USB pre-amp with direct monitor for electret microphone

Dear all,

I run a computer lab with headsets that include electret mics and 4-pin 3.5 mm jacks.
I need to plug them into laptops and let students record their speech so that they hear what they are saying without perceptible latency.
The price is the most important factor.

Can you recommend any solution?

One idea could probably be

  • a usb microphone interface with direct monitoring like Roland Duo-Capture or Behringer U-PHORIA UM2
  • 4-pin to 3-pin (mic and headphones) adapter +
  • two 3-pin 3.5mm jack to 1/4 inch jack adapters

Or maybe you can think of anything better?


You picked the two system most unlikely to work together. Any time you plug a microphone into a computer, the monitoring for that microphone is likely to come back out to headphones late because of digital processing. Built-in echo.

You need a computer to run the microphone because the battery voltage (five volts) to run the electret element comes from the computer.

So to recap. You can’t use a stand-alone MicPre like the UM2 (I have one—terrific MicPre) because you can’t plug the microphone in and you can’t use the computer because of sound delay problems.

There is only one way out I know of and I’m looking for the documents. Are you any good with soldering irons and electronics hand tools? I built two of these, so I know it works. You build an adapter with a little battery in it to make the electret microphone happy and then in construction, you put a male XLR on the end of the interconnect cable and plug that into a UM2. Listen to the UM2. It has zero latency monitoring and it’s certified for musical overdubbing.

That would work. It may be the only way to make that work.

I can find the schematic, but I have pictures of the adapters I built…somewhere. I’ll take another one if I get deep enough in. Dang. I think I even posted them on-line somewhere.


OK. Here’s the adapter. I’m using 4 AA cells for 6 volts instead of the 5 volts that normally comes from the computer. You do have to select the parts so the microphone contacts end up on the right rings for the connector on the left. Or you can customize the connector as you make the box.

That’s the UM2 on the right.

Let me know if you need the schematic.

I bet you’re wondering why nobody makes anything like this. Because it’s more expensive than a new microphone. Everybody who tries this smacks into the same problem. One podcast Producer did make a four-way version of this for four co-hosts. In that case it worked. Nobody makes anything like that short of a full studio.



Thank you for your reply and the ideas!

I’d very much welcome the scheme, Ill try soldering it.

I wonder if your design isn’t similar to - with the difference that the French one allows to avoid buying and changing batteries… UM2 after all does provide power for XLR, only the voltage is too high the the electret.
Check out the links at the top of the above page. I asked around today and somebody suggested using voltage adjustable regulator like TL783C, possible schemata are available here

I’m also curious about electrets we are using. Do you know the parameters of the mic in your setup? Mine has a voltage range 1,5-10V. Which theoretically should run on a single battery in your design. You think in that case the effect would be lower dynamics?

Another thing that might help my problem though I haven’t tested it yet is asio4all + sound recording software like fl studio. I haven’t managed to run fl studio but this solution is claimed to reduce latency to imperceptible lows - which if true would allow to bypass the expensive pre-amp.



There is also one more thing I’m curious about - what is the voltage of UM2 if the phantom power is switched off. Some say that none, some say it should suffice for an electret. I couldn’t find that information on

And here are one simpler and two more advanced ways how to power an electret micrtophone with phantom power including diagrams

I did the Tip, Ring, Sleeve version. Scroll around if the graphic doesn’t fit. I did it before the forum graphic standards were posted. Everything to the right of the dotted lines is adapter. The original design was not for three-pin XLR but for a “dry” 1/8" plug. Tie XLR pins one and three together and connect that to the common sleeves. Put the Clean Sound on XLR pin 2.
That 10,000 ohm bleeder resistor is recommended but not often included. If you don’t put that in the circuit and you put the microphone and adapter together first and only a bit later connect it to the mixer, you could briefly dump five volts into the mixer. Don’t wear headphones while you’re doing that. I’m just doing the maths in my head and that bleeder probably should be 22,000. Ten times the value of the microphone part which is 2200.

I dug out one of the posed graphics.

I haven’t dug into the documents for using 48V phantom Power in an adapter yet, but I can’t help thinking it’s begging loudly for trouble. Phantom Power is phantom by putting 48V on both XLR pin 2 and pin 3 but not on pin 1. Since the show is squirting down the audio cable as the difference between 2 and 3, the battery voltage effectively vanishes. It becomes a phantom [organ music up].

Many adapters work by grounding pin 3. 48V isn’t a phantom any more. I would think if you really offended the sound gods recently, you could blow a speaker across the room. I do know someone who got it to work, though, so there’s hope.

As we go.


My UM2 supplies +46 volts at the XLR when the phantom power is turned on. It drops to dead zero over about a minute (with nothing connected) when I turn it off. I stopped watching at .003v and sinking. Phantom power is required to actually go off when you turn it off. Some legacy microphones can be very seriously damaged by applying phantom power.

That one, for example (the big one in front).

You can reduce a 44BX ribbon to garbage by applying phantom power to it.

Looks grand, doesn’t it? It’s Hollywood. His actual microphone is that little Sony lavalier on his jacket.

As we go.


Audacity can’t easily use ASIO, so that’s not a good Get out of Jail card.

Please note that headset in my illustration has the headphones on one plug and the microphone on another. Many modern, non-digital headsets jam everything onto one plug via extra rings and it’s up to you to figure out what they did.
Screen Shot 2016-09-10 at 22.34.48.png

Do you know the parameters of the mic in your setup? Mine has a voltage range 1,5-10V. Which theoretically should run on a single battery in your design.

I was obsessive. The actual implementation in that illustration uses four AA batteries for a total applied voltage of 6v. The microphone has little or no change in operation from the 5 volts supplied from the computer. It should also give a battery life of forever. Many people jump right to the “transistor radio” 9v battery. As you saw, that’s right on the design edge of some electrets.

I’m not a fan. While I have no doubt you could force that to work, you point out it exceeds the applied voltage of a 10 volt device. It also has a Zener. Zener diodes are avalanche devices. They work by breaking down at a predictable voltage. They do it loudly. C1 is required to suppress the Zener noise. Put a good quality capacitor in there—or better still two. One large sloppy electrolytic like that one and a nice, well-behaved smaller mylar or mica.

Remember, we’re talking about microphone signals. Just a little louder than molecular noise.

The schematic is incomplete. There is also 48v on pin 2. That means the design has pin 3 doing battery work but not pin2, so the 48v is unbalanced and could be noisy.

Pin 3 is, in essence, isolated and floating where pin 2 has the show on it. So Pin 3 is an antenna. Make sure everybody turns their cellphones off.

It’s a given with all these designs that you will never get the 30M cable runs typical of broadcast microphones. You may not make it across the room. But it may be enough to go with.

I need to go look at the other examples.


That one is much better apart from the Zener value which can be chosen. Somebody is paying attention.
I’m going to bed.
Screen Shot 2016-09-10 at 23.16.26.png

In stead of a battery box, I use a phantom-to-pip adapter. Here’s the schematic:

You could use a 9V zener (D1) to lower the output to less than 10V if you think your mic wouldn’t be able to withstand 12V.

The lowest noise is produced with a 5.6V zener.

Higher pip power gives a higher headroom. The output won’t be significantly higher, and the noise floor is a tiny bit lower with a higher voltage to the electret.

And again, we’re designing a Phantom Power adapter. My original design was to a dry (no battery) 1/8" plug, not commercial grade XLR. There was no 48v and no other option.