An INA217 Mic preamp
April 23, 2017 6 Comments
Most of what I build is designed to reproduce sound that is already recorded, whereas this is designed to actually record sound for later reproduction. A slight departure from what I ordinarily do then, but bear with me. 🙂
It’s a microphone preamp based on the INA217 instrumentation amplifier chip from TI. The board layout is actually (another) one of my old designs that I’d managed to forget about for years but for reasons I’m not really sure about I rediscovered it and decided to rework it fairly recently. I think I did make a prototype board of the original back then but just never put it together – which is probably a good thing as I found an error in my original schematic when I did the update 😀
This design is also known as “the $5 mic preamp” (google it) since if you really pare it back to the essentials, it could be built for not much more than 5 dollars in components. My version is much more luxurious though, featuring an on-board XLR/TRS combo jack, configurable gain and phantom power as well as a DC servo and all the EMI-filtering and protection circuitry needed to avoid noise and accidents with phantom power. The only feature for this sort of amp that I have left out is the option to pad down the output with a switch – didn’t need that (and besides, no space left over anyway 🙂 )
I’ve also updated my matching PSU board which uses two small EI-core transformers to provide both the +/- voltage and the 48V phantom voltage. The transformer form factor only allows current for a single amp board, but that is OK. Originally regulating the phantom voltage was a bit of a faff, but since there is now an LM317HV with can tolerate up to a 60V-input, that was the obvious choice for the phantom supply regulator. This also means that for supplying more boards, my previous “Triple-PSU” design should be usable.
I’ve tested the preamp board with both a condenser mic and a cheap-ish Beyerdynamic dynamic mic and it seems to work quite well. I don’t have a proper recording setup at the moment though, but the sound quality is definitely good enough to warrant further experiments. I’ve made some updates to both boards and I’ll release the files when they have arrived so you can have a go yourself 🙂
theslowdiyer 
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Hi There,
I’m looking at your PSU and i am confused how you got 48v from that regulator and the transformer which has 2 sec windings that only give you 2x18v? What am I missing here?
2 x 18VAC in series is 36VAC, which rectified is app. 51V (36 x sqrt2). I think I chose the resistors for app. 46VDC regulated voltage to ensure enough headroom for the regulator not to drop out (and the mics do not care), but the transformer is very lightly loaded so it should put out more than 18VAC per winding in any case. You could also use a voltage-doubler or -tripler with a lower voltage transformer, but to me this is the “proper” solution 🙂
Hey man, great project and all in all the best INA217 based preamp I could find (a combination of simplicity and features. Most of what I found was either too simple or too complex). I’m about to order the PCB’s to build it.
I have reviewed every aspect of your Preamp and PSU designs and everything seems perfect
How is your’s still holding up? Do you have any advice? any consideration?
HI Carlos, thanks for your comment. I haven’t used my amp that much, but I occasionally get good feedback from other users so I hope it meets your expectations. The only tip I can think of is to be aware of the orientation of the protection diodes, because unfortunately it isn’t very clear. Remember to check against the schematic and board layout before soldering anything in place. Good luckwith the project! 🙂