balanced microphone preampPosted by Author makecircuits
Balanced microphone preamp
As I said in my cheapo digital audio project document, I was going to build a microphone preamp to attach a Behringer ECM8000 measurement microphone to line input of my minidisc deck. After scanning the web to see if similar projects existed, I basically found three kinds - ones that used transformers (ie. pretty expensive to start with) and some even followed those with tubes and another transformer (damn expensive). Then there were couple of textbook implementations using SSM2017 chip in pretty much datasheet configuration, and finally, some projects based on op-amps.
After toying with idea to use SSM2017 (Analog Devices has discontinued the part, THAT 1510 is a compatible chip) I put it aside as too much trouble and expense to order the parts via Elfa catalogue which carries the part, as I did not have anything else going at a time. So, op-amp version it was.
Most electronically balanced input preamps have largely similar design for input circuity - the phantom power is fed to microphone via tw1o 6.81k resistors (these should be matched pair), DC is then blocked with caps and op-amp is then wired to amplify the differential signal. A circuity is usually added in a form of zener limiter to protect op-amp inputs from large transients when the microphone is connected/disconnected or power is turned on/off.
Well, it`s pretty much a PAiA preamp - usual input circuity followed by one half of dual op-amp rigged in fixed 20dB amplification circuity, followed by second stage which is adjustable for up to 40dB amplification, for up to 60dB total.
The schematics shows NE5532 op-amp which I was unable to get when I shopped for parts, so I used an ancient JRC4558D op-amp which I scavanged from PCB of old Yamaha synth (some people seem to be very fond of this chip, maybe some day someone will tell me why?). Nice thing about these dual op-amps in DIP8 package is that pin layout is pretty much standard and you can just drop in a different one if you want to. So feel free to experiment, look towards op-amps with low noise and big allowed differential input voltage.
The power supply is a simple affair, supplying +-12V using 7912 and 7812 regulators. As ECM8000 is happy with only 15 volts of phantom power, I added a 7805 which I referred to +12V instead of ground - resulting in 17V output which I used for phantom power. tw1o things - as the circuit uses only a neglible amount of power, the regulators may need an extra load resistor across outputs for proper regulation. about 2k should be fine (the power-on led does it for 7912). Another thing which is actually related benefit - you can pretty safely raise the input voltage for regulators to get more voltage for phantom power, just use higher output voltage regulator instead of 7805 (or raise it using a zener). Make sure you stay below allowable input/output difference limit for 7812, tho (usually 40V).
Unfortunately, precision resistors are need to achieve a good CMRR using op-amp. I did not have these, so I used regular 5% ones and added small value resistors in series to adjust for exact values. On schematic drawing precision resistors are marked with asterisks. I also did not have metal film resistors which you may want to use in first stage due their smaller noise. Anyway, I did not build it for recording mic, so I used usual carbon resistors and even put tw1o regular electrolytic caps in signal path.I will probably replace the opamp some day, but I`m happy with a current version for my purpose as it is.
I initally planned to build the whole thing using scavanged parts, but as I missed some I went shopping. Thn I decided to do it differently this time to see how much the project will cost when built from off-the shelf parts (no exotic stuff like metal film resistors and polymer caps this time, remember). tw1o most expensive parts were a power transformer (3.7W, 230V primary, 2x15V secondaries, PCB mountable version) and a piece of breadboard (I only used less than half of it). Enclosure (plastic, easy to work with) and accessories like power switch, power cord and it`s connector on a back plate were also significant cost raisers. Thw table below shows parts and their prices in Estonian kroon and DEM, as we have 18% sales tax in here, it is added at a bottom of table. The shop where I bought the stuff (Tevalo) is certainly not the cheapest place to buy parts, but as it was saturday others were closed. The point was to see how much the project would cost approximately, so here we go: