Project files: INA217 Microphone Preamp
May 27, 2017 6 Comments
What is it?
Board files for my INA217-based microphone preamp and the matching PSU as shown here. The design is meant to be “configurable” with three different gain options and phantom power selectable via jumpers. The amp also has a full complement of protection features. The matching PSU has three rails via two small onboard transformers for a compact “all-in-one” solution.
How big are the boards?
The amp board measures 3.1” x 1.9” (app. 79 x 48 mm.) and the PSU board measures 3.95” x 2.7” (app. 100 x 69 mm).
What is the status of the boards?
The amp board is version 2.1. Version 2.0 was my update of the original design as showcased in the previous blog post and 2.1 adds a few minor tweaks including an LED to indicate directly on the amp board if phantom power is on or off.
The PSU board is version 2.1 as well for much the same reasons (although the v2.1 “tweaks” consisted mostly of fixing a couple of fairly serious mistakes in component labelling 😀 )
Does it use any special/expensive/hard-to-find parts?
Not really hard-to-find as such, but still worthy of some attention 🙂
- The regulator for the phantom supply regulator must be a LM317HV type which allows for a greater in/out differential. You can use the standard version as well, but a short will then kill the regulator.
- As for the INA217: I am not sure if there are fakes about, but buy from reputable sources just in case. Anything in an 8-pin DIP is an easy target for fakes really.
Anything else I need to know?
- This board adds nearly all the bells and whistles described in this paper from THAT corp on instrumentation amp IC-based microphone preamps. These extra components for short-circuit and EMI-protection are optional, but definitely recommended.
- The board has a Neutrik A-series Combo-jack onboard which is very practical and versatile. Unfortunately it means that if you use the TRS it shorts the phantom voltage to ground if it is plugged/unplugged while the amp is on. Protection features have been added, but this scenario is best avoided so only (dis)connect the TRS while the amp is off.
- See the INA217 datasheet for gain calculations. While you can add a switch to select between the different gain settings, doing so may add quite a lot of noise so it’s not recommended.
- Voltages for transformers: The two transformers will have to be 2×12-15V and 2x18V respectively. They are usually single-primary, so choose the ones that you need. Note that with transformers in this form factor you will not be able to deliver more power than is required for a single mic amp. If you need a triple PSU that can supply more than one amp board, this design should work just fine (with external transformers.
- Replacements for the INA217 are mainly the THAT1510/1512, but there are some differences so I am honestly not sure if they are a drop-in replacement. Refer to the files under “related information” if you want to check for yourself.
Downloads:
Download design files here
EDIT 20th July 2019: Not sure why the BoM for this project did not make it into the download file, but here it is 🙂
Related information:
Note: Always read the “intro post” for additional important information about my designs.
Before you start I strongly suggest you read through the INA217 datasheet. Please also refer to the aforementioned paper from THAT on this type of microphone preamps, this THAT design note and the datasheet for the THAT1510/THAT1512 ICs.
theslowdiyer 
Why not use a pot for the gain?
Very good question. It’s possible, but three reasons why I chose not to:
– A gain pot for this design has to be “reverse log” taper, which isn’t that easy to find in a decent quality.
– The loop that controls the gain of the INA217 is the most sensitive part of the circuit so you want the signal paths here as short as possible. An off-board pot with long wires is not a good idea and an on-board pot close to the feedback loop didn’t really seem doable either. (At least not while keeping the onboard XLR. Chop off that and having a pot on on board edge would probably work fine).
– Last but not least I didn’t really need variable gain. I only have a few mics and so matching the amp to the mic seemed perfectly possible. The switchable gain is there to make some adjustment possible without compromising on the first two points.
All of this is of course solvable, either with a real pot or with a set of resistors and a PCB-mounted switch (like on the Fivefish SC-1), but I prioritised a simpler design that matched what I needed at the time 🙂
Hi. You mentioned the transformers are not able to supply the required power for multiple amps. Is this only due to the ratings of the transformes (one could choose transformers with higher values like Block VC 10/2/15) or is there something else we would need to mind here when considering multiple amps? Thanks.
It is only referring to the rating of the transformers that will fit on the board. If you use offboard transfomers (or if you manage to find ones rated higher than I could), the only limitation then is the current capability of the voltage regulators. However, that translates to many more mic-amps than I think anyone would reasonably need in one box (20 or so) 🙂
Thanks for your reply. What I’m looking for is a four microphone preamp that fits in a 1U rack box and your project looks very promising for that. As the PSU would need to be rated for higher current draw I need a bigger transformer and I was thinking it should be possibe to fit a Block VC 10/2/15 in a 1U rack (considerin also the clearance between the PCB and the frame).
That transformer is probably a bit too tall (34.6 mm) to mount safely in a 1U enclosure, but if you can find a smaller model that should fit OK.