I've looked around and am having trouble finding an answer to a somewhat specific question. I hope that an audio engineer can help.

I have a Dayton EMM-6 microphone I'm using to characterize acoustic transducers. Currently it is being supplied by a 48V Phantom Power supply, and I am unbalancing the signal out of the power supply by way of an XLR to BNC converter. This unbalanced signal is wired into a DAQ. The goal of this is to get the raw voltage data from the microphone and manipulate it.

It technically works, but is hindered by the fact that the DAQ wants a low supply impedance (voltages that are read should be from low impedance sources). If not, the multiplexing in the device will cause ghosting, where signals from one channel begin to appear on other channels. I am reading three voltages simultaneously such that I can also get power supplied to the devices and an efficiency measurement for the acoustic transducers. One supply voltage is read across a 1 ohm resistor (current measurement) and another is read across the transducers themselves (around 40 ohms). I have measured the impedance from the signal to ground of the signal from the Phantom Power supply and determined it is likely 10k ohms. I believe this to be causing the ghosting in the system, which means that until I fix this problem I must record from each channel separately so that signals do not ghost in to the adjacent channels.

After some research, I believe that I need some sort of gain stage (transparent gain) with a low output impedance. A microphone preamplifier fits the bill here (I think), but I have had a ton of trouble finding anything that is relatively cheap, single channel, does not color the signal, and works with a condenser microphone.

With all this said, can any of you with more experience recommend a preamplifier for my application? Under $200 would be awesome. If there isn't really an option there, do you recommend I make my own from scratch?

I am also curious as to whether unbalancing the signal could be causing problems when reading the voltage using a DAQ.

A last note: if I am asking this in the wrong place, a pointer to where I should go would be incredibly helpful!


  • Cheap and doesn't color the signal? You might be able to build something but transparency is not easy to come by, therefore not normally inexpensive. If you DIY with discrete components, you'll have to find a great circuit that somehow has a low component count, and then you'll probably have to buy tighter tolerance products and test them for accuracy to get the most transparency. If you DIY based on an existing op-amp, you'll probably have to pay a premium to get a relatively transparent op-amp. – Todd Wilcox Mar 31 '16 at 2:52
  • Or... I could be totally wrong. This is for an electret mic but is promising and cheap: ti.com/tool/TIPD181 Notice that battery power keeps the cost and complexity low while also eliminating any concern about power line noise. – Todd Wilcox Mar 31 '16 at 3:02
  • Awesome! I'm going to take a look at that and see if it works. – xcnmoore Apr 1 '16 at 15:52
  • I took a quick look, and am slightly worried that that preamp does not supply the correct power to the microphone; 48V of Phantom Power are specified by the condenser microphone. – xcnmoore Apr 1 '16 at 17:20
  • In your question you state you are already using an external phantom power supply. I assumed you would keep using it. – Todd Wilcox Apr 1 '16 at 17:23

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