time between the notes

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 - schematic

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 (left) supplies approximately 45 - 52 volts DC of clean phantom power to the microphones. The gain range is 5db to 40db. The pre-amp is quite small, slightly heavy, and very clean sounding. It is powered with rechargeable 9v (7. 2v nominal) NiMH batteries when recording with my AKG C-480 microphones. The AKG microphones perform well with slightly less than the full 48v DC phantom power those batteries supply. I got the name littlekit from Jon O`Neil as he was calling his pre-amp the Naiant littlebox and mine wasn`t exactly a littlebox, but more like a DIY kit. The fully assembled Naiant littlekit microphone pre-amp is pictured below, right. For more information about the Oade M148 pre-amp that inspired my project, there is an excellent discussion about it with internal and external pictures here. The Oade pre-amp uses discrete transistors and transformers to supply very low added noise gain. Due to the inconvenience of having to open the box up and pull out and recharge the (5) NiMH batteries individually every time I want to use it I haven`t been using my littlekit pre-amp very much. So, I decided to re-engineer it to run on a 7. 4v rechargeable battery rather than 48v. I found a stand alone nominal 6v DC to 48v DC phantom power circuit board in a non-working Graham-Patten DMIC-20 I had in my basement. I did some tests and found it is a nice clean circuit that will supply plenty of current to run the littlekit pre-amp. The best thing about using the new circuit is that I can power it with a relatively inexpensive 7. 4v, 940mAh single rechargeable Li-Ion battery pack (right) that will fit in the box with the new circuit after all the 9v NiMh batteries are removed! I had to add a power jack on the box, (top left photo) so I could charge the new battery pack from the outside with a dedicated Li-Ion battery charger (left). Since I had to take the littlekit apart to drill the hole for the new power jack and make the component changes, I also


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