I decided to test my pre, powered by a 9V battery.
NOTE: I'm testing the pre without any optimization. Basically, I took the preamp designed for use with a 12VDC wall wart, and hooked it up straight to a 9V Battery.
This is the battery I used (bought at a grocery store). Duracell Coppertop 9V battery... costs about $10 for the pair, or $5 each.
CURRENTLY: The preamp is not optimized yet for battery use. Basically, the 48VDC-DC converter is running all the time even if phantom power isn't used. The PSU has an on-board LED mounted inside the case (that no one can see, it just serves as a reminder for me that there is input power)... the LED resistor value is low, so the LED consumes 5mA, the VU meter is running and operating... again with the LEDs using 5mA (low R value), the preamp is using DC servo instead of AC coupling. Again, these were things that didn't matter back then when I'm using a wall wart or a power transformer.
TO DO/FUTURE: To optimize the pre for future battery operation, I need to modify the phantom power switch so that it turns ON the 12V to 48V DC-DC converter only when phantom power is needed... instead of being ON all the time. I need to reduce LED consumption from 5mA to under 2mA... from my testing, the brightness is still good at less than half the consumption. I need to remove one of the opamp chips on the board and let the preamp run using AC coupling, instead of DC servo. That reduces chip count by one. (Or provide a switch so the user can toggle between DC Servo/AC Coupling.)
Again, typically... I expect to see a 12Volts input. But in this scenario, we'll just use an ordinary 9V battery.
We start at 9.1 Volts out of the box, hooked it up to the preamp with all the LED lights blazing, and VU Meters dancing. I used music material, and a movie material with lots of bass and explosions (Transformers
) The VU meters got a real workout.
Total Running Time on a 9V Duracell battery: 225 minutes (or 3.75 hrs) After that, the preamp shut down and stopped, i.e. no sound can be heard. Prior to that, even at 224 minutes, there is still full sound.
Power Consumption did not change whether the gain is at 6dB or higher. It's constant. However, the VU Meters with the bright LEDs, consume a lot of power (5mA per LED). Phantom Power is off on this quick and dirty testing. Single channel testing only.
As you can see from the graph, voltage was falling at a steady rate. And even when the 9V Duracell battery was only giving less than 7Volts, we still got 36 minutes operation out of the preamp (with all the VU meter LEDs running, etc).
ADDENDUM: When the pre shut down, I disconnected the 9V battery and let it rest for 15 minutes. Battery voltage climbed up from 6.5V to just a little over 7 Volts. Then I hooked it up again to the preamp and we got another 30 minutes life out of it.... then it shut down.
Rest the 9V battery for 10 minutes, and we got another 20 minutes operation out of it.
At this point I concluded my testing.
The current consumption ranges from 109 to 117mA. I'm a little surprised about this, since the preamp board is supposed to use only 17mA... and add 5mA for the PSU LED. I don't know how much the VU meter consumes though. It depends on the program material, and on how many of the 5 LEDs are lit up at any given time. Also, the (2) onboard DC-DC converter have a set minimum power consumption for it to operate. So add all those up, you get 109 to 117mA. Adding a second preamp channel shouldn't increase much power consumption, but adding a 2nd VU meter will.
I *think* I will ditch the VU Meter function and opt instead to use a signal and over/clipping indicator, using low-power opamps to do the job. Hmmmm.....
Also, just to put things in perspective... we're converting this ordinary Duracell 9 Volts input (down to 6.5 Volts) internally to a HIGHER split +/- voltages and even +48Volts.
You can't create a higher voltage from nothing, so the current consumption increases as the voltage falls down to maintain the same power equation... you know, Power = Volts x Current
In this case, total power consumption was less than 1 Watt the whole time! Ranging from 0.91Watts to 0.75 Watts.
In field use, most likely you'd probably be using a higher capacity battery (and not a 9V toy battery)... like a Li-Ion with 6800mAh, and not the puny 580mAh of a toy battery. I think it will be safe to say that you'll be able to tape concerts several hours long.
Until next time....
