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 two 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. Two 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 two
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). Two
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:
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