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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

OPA1622EVM Help
OPA1622EVM Help
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Old 3rd January 2018, 06:17 PM   #1
Harry Manback is offline Harry Manback  United States
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Default OPA1622EVM Help

I'm no electrical engineer. I do this as a hobby.

I'm building a preamp from individual components. ODAC, ESP Phono module, relay switched with LED, Alps RK27 motorized pot, and an OPA1622EVM.

Seems to work ok (aside from blowing up 2 OPA1622EVM modules due to my stupidity with short circuits...). My question is: The OPA1622EVM is supposed to be an eval board where you can change gain, cutoff freq, and phase. I see no way to actually modify these things. I want to raise the gain, but to do so, I need to add a resistor in parrallel to change the ratio of r1/r2, but they are SMD components. I realize that this board is for professionals looking to develop with the OPA1622, but, does TI really expect me to solder a resister over an smd resistor to change the gain? I must be missing something.

If any of you have any experience with this module, please chime in.

I also have a few more questions to follow:

1. I have the pot coming off the input and going into the opa1622evm. Is that the correct way to do that?

2. I have a pretty beefy power supply that supplies power to the opa1622, the phono preamp, and a line level converter to feed a balanced signal to the opa1622. If I wanted to avoid another opamp mishap, should I put a fuse inline with each power input line (+/-) or one for the (+/-/G), or is this just a stupid idea? If not, would a .2amp fast blow fuse be small enough to save any of the components, or will 0.2A kill everything before it blows?

Thanks so much for any help you can offer!

Kindest regards,

Darin
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Old 12th February 2018, 10:38 PM   #2
Harry Manback is offline Harry Manback  United States
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Nobody?
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Old 12th February 2018, 11:11 PM   #3
BamboszeK is offline BamboszeK  Poland
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I think TI not really meant stacking the second resistor on top but replacing it with a new one with proper value. Soldering small SMD parts by hand may be pretty tricky. Good eyesight, some technique, and proper tools are very helpful. I find desoldering braid quite handy when working with SMD. You may also try with hot air station.
1. It is perfectly fine.
2. Fuses are usually placed at the primary side of transformers to avoid shock or fire hazard. Placing them at secondary or even after rectifier isn't the best idea imho. Protection would be pretty limited anyway and it may negatively affect the performance of power supply. I suggest being careful with wiring and maybe placing circuit detecting DC at the output to protect headphones. I've been using something like this.
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Old 13th February 2018, 05:22 AM   #4
xx3stksm is offline xx3stksm  Japan
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Quote:
Originally Posted by Harry Manback View Post
I see no way to actually modify these things. I want to raise the gain, but to do so, I need to add a resistor in parrallel to change the ratio of r1/r2, but they are SMD components. I realize that this board is for professionals looking to develop with the OPA1622, but, does TI really expect me to solder a resister over an smd resistor to change the gain? I must be missing something.

Darin
Unfortunately, I think TI wants you to desolder SMD and solder another one, or parallel suitable one. I have much experience about such reworks from my teens, which can make it possible to solder 0603 SMD even in my old age. But I'm sure it's impossible for a nonexperienced people to do so. It could be helpful if 3216 SMD were used.

If you have simulation software like SPICE, you can find preferable values without soldering. After fixing the values, paralleling over an existing SMD is easier than desoldering and soldering. You need a tweezer and goggles (a magnifier). But I'm not sure you can finish the mission.
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Old 13th February 2018, 05:31 AM   #5
xx3stksm is offline xx3stksm  Japan
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Conversion PCB from QFN to DIP like this is another choice. If you have mother board for an opa1622evm module, DIP module is easy to handle.
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Old 13th February 2018, 05:31 AM   #6
tomchr is offline tomchr  Canada
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As you mention, the TI EVMs are set up to allow engineers to evaluate the OPA1622. TI assumes that those who wish to change components are equipped with the necessary soldering and de-soldering equipment.

If you would like to get into electronics as a hobby and expect to build much with SMD components, investing in a good soldering iron and a pair of hot tweezers for de-soldering would be well worth it.

Given that the OPA1622 EVM uses 0603 size components, you should be able to remove the resistors with a soldering tip wide enough to reach both ends of the component. Just toss the part once you get it off. Then clean up the solder with solder wick and solder a new resistor to the board.

A fuse is not likely to be able to save the IC. The IC usually fries before any fuse.

Tom
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Old 14th February 2018, 03:49 AM   #7
Monte McGuire is offline Monte McGuire
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OPA1622EVM Help
The biggest help with an EVM is that it provides a moderately sensible power supply and bypassing system with connectors for input output and power, and some pads to allow you to fit your own feedback networks, or other circuitry. It's supposed to replace the week or so needed to get a nice surface mount PCB designed from scratch. It will never be exactly what you want, but it can save a day or two in silly effort just to power the thing up in a sensible environment.

You can always dead-bug a design using surface mount to DIP adapter boards, and TI sells a nice set for very low money ($10+shipping): DIP-ADAPTER-EVM DIP Adapter Evaluation Module | TI.com

I've used these with some SO8 packaged op amps, and while you'll still spend a bit of time getting the basics wired up, you can use through hole resistors if you want, and do other goofy things you may need to do to get a simple prototype scratch circuit together.

I'd try to get some tools to allow you to do basic SMT soldering - a tweezer soldering iron is very useful, as well as a conventional fine point iron. A hot air wand can be useful too, but only if you have to play with DFN and QFN packages and have a PCB or adapter with proper footprints for them already.

SMD caps and resistors can be very nice to use. A 1206 or 0805 cap can fit between the pins of a 0.1" part quite easily, so they're nice even for old devices built with through hole PCBs.

Best of luck!
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Old 7th March 2018, 08:22 PM   #8
Harry Manback is offline Harry Manback  United States
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Thank you all for your suggestions! I appreciate the feedback.
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