Electrete_Condenser_Microphone_Line_Level

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.

Overview

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 together.

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.

The Circuit

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 problems.

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.

Assembly

Photo 2. The preamp circuit fits inside an approximately 3 cm x 4 cm x 2.5 cm plastic box.
The tie.

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 sources.

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 damage.

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.

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