Portable battery-powered mic preamp

I am SUPER INTO the current trend of tiny musical instruments (OP-1, Volca series, Pocket Piano, etc). I’ve built a few accessories to complement my collection. Some people at the Operator-1 forum have expressed interest, so I thought I’d share the designs. I hope to provide enough information that you can make your own.

In this case, I wanted to use a pro microphone with the OP-1. So I designed a small, battery-powered mic preamp with an 1/8″ output.

mic_pre

Warnings!

I am not a professional electrical engineer. Use these designs at your own risk.

The design has no phantom power, and therefore will only work with dynamic microphones.

I had a bit of a brainfart when I designed the output stage, and the result is that it won’t work plugged into a balanced input. The preamp works great with the OP-1, but you might get funny results if you plug it into a pro mixer or such.

The provided PCB uses some surface-mount components, which require a different soldering technique from through-hole electronics. If you can do through-hole soldering, you can probably learn surface-mount, but it is different. I highly recommend using a hot air tool, but it is theoretically possible to do it with a plain old soldering iron as well.

Schematic

Click for full size:

micpre_schematic

PCB

I shared the project on OSH Park, where you can download the .brd file or order PCBs.

Design notes

I can’t really take any credit for this design as it’s more or less straight out of the INA217 datasheet.

Power supply

The TLE2426 is a nifty IC for converting a single voltage into a bipolar power supply. It finds the midpoint voltage of its two inputs, and then you can pretend that’s ground. Your 9V battery then looks like a +/- 4.5V bipolar supply.

D1 is for reverse protection. D2 is a power indicator. If you use the same LED I did, it’s SUPER bright, so feel free to replace R5 with a slightly bigger resistor. You can swap in your favorite color of LED instead, just adjust R5 to match. C5 is for smoothing the power supply. This circuit is not very power hungry, so you don’t need much; 47μ is probably plenty. I happen to have a giant bag of small 100μ caps so that’s what I use.

Preamp

This is a totally standard application of the INA217 preamp.

R6 is the gain potentiometer; it should have a reverse-log (also called reverse audio) taper for smooth control. Unfortunately, it’s backward: fully clockwise is minimum gain, fully counterclockwise is maximum gain. I could not find a suitable arrangement that gave me both a nice gain curve and conventional operation, so I chose the gain curve.

R2 sets the maximum gain at about 60 dB.

Output stage

There’s a 1/4″ and 1/8″ output, each of which has its own op-amp buffer. Even though the schematic says 4227, you should use a rail-to-rail op amp like the TI 2374 instead. (This is a case where I didn’t really know how to use Eagle yet.)

It was kinda dumb of me to give the tip and ring of the 1/4″ output independent buffers with the same signal. This is really confusing to a balanced input. I never bothered to fix this, because I mainly just connect the 1/8″ output to a stereo line input and that works great.

Parts list

You can get almost everything from Mouser. There are a couple random things that I got from other suppliers, mainly because they matched the parts that happened to be in my Eagle library. Feel free to substitute, just check the data sheets carefully.

Connectors

9V battery clip Digi-Key
8-pin DIP socket Jameco
Neutrik XLR/TRS combo jack Mouser
1/4″ output jack Sparkfun
1/8″ output jack Mouser
Switch Mouser

ICs

TLE2426 rail splitter Mouser
TLV2374 op amp Mouser
INA217 instrumentation amplifier Mouser

Passive

10K reverse log pot Mouser
2x 2.2K 0603 resistors Mouser
10 ohm 0603 resistor Mouser
1.5K 0603 resistor Mouser
4x 1μ capacitors Mouser
100μ capacitor Mouser

Diodes

Indicator LED Mouser
1N4148 diode Mouser

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