Here’s my DIY solution to powering the Sound Devices MixPre-6 audio recorder. The official “L Series” battery sled (MX-LMount) works great, but I wanted something to meet my specific use case. It’s a flat “backpack” that hooks under the MixPre via claws on the front and back, with a short wiring harness that connects to the existing 4xAA battery sled on the back of the recorder.
Updated to v2 (June 03 2020): New side cleat (mixpre-batt-side-cleat-v2(print2x).stl) is more durable and has larger hook end so recorder can stand on it’s end without falling over from the offset weight of the battery pack.
Note: You may ask if it’s safe to send 8.4v into the battery sled connector (since the AA batteries would provide a lower voltage). AFAIK no regulation is required, and I use this setup often. The battery logic in MixPre recorders changes based on input voltage, and this setup properly triggers the Li-Ion battery profile. (I think you can over-ride in the menu though.) The official L-mount adapter (MX-LMount) has active electronics, but there doesn’t seem to be any regulation (at least at 2S lipo voltages). I have tested one with a charged L-mount battery and it passes 8+ volts from input to output. I think the electronics are a set of MOSFETs with some clever logic to isolate the 2 L-mount packs from eachother? Otherwise the lower one could discharge the higher one? Probably also prevents a short on one input from frying the other battery. We don’t need any of that with a single 2S pack (though you DO want protection from under/over voltage and excessive current – normal Li-Ion protection board safety stuff.)