On my last Mouser order, I threw in various hyped ICs. I got around to building the INA217-based "$5 Preamp" that's all over the web (and here). My immediate need is for a mobile preamp for my Tascam DR-40 (a 4-ch handheld recorder with noisy built-in preamps), so I'll be running it off two 9Vs to get away without any power regulation.
I drew the schematic and the board in Kicad, basically true to the example circuit in the spec sheet, though without phantom power or ESD protection diodes, and with a balanced output (an opamp voltage follower and two matched resistors). Upon actually etching it, I realized it's a bit large and sparse, so I'll probably revise it to be a little more compact.
The files are on a Github repository. The kicad files are under kicad/ and PDF exports are under kicad/exports/
https://github.com/chapatt/ina217-mic-pre
It sounds transparent, as far as I can tell, and it behaves approximately to spec, in terms of gain factor and max input/output with a given supply V. I'll make a quick video with a 'scope and post the sheet metal enclosure template this weekend.
I drew the schematic and the board in Kicad, basically true to the example circuit in the spec sheet, though without phantom power or ESD protection diodes, and with a balanced output (an opamp voltage follower and two matched resistors). Upon actually etching it, I realized it's a bit large and sparse, so I'll probably revise it to be a little more compact.
The files are on a Github repository. The kicad files are under kicad/ and PDF exports are under kicad/exports/
https://github.com/chapatt/ina217-mic-pre
It sounds transparent, as far as I can tell, and it behaves approximately to spec, in terms of gain factor and max input/output with a given supply V. I'll make a quick video with a 'scope and post the sheet metal enclosure template this weekend.
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Thanks for the input! I edited the post to note the omission of the diodes. And true about the capacitors--the datasheet does note that if phantom power is to be turned off, the capacitors should be non-polar.
In my case, I'm not injecting phantom power on the output, thus probably don't even need the caps on the output, I'm thinking? Do they have any other purpose?
My input, however may receive phantom power, so I need the coupling caps on the input, plus they form a high pass filter. The fact that there isn't necessarily 48 VDC on the input I guess does mean they should be non-polar.
Ah, I'm reading up on that now. I'll fix the schematic. Thanks.
And I reversed things in my comment here:
I meant to say I'm not injecting phantom power on the input, but I probably should keep them because they form a high pass filter. My output may receive PP, so I need the coupling caps.
You do not want significant high-pass at a microphone input which draws any input current, unless you consider the effect on low-frequency hiss (random rumble).
47uFd+2k2 is 2Hz and sufficiently low for most purposes. However if you need a 50hz or 150Hz hi-pass to clean wind rumble or clean-up a mix, do that *after* you gain-up the signal some.
There sure can be large DC across caps inside a NFB loop.
However I think Mooly's point is that e-caps distort and NP e-caps are said to distort less.
Also bipolar types have less distortion, and this is more important when they are not
used within a nfb loop.
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