Todd - Since you're going custom anyway, screw the sled, and use the Hirose connector. Get yourself an outboard power battery that has a variable voltage setting (I've been using the 32,000mA PowerAd) and make a cable that goes from that to your MixPre. Simple and sweet. This will give you power as if it were plugged into the wall, and can handle a festival without a problem. I am currently looking for affordable right angle 4-pin Hirose connectors because the straight ones stick out too much and bend; I suggest you use one too. Let me know what you do and if you want me to get you connectors if I can get a bulk deal.
I thought it was just the 10T that has the hirose connector?
I stand corrected. I thought all SD equipment used the Hirose connectors. Apparently they are using USB-C connector for this unit (probably due to the intended use for this machine). That said, I think Todd is on the right track by having a sled adapted for his needs, but
there are available sleds for larger batteries that they sell as accessories.
From the manual:
Powering
Flexible Powering Options
In designing the MixPre-3, we felt that it was important for it to be able to
operate in a wide array of applications and environments. To support that goal,
power can be supplied from several possible sources. Some possible powering
scenarios are:
• Four AA alkaline or NiMH rechargeable batteries installed in the provided
4-AA battery sled.
• Eight AA alkaline or NiMH rechargeable batteries installed in the optional
8-AA battery sled accessory.
• One or two Sony-type L-mount batteries installed in the optional L-mount
battery sled accessory.
• USB-C to USB-C cable connected to a USB-C host computer or AC wall
adapter.
• USB-A to USB-C cable connected to a USB-A host computer or AC wall
adapter.
• Dual USB-A to USB-C cable (included) connected to a USB-A host computer.
Use this cable option if your computer is unable to provide adequate power
for the MixPre-3 with only one USB-A port.
• Optional MX-CHARGE AC wall adapter to USB-C power supply.
The master power switch for the MixPre-3 is located in the upper right corner of
the left side panel.
To preserve battery power, USB-C power sources take priority over batteries if
both are connected with one exception - if the USB-C power source does not
meet the full power requirements of the MixPre-3, you can choose to power
from battery or continue to power from USB-C. We have designed the MixPre-3
so that even with an under powered USB-C source, you will still be able to use
two of the three channels with 48V phantom mics. The third channel can still
be used with dynamic mics and line, aux, or USB sources. To achieve this we
automatically do the following:
• Dim LCD and LED Brightness
• Disable USB-A port
• Disable HDMI TC port
For protection against accidental loss of power (not something you particularly
want to happen during recording), the MixPre-3 incorporates circuitry to
automatically switch over from USB-C to batteries if the USB-C power source
accidentally gets pulled.