The MixPre-3 and MixPre-6 are high performance audio instruments. These diminutive products offer audio quality that equals, and often exceeds, rack-mounted, mains-powered audio gear. To get to this level of performance they require more power than other products in their class. This article discusses different battery types and their effect on MixPre battery runtime.
Great Audio Performance Requires Power
The MixPre-3, MixPre-6, and MixPre-10T offer Sound Devices Kashmir microphone preamplifiers. Kashmir preamplifiers offer a substantial amount of ultra-low-noise, ultra-low-distortion gain, up to 96 dB! For applications with low-output dynamic microphones or when distant-miking in quiet ambient environments, high gain is a requirement. Lower gain preamps, like those found on many handheld recorders, won’t work in these applications.
The MixPre Kashmir preamps provide full-specification phantom power (up to 10 mA at 48 V) for true condenser microphones. The combination of their high gain, low noise, low distortion Kashmir circuitry plus full specification phantom power means that the MixPre recorders require more power than recorders with lower gain, lower performance preamp topologies.
The MixPre-3 and MixPre-6 at idle ( powered but not recording or powering phantom microphones) consumes roughly 4 watts. Power consumption increases when the unit is recording and when phantom power is activated. The MixPre-6 can draw close to 6 watts while recording and using four high-performance phantom powered microphones.
AA Battery Basics
The MixPre recorders are powered from four AA (LR6) batteries with the included battery holder (MX-4AA). The optional MX8AA, doubles the battery count to eight. The MixPre can also be powered by L-type camcorder batteries when using the the optional MX-LMount sled.
AA batteries are a convenient and readily available power source. However, not all AA batteries are the same. In higher power applications such as in the MixPre, the right type of AA cell has a significant effect on battery runtime. AA (LR6) batteries are available in numerous types. Three common types are used in the MixPre: alkaline primary, NiMH rechargeable, and lithium primary. Each of the AA battery types has pros and cons.
Alkaline batteries can power the MixPre, but are not recommended. While alkaline batteries are well-suited for low current (less than 25 mA), intermittent use applications such as remote controls, computer mice, and low power flashlights, alkaline cells don’t perform well in higher power applications.
When looking at a battery as a voltage source and a resistor, alkaline batteries have higher internal resistance than either NiMH or lithium cells. In a lower power application this high internal resistance isn’t an issue, and the battery can deliver close to its full rated capacity. In higher power, continuous applications like in the MixPre, the higher resistance of alkaline batteries works against it, dropping the voltage, increasing its current draw, and quickly depleting the battery.
Typical alkaline AA-size alkaline batteries are rated for 2500 milliamp hours (mAh) at 1.5 V. Again, at lower current loads it can provide the rated power. With higher loads, the mAh capacity goes down significantly. For example, alkaline AA batteries provide only 1300 mAh with a 500 mA load.
AA alkaline pros for use in MixPre:
- Inexpensive per cell
AA alkaline cons for in MixPre:
- Short runtime because of high internal resistance and discharge characteristics that don’t match up with MixPre’s steady power requirement
- One time use
- Since these don’t have as long runtime, there is false economy using alkaline batteries.
Rechargeable (and precharged) Nickel-metal hydride (NiMH) cells are a good choice for powering the MixPre. They provide a measurable increase in battery runtime compared to AA alkaline at a lower cost of operation.
AA NiMH batteries are 1.2 V nominal cells. Most quality NiMH batteries are rated between 2000 and 2800 mAh. At first it may seem that an alkaline battery (1.5 V @ 2500 mAh) should offer comparable, or longer runtime than the NiMH (1.2 V @ 2800 mAh). That is not the case with the MixPre. The difference is in the cells internal resistance. Because the internal resistance of the AA NiMH is considerably less than the alkaline cell, the NiMH battery can provide nearly all its rated capacity in higher power devices like the MixPre.
AA NiMH pros for use in MixPre:
- Can deliver good runtime in higher power devices like the MixPre recorders
- Inexpensive because of its ability to recharge
- Good availability at retail. Precharged NiMH batteries ready to operate.
AA NiMH cons for use in MixPre:
- Initial purchase price higher than alkaline cells
- Less runtime than lithium AA
- Recharging needs to be considered part of the workflow
- Below 1 degree C capacity falls off
- Off-brand cells rated performance can be inconsistent from battery to battery.
Of the three main battery types, lithium primary AA batteries provide the MixPre the longest runtime. A lithium primary battery (non-rechargeable) delivers nearly all of its available capacity (1.5 V @ 3500 mAh) at high current draw. This is ideal in higher power devices like the MixPre.
There are several “lithium” chemistry batteries that appear similar to standard AA 1.5 V lithium batteries but are not suitable for use in the MixPre. Some of these are rechargeable and some of these operate at higher voltages. Using batteries with a voltage higher than 1.5 V may damage the MixPre. Use 1.5 V lithium batteries in the MixPre, such as Energizer Ultimate Lithium.
AA lithium pros for use in MixPre:
- Longest runtime of all AA battery types, considerably longer than alkaline
- Very long shelf life
- Temperature invariant
- Quickly becoming more popular and available
- Price continuing to come down
AA Cons for use in MixPre:
- Higher per battery cost than alkaline (much) or NiMH
- One time use
Runtime Estimates for AA Batteries
The following example shows the runtime difference between battery types. Note the listed test conditions:
- AA batteries, 8 using MX-8AA
- Recording active
- 96 kHz sampling
- Tone oscillator active
- Time shown in minutes
|Battery Type||4 ch, no phantom (minutes)||2 ch with phantom (minutes)|
From the chart above lithium batteries offer significantly longer runtime in the MixPre than alkaline or NiMH batteries. NiHM batteries provide at least a 2x increase in runtime compared to alkaline batteries.
Note: Using 4 alkaline batteries in the MX-4AA to power the MixPre will give much less than half the runtime listed above for 8 alkaline batteries in the MX-8AA, since the two groups of 4 batteries in the MX-8AA are wired in parallel, thereby halving their effective internal resistance.
For applications requiring maximum runtime, power the MixPre from 7.4 V Sony L-type lithium-ion (LIB) camcorder batteries using the MX-LMount. Two high-capacity L-Mount LIB batteries can power the MixPre for nearly a day.
AA Battery Discharge Charts
The following battery discharge charts show voltage versus time with the different battery chemistries in the example above. Note the different in voltage drop over time between the three battery chemistries.
AA Alkaline AA NiMH AA Lithium