Project files: INA217 Microphone Preamp

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 (link) 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.

(Yet another) anniversary!

Yes, it’s that time of the year again – and this year it’s the fourth anniversary of the blog 😀

Not a lot to say that I haven’t already said the last couple of years, but I still expect to continue writing as much as time allows. I am also still very excited and greatly appreciative of your questions and comments, so keep it up 🙂

Picture below is of what is (currently) sitting near the top of my project pipeline, namely four 4U diyaudio special-edition pre-drilled heatsinks. These are specifically intended to accelerate (as much as possible) the completion of my Pass VFET project as well as one other Pass project using boards from the diyaudio store that I have wanted to do for some time now 🙂

Project files: Little helpers – Alps PCBs III

What is it?
These are “little helper” boards for the Alps RK168xx series of motorised potentiometers. These pots are not quite as good quality as the standard Alps “Blue Velvet” RK27-series, but they are cheaper and smaller. They are also used in many commercial products, so they should work fine for many diy projects. The motor also means that the pots have a nice mechanical feel to them 🙂
There are two board versions, a 2-channel (for stereo with the RK16812) and a 4-channel (for balanced amps with the RK16814). Alps also makes a six-channel version of the pot and adjusting the footprint to fit these should be relatively easy, but I have no need for these now so I couldn’t be bothered 🙂

How big are the boards?
Both boards measure 1.85″ x 2.0″ (app. 47 x 51 mm) and the rear mounting holes are in the same place on both boards.

What is the status of the boards?
Both are version 1.0 since they are exactly as my prototypes.

Does it use any special/expensive/hard-to-find parts?
Mostly there’s only one real part on the board and that is the pot itself, so not really 😀

Anything else I need to know?

  • These are “preamp” style boards have a ground plane and a ground pad that can be used if you grounding scheme requires the shaft of the pot to be grounded. Use a piece of wire connected from the ground pad to either one of the screws on the back of the pot or soldered to a ring terminal wedged between the pot and the chassis. You can also use the grounding pad on the bottom instead.
  • The boards can also be used to make separate, passive preamps. In this case, a 10k potentiometer should be used.
  • The screw clamps are standard 5mm pin spacing types, but of course it is possible to solder bare wires to the boards as well.
  • The basic Eagle footprint for the RK168 was one I found in a diyaudio-thread, so I can’t take credit for that. All I have done is modify it to match the Quad-version as well.

Downloads:
Download design files here

Related information:
Note: Always read the “intro post” for additional important information about my designs.

As usual, please remember to consult the manufacturer’s datasheet as well.