O2 headphone amp with 4x 8.4V cells
Here is a modification of RocketScientist's O2 headphone amplifier that allows 4 of the 8.4V NiMh cells to be used in the taller B3-080 chassis. Two cells are in parallel on each power rail for double the run time.
The first photo below shows that two 8.4V NiMH cells fit just fine on top of each other on a standard O2 PCB in the B3.080 case. The battery terminals must be left off the PCB to allow them to fit. Double stick tape between the cells and between the bottom cell and PCB is a good idea to keep things in place. The contacts are pointing out the back in the photo just for the picture. They go in the other way, the standard direction.
The stacked batteries in each position use a 2-cell 9V battery clip, Mouser 534-227, with the shorting wires on the clip cut. The clip normally connects the 2 cells directly in parallel for use with alkaline batteries. Here we add diodes. so the shorting bar must be cut and two more wires soldered on. Don't put the 8.4V NiMH cells directly in parallel without the diodes added, as shown in the schematic.
The schematic below shows how it all is wired. The only change to the two existing batteries is the addition of a Schottky diode in series with the 220R 1W resistors. Just cut one end of the resistor short, then place the diode in the PCB hole on that side of the resistor (tombstoned). These are 3A Schottky's to account for thermal derating due to proximity to the power resistor. 1N5822 or SB340, Mouser #511-1N5822.
The two new batteries in parallel are wired the same way, with new diodes and a diode in series with the new 220R resistor. But the two additional 220R resistors are Caddock T0-220 mount units that can be mounted on the rear panel of the B3-080 chassis, with the diodes soldered to the leads. Mouser #684-MP820-220.
The diodes allow each of the 4 8.4V NiMH cells to charge independently, then discharge independently to account for the differences in the cells.
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