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Old 17th May 2013, 06:57 PM   #11
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I'll need some time to chew on what you have said. But here are a few more bits of info that may or may not be relevant.

- The opamp is OPA2132 (not OPA2134)
- The noise is kind of like a wide band white noise
- The opamp is socketted, and the circuit is built point to point on a perf board
- I swapped the opamp for LT1358, this opamp seem to make the least noise, but the noise is more like a motorboat noise
- I tried other opamps, LT1208, LT1459, OPA2107, but they make even more noises

The power supply is a 2 stage LM317/LM337 bipolar supply. Out of the rectifier I got about 29V. The first regulator stage is set to about 20V, the second stage is set to about 15V. One of the reason I do that is because the voltage of the transformer is too high and I worry about too much heat dissipation on the regulator.

The final capacitor on the PSU is 2x47F on each rail, mounted about 1 cm from the opamp and about 5 cm from the regulator. In my earlier version, I used 2x220F on each rail. I didn't do much careful assessment, I only listen to it through headphone and I didn't hear anything out of the ordinary. I took those caps out because I needed them for something else.
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Old 17th May 2013, 07:30 PM   #12
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I tried a 4.7F cap across pin 4 and 8, no difference.

As for lay out, here is the top side.

Click the image to open in full size.

Bottom side. White wires are ground, black wires are supply rails.

Click the image to open in full size.

As you can see, I am building this on an old preamp. I tap the input, output and ground from the existing board underneath. I can see that the grounding may be less than optimal. So I disconnected the input, so the only thing connected to the ground plane is the output ground and the PSU ground, no difference.
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Old 17th May 2013, 07:39 PM   #13
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You're supposed to connect 0.1uF capacitors from each power pin to ground, preferably MLCC's. I don't understand what you mean by "across pins 4 and 8", and you do not EVER use electrolytics there.
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Old 17th May 2013, 07:44 PM   #14
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The OPA2132 is similar to the 2134 either of which are perfect for this application. This problem lies elsewhere but I really don't know what to suggest. A scope would help to see what was going on.

As you mention adding the input and output connections to points already present on the old PCB, could there be any possibility of interaction with existing circuitry such as connecting the opamp output to something present and active on the board.

Do DC voltage checks show anything amiss. There should be essentially 0.00 volts on pins 1 and 7 (the output pins).
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Old 17th May 2013, 07:47 PM   #15
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by dirkwright View Post
You're supposed to connect 0.1uF capacitors from each power pin to ground, preferably MLCC's. I don't understand what you mean by "across pins 4 and 8", and you do not EVER use electrolytics there.
There's a multitude of ways to decouple a circuit, everyone has there own ideas too. A single electrolytic in practice will kill any oscillation and prove the point... if that were the issue here... but its not.
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Old 17th May 2013, 08:05 PM   #16
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From the picture, you can see that I removed the fuse on the PCB and tap the transformer output from there. So in theory it is impossible from anything to be powered on the old PCB.

And for the signal input and output, I made sure to remove everything in their paths. I tap the input from the input cap that I removed. I tap the output from the output relay that I also removed. Although to I can check again.

Unfortunately I don't have a scope. At this point I am tempted to get a cheap NE5532 preamp kit from eBay which should work. Or build a B1 buffer with the existing PSU. But my self confidence is at a low point now that I can not get this working properly.



Dirkwright,

I tried a 47nF film cap, but I will humor you and try 100nF MLCC later.
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Old 17th May 2013, 08:28 PM   #17
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100nF MLCC, no luck.

I forgot to mention, there is no DC on the input and the output.

I'll try desoldering and soldering the resistors back.
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Old 17th May 2013, 08:38 PM   #18
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Quote:
Originally Posted by Navyblue View Post


The reason I am asking is, recently I built a simple single stage non inverting opamp preamp. I have the the power supply capacitor (2x47F) after the regulator mounted close to the opamp, at about 1 cm away. The I think it is oscillating (it hisses quite badly). My guess is it has to do with the power supply decoupling, otherwise I am quite stumped on how can something so simple go so wrong.

The opamp used is OPA2132, which has a reputation on being well behaved. But I ended up swapping it for LT1358 as it seems to be a tiny bit more stable.
Stop guessing!!!!

You don't even know for sure that it's oscillating. You have to use a high frequency scope to see that for sure. Another clue would be that the opamp gets really hot.

You can't figure out a solution until you are certain of the problem.
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Old 17th May 2013, 08:39 PM   #19
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Quote:
Originally Posted by Navyblue View Post
100nF MLCC, no luck.

I forgot to mention, there is no DC on the input and the output.

I'll try desoldering and soldering the resistors back.
If you're too cheap to buy the proper parts then you can't expect to get good results.
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Old 17th May 2013, 08:51 PM   #20
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I meant I tried 100nF MLCC like you suggested, no difference.
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