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INA217 Mic Preamp Help
INA217 Mic Preamp Help
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Old 13th June 2019, 07:45 PM   #1
Waaaaaark is offline Waaaaaark  United States
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Default INA217 Mic Preamp Help

Hi, all, I hope this is an appropriate subforum for this. I've put together a mic preamp that utilizes the TI INA217. The power supply kit comes from FiveFish Audio and I've set it to supply 15v on both rails. Everything powers on just fine, but after a short while, some strange feedback starts happening that I can't control with any of the switches and doesn't go away until I power the unit off. I'll include some pictures of how I have the thing assembled so far. Any help or clues would be very greatly appreciated!

Also, how can I attach a sound file? I recorded the feedback that's happening; it sounds like low-frequency oscillation. When it starts happening, my LED under the V+ rail dims considerably and the LED VU meter on the preamp channel lights up in time with the noise.
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Old 13th June 2019, 07:52 PM   #2
Mooly is offline Mooly  United Kingdom
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INA217 Mic Preamp Help
If LED's are dimming then that points to a rail dropping. Before worrying about whether its an audio fault or not, monitor the rails and see what happens. Do the regulators get hot? Do any of the electrolytic caps get hot (one fitted wrong way around)?

Sound files are best attached using something like Dropbox. You can attach files directly in the same way you do for pictures but you have to put them in a zipped folder first and stick to the forum size limits which is just 976.6kb. A low bit rate MP3 is do-able though.

Check the rails first, and check for anything getting hot.
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Old 13th June 2019, 07:59 PM   #3
Waaaaaark is offline Waaaaaark  United States
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Thanks for that. Being quite new to working with electronics, I have to ask, should I power it off before feeling the regulators? I feel some heat coming from the unit, but didn't look into it yet. I'll also double check my cap orientation.

To check the rails, should I use my MM at the PSU, checking between V+ and GND? When I calibrated it, I know it was right at 15V, so I wonder why it would be dropping?

Here is the link to the sound I'm getting
Dropbox - preamp-feedback.mp3 - Simplify your life
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Old 13th June 2019, 08:10 PM   #4
Waaaaaark is offline Waaaaaark  United States
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Okay, so I just checked the V+ rail when its making the noise, and the voltage jumps up to 33V and up. So it starts at just below 15V where I calibrated it and it just goes up. Nothing seems so hot that it's worrisome.
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Old 14th June 2019, 01:29 AM   #5
Mark Tillotson is offline Mark Tillotson
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Oscillation? Is everything decoupled?
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Old 14th June 2019, 02:56 AM   #6
PRR is offline PRR  United States
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> the voltage jumps up to 33V

Bad solder joint. Start near the regulator, but could be elsewhere.
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Old 14th June 2019, 07:27 AM   #7
Mooly is offline Mooly  United Kingdom
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INA217 Mic Preamp Help
The recording sounds... well it sounds like anything could be happening but the clue is in the measurements you made.

PRR's suggestion is a good one, if there is a bad connection on a regulator then it loses its reference points and the output can shoot up.

I'm assuming this is a dual rail supply with plus and minus 15 volts?

A diagram would be a big help in diagnosing this and in helping you pick test points.
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Old 17th June 2019, 06:01 AM   #8
Waaaaaark is offline Waaaaaark  United States
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Hey friends, sorry for the delay. The VU meter boards on both channels were acting up, so I disconnected them and the problem went away. The only thing I can think of was that I had to substitute a couple of transistors since the ones listed on the BOM were obsolete. I still wonder how the voltage could have gone that high...

The noise is gone and the pres work, but after comparing them to the Saffire pro 40s I have as backups, there's barely a hair's difference in sound. I'm thinking I'll try the THAT 1510 and/or implement some io transformers. Any input on that would be great (how do I add more character to my pre?). If there's really nothing to be done, I'll just re-use the power supply and build some discrete pre-amps.
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Old 17th June 2019, 02:22 PM   #9
Mooly is offline Mooly  United Kingdom
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INA217 Mic Preamp Help
Its pretty near impossible to diagnose at a distance why the voltage is shooting up. You would have to test the PSU in isolation and see whats going... and we would need full circuit diagrams to come up with suggestions.

Maybe the VU draws current beyond what the regs can support although that shouldn;t cause excess voltage. Also do the VU's have another supply beside the regulated 15 volts... all questions we would need to know.

Sonically the INA217 is really the deciding factor and you are not going to change the basic character of that. It is what it is... excellent when used correctly
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