After quite a bit of fooling around, I finally got the USB D-104 Mic project working.
This is a D-104 Mic that will plug directly into a computer via USB. Everything is self-contained within the microphone or base itself.
The computer recognizes the device as a D-104 USB Microphone
The condenser element is a whopping 34mm, AKA 1.34 Inches in diameter. So it's pretty big. The capacitance of the mic element is approximately 50pF.
A big challenge was constructing an amplifier that would have a sufficiently high input impedance as to not load down the element. I used a 1 gig ohm input resistor, looking into a j-fet with very low gate c (approx. 4pF). The gain of this amplifier is approximately 40, and the amplifier is co-located within the head to keep stray C to a minimum. The output voltage from the amplifier-element combination with very loud speech, close talked is over 6V P-P. This is excessive, and the normal output is about 1V P-P.
The element requires high voltage - much higher than 5V which is the USB voltage available. So, a 5VDC to 24VDC converter is used to get sufficient voltage to run the element, and the associated analog circuitry.
In the base is further analog amplification, to bring the level from the head (with normal-loud speech) up to around 4V P-P for the input to the Analog to Digital converter, which is part of the microprocessor unit located in the base. The microprocessor is a Microchip PIC which contains the A/D converter, hardware interface for USB and many other input/output combinations. I wrote the software that runs on the microprocessor.
One of the pins on the chip is used to facilitate transmit/receive control from the switch in the mic handle. When the switch is open (receive), the software in the microprocessor detects the T/R switch is open, and writes all zeroes (0s) to the USB data buffers. This allows software to detect the presence of anything other than zeroes as a key to transmit/receive. The mic otherwise behaves like a standard USB microphone.
I included a line level analog output for in-shack operation and direct connection to an audio console or other analog input.
Included in the head is material facilitating a pop filter thereby reducing breath pops, etc. to a reasonable level.
Audio fidelity is excellent - Listening, I believe it is better than the Behringer B1 I usually use !
The element is unidirectional (which is what I wanted), so off-mic sounds are significantly reduced.
This is the 2nd USB D-104 Mic I've put together. The other one uses a Radio Shack electret condenser element.
I like the look and feel of the D-104. Maybe Bob Heil might be interested in producing something like this ?
Anyway, a challenging, but fun project. Here are some pictures:
The base components
The head:
The cover with pop filter:
The overall system - with the mic plugged into a laptop, and actually transmitting via remote control to the transmitter: