MICAMP - 2
|After the successful construction of my microphone
I decided to create a new one. This would be smaller and have even better performance.
I am very familiar with the technology used in professional audio.
I have worked for many years with the design of audio routers for broadcast stations.
I see that amateur radio would benefit from adopting some of this technology.
Here are some:
Audio outputs using 3p. XLR male. p1 - GND, p2 - positive and audio, p3 - negative audio,
Output impedance should be less than 66 ohms.
Audio input using 3p. XLR female. p1 - GND, p2 - positive and audio, p3 - negative audio,
Input impedance will be bigger than 10 kohm.
Here is my new microphone amplifier. It has 48 volt phantom power for condenser microphone.
I'm starting the project to explore the junkbox after the most important parts.
Transformers, fuse and power switch are mounted at the back panel.
12.6 Volt DC for the tube,
+/- 15 Volt DC for the line output amplifier,
330 Volt DC with my special hum killer regulator.
48 Volt DC for microphone phantom voltage.
|I am using DC filament voltage.
With DC, I avoid hum.
A microphone amplifier has high gain could not have hum.
|One female XLR microphone is used as input.
XLR male used for balanced line output.
Phono jack for the unbalanced line output.
Here is the tube sockets, input transformer and volume and gain pot meter mounted.
|To the left I have line output circuit.
The NE5532 is used for line driver for RCA connector
and for volume control.
The THAT 1646 is used as a balanced line driver.
This is the best line driver for professional audio.
Here, the power and amplifier assembled in the box.
|Here is the phantom power circuit.
Two 6K81 resistors are connected from 48 Volts DC to the positive and negative microphone input.
To the left I have Mic transformer. It has aluminum screen.
|This is from the same area.
I have here exchanged the microphone transformer with a new type.
When I tested the amplifier, I hear a terrible hum.
The cheap input transformer pick up the magnetic field from the main transformer.
Here I have mounted a shielded Sennheiser transformer. Perfect, no hum.
I can now begin to take down the Soundcraft Mixer and replace it with my new microphone amplifier. Saves space.
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