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Adapting a USB sound card to work with a studio condenser mic

Neewer is a Chinese brand for good quality affordable audio equipment. Their condenser microphones NW-700 and NW-800 are reasonably priced and really good. Those are cardioid-directional low noise microphones with a wide dynamic range and they can compete in sound quality with professional microphones. It's possible to get this microphone to work really well with a simple USB sound card.

nw-700 mic

nw-700 frequency response

Condenser microphones need a power supply to function. Almost all computer microphones are condenser microphones and your PC provides about 2.5V to 3V DC via a 3KOhm resistor on it's Mic port. Those 3V are good enough for the typical small headset microphones and other computer gadgets. The problem is that the Neewer NW-700 needs a higher voltage. The NW-700 will work a bit with the 3V but the volume will be very low. It does just not work properly with so little power. Professional studio condenser microphones are powered by a 48V DC phantom power supply. It's box with it's own power supply and you put it in line with the microphone. It's the same concept as the 3V from the sound card just much more juice. The NW-700 can work with such a phantom power supply.
48V DC phantom power

The problem is that this setup is bulky. It's OK for a studio setting where things are installed permanently but it does not work well if you want to carry a NW-700 together with your laptop. It's possible to buy overpriced portable battery powered phantom power supplies but there is a much better solution: Modify a small USB sound card. If you read the data-sheet of the NW-700 then you will find that it can actually already work properly with just under 5V DC.

The soundblaster cards and today's sound cards

Creative Labs soundblaster cards were very popular cards in 1990's and they came up with the idea of a small bias voltage to power the microphones. The connectors where 3.5mm and had 3 connections for a single Mono Mic: 1(tip)=signal, 2(ring)=5V bias voltage, 3(sleeve)=Ground. Today's sound cards continued the idea of bias power but they work with only 2 wires (still same connector) and about 3V bias voltage.

The soundblaster way most sound cards today
Left: how it used to be in the soundblaster cards, Right: what you will find in today's cards (especially small USB sound cards)

A USB sound card that provides 5V DC to the mic

I bought on ebay a USB sound card that looked like it might still have space in it's enclosure for a few extra components:
A usb sound card with a big enclosure

Any USB sound card looks internally basically like this:
usb sound card internals

A sound chip provides about 2.5V to 3V DC via a 3 KOhm resistor to the microphone port. There is as well a capacitor connected to the mic port to separate the audio signal from the DC voltage.

usb sound card, locate resistor

A Ohmmeter is useful for locating the correct resistor. This card had actually only one resistor. The other components are a LED and capacitors.

What we need to do: remove the existing resistor and add a new one that connects to noise free 5V DC. USB offers 5V DC but there might be some noise so we add a small low pass filter (270 Ohm resistor + 10μF capacitor). Like this:
modifications to provide 5v to the mic

The components in the dotted box are the ones that were already on the usb sound card and we remove the resistor.

List of required components:

I opened the casing of the sound card by prying it open with a knife and then I soldered the extra components to the board. The brown box in the corner is the 10μF electrolytic capacitor (belly up). You notice that there is a green LED and an extra resistor in the middle. This sound card had an annoying bright red LED that was blinking all the time. I replaced it by a green one and dimmed it down with the extra resistor.
The modified USB sound card

The NW-700 microphone works great with this modified sound card and it costs only a few cents in materials. The output "phantom voltage" is 5V without any mic plugged. Under the load of the condenser microphone it will go down a bit to around 4V but that is OK for the NW-700. Sensitivity and sound quality are very good.

The modified USB sound card, case back on

USB sound-cards with higher DC bias voltages

I did a survey of several USB sound cards prior do deciding to modify the card. I used a digital voltmeter and measured the DC bias voltages on the mic port of the USB sound card. In other words this is the max voltage with no load. I did not analyze how much the voltage goes down under the load of the NW-700 Mic and what kind of internal pull-up resistor the cards use. It is however very likely that all of them use around 3K Ohm for the pull-up resistor.

This bias voltages is very important for any condenser microphone and its sound quality, yet not a single USB sound-card specified this voltage on the package or the card itself. You have no way to know what the bias voltage is unless you take a voltmeter with you to the store.
It is possible to use the NW-700 condenser mic with cards that provide about 4.5V DC but the quality of my modified card is still better. Computer gadgets that function already with lower voltages can still be used on my modified sound card without suffering any damage. I did not find any equipment that started to malfunction in combination with my modified card.

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