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Universal buffer/headamp based on OPA1622
Universal buffer/headamp based on OPA1622
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Old 5th May 2016, 07:56 PM   #11
OlegSh is offline OlegSh
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Universal buffer/headamp based on OPA1622
Hi agdr,

I already soldered the regulators on four dual polarity boards with success. I fried one board at first and thrown away two chips while learning but in the end it worked with the solder paste and the hot air solder station for 40$ from ebay. The OPA1622 is a bit more challenging but I intentionally put a big via in the thermal pad to heat it from below. I guess it should work this way.

Regards,
Oleg

Regards,
Oleg
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Old 5th May 2016, 11:54 PM   #12
agdr is offline agdr  United States
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Universal buffer/headamp based on OPA1622
OlegSh - good to hear it worked! I took delivery of this one from eBay yesterday. Never have hot air soldered in my life! This will be interesting. From what I read apparently folks are using hot air all the time to rework xboxes and such.
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Old 6th May 2016, 01:05 AM   #13
Shaq888 is offline Shaq888  Ukraine
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Quote:
Originally Posted by agdr View Post
how did you solder your OPA1622?
As usual - upside down!

Click the image to open in full size.

Although this is just a test jig. Normally, if I had a PCB made for the footprint, I would use a solder paste and a hot air station.
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Old 6th May 2016, 02:00 AM   #14
Shaq888 is offline Shaq888  Ukraine
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I have to say that common mode distortion, even with imbalanced impedances, are not bad at all. However, you sticc can see the difference. There are a few spectra. VS=+/-10V, G=+1, 3k3 load, 2Vrms(+6dBVrms) signal, 1kHz, 10kHz and 20kHz. In both cases source resistance is 1k. In one of them I balanced resistance next to inverting input, in other I connected it directly to the output.
http://i.imgur.com/Ykcs0Pk.png
http://i.imgur.com/6Or82PF.png
http://i.imgur.com/LjGGEd8.png
http://i.imgur.com/JHC6MUY.png
http://i.imgur.com/bSf7yOT.png
http://i.imgur.com/r1qnaPO.png

Last edited by Shaq888; 6th May 2016 at 02:03 AM.
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Old 6th May 2016, 03:43 AM   #15
agdr is offline agdr  United States
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Universal buffer/headamp based on OPA1622
Quote:
Originally Posted by Sergey888 View Post
As usual - upside down!
Dead bugs!

Good to know about using the hot air! From what I read the surface tension of the solder more or less snaps the chip into place over the pads. Should be fun to watch.
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Old 6th May 2016, 12:27 PM   #16
OlegSh is offline OlegSh
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Universal buffer/headamp based on OPA1622
Thanks for posting your measurements, Sergey! Now I can sleep well

Agdr, I would not rely entirely on the surface tension to position the parts during soldering. It will happen for the chip especially with the thermal pad but bulky two terminal parts may still be misplaced after reflow. So it is better to put them straight initially.

Last edited by OlegSh; 6th May 2016 at 12:29 PM.
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Old 6th May 2016, 01:11 PM   #17
Shaq888 is offline Shaq888  Ukraine
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No problems.

If you are going to use it with pot in non-inv config, you can do some optimizations.
Use low value pot, like 4.7k.
Than took the highest possible output resistance, when it is in the middle, and divide it by two. Than add resistance of EMI filter. This is approx resistance you need to have connected to the inv. pin to have OK-ish imbalance in any position of the pot. Also you can take a bit higher value, if you think that min and max positions of the pot wont be used a lot. Also in min position distortions will not be high due to low common mode signal.
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Old 6th May 2016, 01:18 PM   #18
OlegSh is offline OlegSh
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Universal buffer/headamp based on OPA1622
Quote:
Originally Posted by Sergey888 View Post
This is approx resistance you need to have connected to the inv. pin to have OK-ish imbalance in any position of the pot.
Just to make sure, are you talking about the feedback resistor?
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Old 6th May 2016, 01:25 PM   #19
Shaq888 is offline Shaq888  Ukraine
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Yep, take them in parallel. Also don't forget about cap in feedback, the same value you will be using for emi filter. Dont make it large, since cutoff will depend on pot position.
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Old 6th May 2016, 01:26 PM   #20
OlegSh is offline OlegSh
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Universal buffer/headamp based on OPA1622
Understood, thanks!
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