time between the notes

Posted on Feb 4, 2014

The circuit board I purchased didn`t have a working 48v phantom power voltage converter on it. That was completely fine with me because I wanted to build a 48v DC phantom powermicrophone pre-amp which would be powered by five 9v batteries. It was good luck that Jon`s circuit board plus power switch and the (5) 9v batteries fit nicely in a Hammond

time between the notes
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1590BB enclosure I already had (below, right). After hearing some recordings made using the M148 microphone pre-amp designed and built by pioneering taper Doug Oade, I was intrigued by the possibility of using batteries alone to provide clean phantom power to condenser microphones in the field the way his pre-amp did. The M148 is nicknamed "the brick" by users partially due to the weight, as it is loaded with several small but heavy sealed lead acid batteries wired together to deliver 48v DC to the microphones. Those batteries are also used to power the transistor front end of the pre-amp. The partial littlebox circuit board I bought from Jon (left) was already stuffed with components. All I had to do was install a SPST power switch, a power indicator LED, and drill holes in the enclosure for the input & output jacks and the two gain knobs. I found some nice 9v battery connectors at Radio Shack. In the end, all the pieces fit together rather well in the small Hammond 1590BB aluminum enclosure (top left & above right). This schematic shows the microphone pre-amp circuit Jon designed and built. My pre-amp uses the INA111 instrumentation amp rather than the AD620 he used in the early littlebox pre-amps. It was my choice to go with the INA111. I am more familiar with that chip because I know what it sounds like in other IC based microphone pre-amps I`ve used. With five 9v batteries installed, my finished littlekit pre-amp...

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