Electronics > Projects, Designs, and Technical Stuff
Ground loops, computers, and headaches
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Hiemal:
One of my latest projects was to design and make a microphone pre-amp supplied by USB power.

I built it, using two 34063's for a +18 and -18 rail, and a LT1171HV to supply the 48v phantom power for the microphone.

The main amplifier is an SSM2019; a chip with quite nice specs. I basically copied the design on this https://www.analog.com/media/en/technical-documentation/data-sheets/ssm2019.pdf on page 7 at the bottom. I'm using a capsule from an ADK 51s microphone; one of the leads goes to the center of the Mylar membrane and the other connects to the outer case of it.

I tried last night to attempt to record and see if I could get any sound from the amplifier, hooking it up to my computer via a 3.5 mm audio jack.

Upon recording, (turning the gain up to the max of about 200 ish) I just get a TON of noise, and 60 hz interference gobblygook, and absolutely no sound from the capsule itself.

My questions are;

Is this a ground loop issue? Through the noise I can't hear ANYTHING from the microphone so...

Am I connecting the microphone capsule up correctly? Or do I need a different design? As I said before, one of the capsule leads connects to the mylar membrane itself, and the other to the outer brass case.

Do I need to get an audio transformer?

If any of this is unclear, or if you feel like you need more information I could try to put together a schematic.
coromonadalix:
Do you have a schematic of your project ?
Hiemal:
Lemme put one together, it'll take a few.
Hiemal:


Here's the power supply section.



And the SSM2019 section. All as accurate to what I made in person as I could get.

Edit: Forgot to put in a 200 pF cap between the SSM2019's terminals. It's there IRL, just forgotten in the original schematic. Added!
Hiemal:
So doing a lot more digging, i Believe i I had a fundamental misunderstanding of how to make this work properly...

The SSM2019 isn't intended to be hooked up *directly* to the microphone capsule, it's meant for amplification AFTER the initial pre-amplifier. Every SSM2019 design I've found online, outside of the datasheet all have inputs from an XLR connector; not from a microphone capsule itself... So I think I need to make a more basic JFET amplifier first.
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