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Condenser Microphone Line Level Preamp Project
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This preamp lets a condenser microphone
work with line level inputs such as those found on Macintosh computers.
Photo 1. A small hole was cut into the top of
the box, near the far end, so the pilot LED can be seen.
I have a some Macintosh computers that have line level audio inputs and
no microphone inputs. I also have a USB-to-audio interface, which I
bought for the iBook, which has no analog audio I/O at all. The iBook
is dedicated to a particular use, but I wanted to use the other
computers for Voice Over Internet Protocol, which is provided by
services such as Skype, iChat, AOL AIM, Google Talk, and others.
I searched the IT mall in the city I live in Thailand and didn't find a
USB-to-Audio interface, and its not really worth ordering from the
United States because of the international shipping charges and the
hassle of having to go to the post office and pay the import duty. What
I really needed was a preamp, and since I had come to a lull in a large
project involving RF and microcontrollers, I was ready for a quick
analog project for a change of pace, I went ahead and put a preamp
Here is what I had to work with: The computer needs at least 180
millivolts peak-to-peak and the condenser microphone in my headset
produces about 20 millivolts peak-to-peak into a 2.2k load with normal
speech levels. There are USB ports on the machines, so I can use that
for +5 volts. After a quick look in my parts boxes, I decided to
put the input jack and output plug on the end of wires so I would not
have to tediously mount them on the enclosure. The resulting goal was,
therefore, a condenser microphone preamp with 19 db gain that is
powered from a USB port that can be closed in a small box to hang off
the computer and attach to the microphone lead on the headset.
Figure 1. The pilot LED is
powered from the USB connector, before the power supply filter.
The preamplifier itself is a single NPN transistor, and it gives about
28 db of gain. The electrete microphone element includes an
inverting FET (Field Effect Transistor) amplifier, and the drain of
this FET is the the output of the microphone element. The NPN
transistor in the preamp is connected as an AC transconductance
amplifier, meaning that its output is a voltage proportional to the AC
input current. The scale factor is set by the 100k feedback resistor.
With this value, the amplifier's output is 1 volt peak-to-peak for each
10 microamps peak-to-peak of input current.
The drain of the FET in the microphone element needs to be pulled high
through a 2.2 k resistor connected to a sufficiently high voltage.
That's the purpose of the 2.2k resistor on the microphone input. The
other end of the 2.2k resistor connects to the power supply filter,
made of the 100 Ohm and the 330 microfarad capacitor. The output of the
power supply filter also provides power to the amplifier circuit.
Since the collector of the NPN transistor is at about +1 volt, I was
concerned that a load resistor inside a computer, which connected to a
positive power supply could cause the 4.7 microfarad output
signal coupling capacitor to be subjected to revers voltage, possibly
resulting in the preamp not working and the eventual destruction of the
4.7 microfarad capacitor. I found that many sound cards put the load
resistor on the "Ring" connection of the audio input connector. I only
connected the output of the amplifier to the "Tip" connection.
It does not make sense that there would be a load resistor for a
condenser microphone at audio input that does not have sufficient gain
to work with a condenser microphone, but there are some switchable
inputs, and the precaution of connecting only to the tip would prevent
A green LED is connected across the 5 volts from the USB connector. The
2.2k resistor doesn't light the LED very brightly, but it is just not
in my nature to spend a lot of current on indicators unless there is a
real need for them to be bright. And this indicator is only to show
that the power is flowing from the USB connector.
Photo 2. The preamp circuit fits inside an
approximately 3 cm x 4 cm x 2.5 cm plastic box.
The circuit was assembled on a small piece of pre punched phenolic
circuit board. The layout of the components is not critical, but
its a good idea to keep the connections to the base of the transistor
short to minimize coupling of this sensitive node to external noise
A sutiable small black plastic box, which cost only 8 baht (about
US$0.25) at NPE (Nattapong Electronics) store in Bangkok was pressed
into service as an enclosure. I cut a small hole in the top so I could
check on the green LED.
The microphone input jack and the output jack are at the end of wires
rather than being mounted on the box. This lets the box dangle at the
back or side of computer. A USB connector leaches about 4 milliamps
from the USB +5 volt power supply to power the preamp. The connectors
are tied to the circuit board with small gauge enameled copper wire.
The holes through the leads exit the box were carefully made to be
fairly tight, using a soldering iron. Plastic wire ties were
added to act as
additional strain reliefs (see Photo 2).
I spent more time putting the audio connectors on the cables than I did
building the circuit board. The audio connectors were carefully
assembled with strain reliefs upon strain reliefs. It seems that these
are the points in the circuit most subject to failure from physical
The cables for the audio connectors was taken from a cheap USB mouse.
The USB connector and the cable that it is molded onto was taken from
Apple's first USB mouse. Anybody who ever used this ergonomically
disasterous mouse would agree that the cabel and connector are much
better used in this project than in the mouse.
After building the circuit and testing it at least twice, I put the
circuit board into position in the plastic box, then applied a drop of
Cyanoacrylate adhesive to each side fo the bottom section of the box to
fasten it closed.
It works great. Its a little more sensitive than I had expected, but I
just turned down the gain control on the audio preferences panel in the
The whole thing cost next to nothing, it saved
me from the expense of buying a USB-to-audio interface, and I got a
sense of satisfaction building and using it.
Find updates at http://www.projects.cappels.org
Contents ©2007 Richard Cappels All Rights Reserved. Find updates
First posted in June, 2007
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