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.

Schematics

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.

Parts

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:

Balanced microphone preamp


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