time between the notes

Description: The circuit board purchased did not include a functioning 48V phantom power voltage converter. This was acceptable as the intention was to create a 48V DC phantom power microphone preamplifier powered by five 9V batteries. The circuit board, along with a power switch and the batteries, fit well into a Hammond 1590BB enclosure. Inspired by recordings made using the M148 microphone preamp designed by Doug Oade, the goal was to utilize batteries to provide clean phantom power for condenser microphones in the field. The M148, known as "the brick" due to its weight from multiple sealed lead acid batteries, delivers 48V DC to the microphones and powers the preamp's transistor front end. The purchased circuit board was pre-assembled with components, requiring only the installation of a SPST power switch, a power indicator LED, and holes drilled for input/output jacks and gain knobs. 9V battery connectors were sourced from Radio Shack. The completed preamp supplies approximately 45-52 volts DC of clean phantom power and has a gain range of 5dB to 40dB. It is compact, slightly heavy, and produces a clean sound, powered by rechargeable 9V NiMH batteries when used with AKG C-480 microphones. The project was named "littlekit" to differentiate it from Jon O'Neil's Naiant littlebox preamp. Due to the inconvenience of recharging the individual batteries, there was a decision to redesign the preamp to operate on a 7.4V rechargeable battery. A standalone nominal 6V to 48V DC phantom power circuit board was repurposed from a non-functional Graham-Patten DMIC-20, providing a clean circuit capable of supplying sufficient current for the preamp. This new configuration allows for the use of a cost-effective 7.4V, 940mAh single rechargeable Li-Ion battery pack, which fits within the enclosure after removing the 9V NiMH batteries. A power jack was added for external charging with a dedicated Li-Ion battery charger.

The circuit design centers around a microphone preamplifier that utilizes an instrumentation amplifier, specifically the INA111, chosen for its known performance characteristics in similar applications. The preamp is configured to accept a phantom power input of approximately 48V DC, which is essential for powering condenser microphones. The circuit layout incorporates a power switch for user control, along with a power indicator LED to signal operational status. Input and output jacks are strategically placed to facilitate easy connections to microphones and recording devices.

The gain control is implemented through variable resistors, allowing adjustment from a minimum of 5dB to a maximum of 40dB, accommodating various recording environments and microphone sensitivities. The design emphasizes low noise operation, critical for high-fidelity audio applications, ensuring that the preamp does not introduce significant distortion or unwanted artifacts into the audio signal.

The transition to a 7.4V Li-Ion battery system simplifies the power management aspect of the preamp, eliminating the need for multiple 9V batteries and the associated maintenance issues. The selected DC-DC converter circuit board is capable of stepping up the lower voltage to the required 48V while maintaining efficiency and stability, which is crucial for reliable performance in field conditions.

Overall, this microphone preamp circuit exemplifies a thoughtful integration of components and design principles aimed at delivering high-quality audio performance in a portable format. The result is a manageable, user-friendly device suitable for various recording scenarios, particularly in field applications where traditional power sources may not be available.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

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