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Marmotus 15th September 2010 03:59 PM

Oscillating swapped OP
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Hi All,
leading with old audio stuff is fun and surely always rewarding as there were very good designed ones. For instance Aiwa P50 power Amp, it's a wonderful miniature, toroidal, discrete outputs and chip drivers, mine stood 0,2 ohms load (a shorted cable) for tweeters for a year till I realized the situation after some black outs at full power!!! It's companion Pre was gone few years ago to a friends home, and he lend me a little older Aiwa C22 Pre.... this last combo sounded, mmmmm 80's hifish, but after see the pre simplistic aproach I decides to make the least invasive change to give some life to that unit. It's simple layout employs 3 4558 OPs, one for phono, second for a filter stage and third for tone control, I changed them for NE5532AP... and Wild Oscillation fest begun. After studying things a bit (of course I`m far from knowing electronics, but I'd like to ..) I've found that the tone control OP was oscillating and if I set Treble control to + 1 , It stopped completely and let me realize how good it sounds now!! is like a Mark Levinson compared to a 60's Spika AM radio!! that old and good workhorse 5532. So I wonder if the lower input impedance of the 5532 is part of the problem in that feedback circuit, and if so wich could be the solution to leave them there! To those who knows, I've already made the decoupling thing to ground, hoping could be some EMI or PS fault, but no change at all. Attached is Tone amp circuit. I have to say that those two Aiwas are producing a warm, wide SS and engaging SQ presentation as best equipment heard ... Thanks a lot!!

Leon08 15th September 2010 07:11 PM

I don't see why it should oscilate.How do you know that it oscilates?

Frank Berry 15th September 2010 07:15 PM

The 5532 has a better high frequency response than the 1458/4558 IC's.
You may need to add bypass capacitors from pin 4 to ground and from pin 8 to ground.

Leon08 15th September 2010 07:20 PM

Or try another Op-amp.NJM4580 for instance.I use them too and never had a problem with oscilations.

lineup 15th September 2010 07:21 PM

do not forget those 100nF caps from pin4 and pin8 to GROUND.
And when I say PIN, it really means the caps should be as CLOSE as possible to those supply pins of NE5532.

NE5532 is a stable Op-Amp. In all gains from Gain=1
It is internally compensated with small caps.
This is why it is slower than NE5534.
Only if we do not handle it correctly it may be a part of oscillating circuit.

Xoc1 15th September 2010 09:42 PM

It would be good to see the entire schematic if you have got it.
The NE5532 should be OK but as stated you need to make sure that the decoupling of the power rails to the op-amps is good. This will be the case for any op-amp upgrade that you make.
I would try to add some 100uF electrolytic capacitors from the power rails to ground, as well as the 100n caps. If there are no handy ground points near the op amps, you could aways solder a 100n capacitor directly to the op amp right across the power rails from pin 4 to pin 8.
Since you want to make a good job of this and get the best result possible try to use low ESR electrolytics, and for the 100n caps try polyester (MKT) types.

Marmotus 16th September 2010 08:05 PM

Thank you all, very much!

Leon08; as commented, pre set flat produces a hiss sound (what you hear), but power amp's led indicators get full power reading (ultrasound) and protection shut it down in seconds... if you read my post, It used to stand a near short (audio frecuencies) for a long time ..

Frank Berry; It's a huge difference, actually I didn't thought 5532 was so good, fist I made such decoupling and no change, now I put from 4 to 8 0.1 mf and still the same effect...

lineup; probably I should start again, getting a closer ground points or something like (maybe a direct wire??)

Xoc1 I'll get some bigger caps and try!


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