07:37 PM   #10
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I got great results from that machine using OP275`s one of the sweetest sounding opamps, very valve like.
Cheers George
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