Secrets of the DXA-8's power needs

[David Ennis]

I've seen comments from time to time about the DXA-8's appetite for 9V batteries. I thought I'd share my experience.

My first DXA-8 would not last two hours with a fresh Duracel even when not supplying phantom power, and it generated a lot of noise in the audio as the volume trailed off over the last 10 minutes or so of battery life. BeachTek was willing to exhange it, but B&H did so themselves.

The one I have now will power both of my AT3031 mics nonstop for 3 hours with my full confidence. It delivers full volume without noise right up to the point where the battery suddenly dies near the four hour mark. I'm sure I could reliably get four hours if I rested the battery after two hours. I love it.

Still, I was curious. I know that the voltage of a battery drops when it delivers power to a load due to internal resistance. I know that 9V batteries don't store much charge compared to an AA, and that the voltage quickly drops off in use and spends most of its operating life at less than 8 volts. But what is the minimum voltage the DXA-8 will operate at? Could I run wires out of the DXA-8 and power it from a 7.2V rechargeable Lithium Ion battery? That would last for days!

So I decided to do a test. I drilled two 3/32" holes in the outside wall of the DXA-8's battery holder, one at the lower left and one at the lower right. Cosmetically and functionally, they do no harm. Through each of the holes I ran in a length of #22 or so hookup wire and wrapped the stripped end around one of the respective battery terminals. I snapped the battery into place, and there was no problem accomodating the thickness of the wires. I inserted the assembly into the unit as usual, and attatched the stripped ends of the wires on the outside of the unit to a multimeter.

I plugged one of my AT3031s into each channel, plugged the Beach's miniplug output into my GL2, flipped on phantom power and the limiters in both channels, set the gain knobs to midway, and set my GL2 to manual audio mode. The mics were set up near my playing television and I adjusted the GL2's gain controls for a nominal -12dB. Before turning on the DXA's power switch, the fresh Duracel had read 9.46V. This dropped to 9.18V when I turned the unit on and proceeded to fall initally at the rate of 0.01V every ten seconds or so, but that rate slowed down. Here's how it went:

Time------Voltage
(min.)
0 --------- 9.18
15 -------- 8.62
40 -------- 8.13
60 -------- 7.89
95 -------- 7.56
120 ------- 7:40
180 ------- 7.02
210 ------- 6.75
240 ------- 6.55
255 ------- 6.12
260 ------- 5.90

For those who use Excel, when I plot the data I find that a 3rd order polynomial trendline fits it perfectly. If I were BeachTek I'd burn in every unit with such a test.

The limiter LEDs started to dim noticeably at 7.0V and were pretty much unresponsive even to a tap on the mic housings at 6.7V, but there was no letup in audio quality, as monitored by headphones, or volume. At just below 5.9V the audio suddenly died, and the voltage abruptly crashed to about 3.2V.

Clearly, a hearty external 7.2V supply like a Li ion pack would work. But I'm guessing that 6 alkaline "C" or "D" cells connected in series (giving 9 to 9.7V ) would probably last longer due to the higher initial voltage. Harry Kaufmann at Beachtek says that they have considered making an external power pack, but that they didn't believe the demand exists. I'm not so sure. Yes, a lot of events are less than two hours, and a fresh alkaline will cover it for most of us. But then what do we do? Many of us throw away batteries knowing that they still have a lot of life in them because we don't trust them for another gig. And some people would like to be able to shoot a lot longer without thinking about Beachtek power.

[David Ennis]

I decided to probe a little deeper and set up to measure current, voltage and power drawn by the DXA-8 with and without supplying phantom power to AT3031 mics.

I found that the DXA-8 is a constant-power device. That is, as voltage falls, the circuirty causes current to increase such that P = V*I stays constant. That's good, because according to Duracell, that mode of operation provides the greatest service life from an alkaline battery.

My DXA-8 draws 318 miliwatts when no phantom power is in use. Each of my AT3031s causes the DXA-8 to draw an additional 210 miliwatts. Limiter switch settings and gain control settings have no effect on this. This is consistent with Beachtek's specs, and the data in my preivious post is consistent with Duracell's specs on their 9V Coppertop.

It is also consitent with Audio-Technica's specs for the AT3031 if we allow that the DXA-8's phantom supply is about 70% efficient. That is, about 30% of the energy is lost as heat. That's reasonable.

Bottom line #1, if your DXA-8 won't power two condenser mics for 2 hours comfortably. something is clearly wrong enough that you should pursue it.

Bottom line #2, whatever useful life you're getting from a 9V alkaline would be multiplied by about 15 (or closer to 30 if you're throwing away partially used 9V batteries) with an external battery pack consisting of 6 alkaline "C" cells. Such a battery pack should be easy to make and use, and I intend to do it.