opa827 bypassing,pioneer A-8 tone controls - diyAudio
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Old 4th September 2011, 06:47 PM   #1
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Wink opa827 bypassing,pioneer A-8 tone controls

just got my hands on some opa827s and put them on a browndog adaptor.

sounds great in my cmoy,but terrible in the tone control circuit on my A-8 (which id like the use it in)

the decoupling on the tone control board just consists of 2x 47uf/25 elna silimcs

..but theyre approx 6cm away from the opamp,so im wondering if the muddy sound is causing by oscillation, the 2107 i was using is just 4.5mhz but these 827s are rated at 22mhz.

so.. in terms of bypassing what would you guys recommend?


1. small tantalums/npo ceramics actually on the browndog
2. tants/npo's by the electros
3.just move the electros closer
4. move electros closer AND put extra bypass caps on


also what value could anyone recommend for the tants/npos given that the electros are 47uf?

thanks for any info
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Old 4th September 2011, 06:51 PM   #2
Mooly is offline Mooly  United Kingdom
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The joys of swapping opamps,
Swapping Op-Amps... you have checked to see it's stable haven't you ?
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Old 4th September 2011, 07:04 PM   #3
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wish i could mooly.. no scope! but judging by the sound it is oscillating very so slightly,i think the 47uf elna's are just about holding it off.

the circuit definetley was not made for something like this,the originals were jrc4558s.

so any ideas about bypassing these buggers?
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Old 4th September 2011, 07:16 PM   #4
Mooly is offline Mooly  United Kingdom
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If it used 4558's then swapping for TL072's would be a big improvement. They would work straight off with no stability issues. I would guess an OPA2604 would almost certainly be OK too.

Without seeing how the opamps are configured it's impossible to say, and measurement is always advisable to confirm all is well. If the opamps are oscillating then adding a small cap of around 8.2 pF from the output of each opamp to it's inverting input would probably stop it.

You can also try a small cap of around 1 to 10uf directly across the supply pins of the adapter board.
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Old 4th September 2011, 07:33 PM   #5
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cheers mooly i'll give that a go,looks like i'll have to order some NPO's

tried tl072's terrible sounding opamps,op275/opa2107 seem to be the best for me in different builds

if its any help heres a schematic:

Click the image to open in full size.

http://www.imagebam.com/image/f3fb51148067282

cheers

Last edited by lensmanMk2; 4th September 2011 at 07:36 PM.
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Old 4th September 2011, 08:57 PM   #6
wahab is offline wahab  Algeria
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Why not test NE5532 , these are unity gain stable and still used
in recent good quality gears..
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Old 4th September 2011, 09:10 PM   #7
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ne5532 are jellybean,and pretty primitive soundwise compared to modern opamps,ive got a parts box full of them
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Old 4th September 2011, 09:35 PM   #8
wahab is offline wahab  Algeria
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Quote:
Originally Posted by lensmanMk2 View Post
ne5532 are jellybean,and pretty primitive soundwise compared to modern opamps,ive got a parts box full of them
The 5532 has a THD of less than 0.0003%....
You can hear such low ratio and instantly hear the difference with a "good"
op amp that produce three time less distorsion ?..

Last edited by wahab; 4th September 2011 at 09:38 PM.
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Old 4th September 2011, 09:43 PM   #9
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not on speakers no,but on a good pair of headphones i can. and so can a lot of other people.

try building a CMOY and rolling a few opamps (with a good pair of headphones)

anyway i didnt come here to argue i was after some help bypassing an opa827! so kindly go and troll another thread.. thankyou
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Old 4th September 2011, 09:54 PM   #10
wahab is offline wahab  Algeria
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Quote:
Originally Posted by lensmanMk2 View Post
not on speakers no,but on a good pair of headphones i can. and so can a lot of other people.

try building a CMOY and rolling a few opamps (with a good pair of headphones)

anyway i didnt come here to argue i was after some help bypassing an opa827! so kindly go and troll another thread.. thankyou
Didnt you read mooly s thread ?..

You lack politness as well as intelligence, since using 5532,that you have spared, is a good way to check if it s not the circuit that dont suit the 827....
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