Oscillating swapped OP

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!!
 

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Yes,
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

Member
2008-11-08 8:25 pm
Devon UK
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.
 
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!

Cheers!!!
 

stofbj

Member
2011-11-19 7:44 pm
Hi,
I know this is a really old post, but did you ever get this sorted? I have a second C22 as a spare to experiment with, which has the exact same issue with the 5532, lights on the P22 power amp go all the way to max, disconnected it very quickly encase it damages/ over heats the power amp. I usually us the Burr brown (OPA2134) and never had any issues with them in other amps. I kept the old op amps so if all else fails I will go back to them.
 

stofbj

Member
2011-11-19 7:44 pm
Just to update encase someone else tries to upgrade their AIWA SA-C22 pre amp, the
Burr Brown OPA2134PA worked a treat, but I had to removed the filter caps, presume these chips have caps built in. However the 2134 chip did not work in the phono stage, with or without caps on the power rails to ground. Had to put original chip back in, the 2134 caused too much gain or oscillations.