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TechTalk: Microphones
With the DAC+ ADC and DAC+ ADC Pro HiFiBerry offers sound cards that can also be used for sound recording. While many users just use line-level sources like CD players, synthesizers or similar devices, you can also connect microphones to it. This post introduces you to the specific technical details of connecting microphones.
Microphone types
Dynamic microphones
This type of microphone is basically constructed like a small loudspeaker. However, the membrane is not used to emit sound waves, but it is moved by sound waves. It’s called diaphragm here.
Source: Wikipedia, Moving_coil.JPG, (C) CC BY-SA 3.0
In general any classic passive loudspeaker itself could be used as a dynamic microphone – just not good one.
When the diaphragm is moving, the coil moves in the magnetic field and therefore induced an electric current. This will be picked up by the microphone pre-amplifier.
A popular dynamic microphone is the Sure SM58 that’s on the market for almost 50 years now, but there are lots of other dynamic microphones available for all kinds of applications.
Source: Wikipedia, Micro_Shure_SM58.jpg, (C) CC BY 2.0
Condenser microphones
This type of microphone works differently. The diaphragm is here part of a capacitor. When it moves, the air gap of this capacitor changes and therefore the capacitance changes. To work well, the diaphragm is usually quite large (otherwise the capacity would be very low).
Source: Wikipedia, Condenser_schema.jpg, (C) CC BY-SA 3.0
This type of microphone does not induce any current. When you directly connect it to an amplifier input, nothing would happen as no current is flowing. An additional voltage needs to be supplied first.
Source: Wikipedia, Voltage.JPG
Many professional condenser microphones require a voltage of 48V to work. This is called “phantom power”. Our HiFiBerry DAC+ ADC cards do not provide this voltage. However, you can still use this kind of microphone with an external phantom power supply like this:
This will just be connected between your condenser microphone and the audio input.
A very popular choice seems to be the MXL-770. However as with dynamic microphones, there is a huge range of condenser microphones available on the market. It’s usually best to talk to your local musician store.
Electret microphones
This is a type of microphone that many computer users are familiar with. These microphones are usually very small. Internally they are similar to condenser microphones. However, they require a much lower supply voltage to work. It’s usually called “bias voltage”. This makes it easier to integrate it into computer products. The DAC+ ADC Pro can provide the so-called bias-voltage to an electret microphone.
Without supplying a bias voltage to the microphone, your recording will just be silent. As mic bias has some negative impacts on “normal” audio sources, it’s turned off by default. There are two things you have to do to supply a bias voltage with the DAC+ ADC Pro.
- Close the mic bias jumpers left and right of the phone jack.
- Enable mic bias in the mixer settings:
amixer sset "ADC Mic Bias" "Mic Bias off".
The DAC+ ADC does not support mic bias.
While this type of microphones are popular for headsets, the audio quality is usually not good enough for music applications.
Connection
Professional microphones often come with huge XLR connectors that you can’t directly connect to the small 3.5mm input jack of our sound cards. However, you don’t need any additional electronics, a simple adapter cable will do the job.
We offer these in our shop as 1m Stereo splitter or 3m Mono cable. If you want to buy these locally, make sure, you look for an balanced to unbalanced cable. Your local music store should be able to help you with this.
Other types
There are other microphone types like piezoelectric, MEMS, laser and many more. However, these are designed for very specific use cases and you probably won’t use anything like this. If you’re interested in more details, have a look at Wikipedia.
Amplification
Next thing that you notice is that the volume will be probably quite low. While sources like CD players provide a high output voltage of up to 2 Volts, microphones only output very small voltages of a few Millivolts. This requires an additional amplification of the signal. Traditionally, additional microphone pre-amplifiers have been used for this. However, the DAC+ ADC and DAC+ ADC Pro have some amplification integrated already. While external pre-amplifiers can sometimes provide a better audio performance, for many use cases, the internal amplifier is already quite good.
DAC+ ADC
On the DAC+ ADC, you can change the board configuration from no amplification (default) to 32db (40x amplification) using jumpers. Check out the DAC+ ADC data sheet for details.
DAC+ ADC Pro
On the DAC+ ADC Pro, you can set the input gain via alsamixer:
amixer sset ADC 40db
40dB (100x amplification) is the maximum amplification that the DAC+ ADC Pro offers. Depending on the microphone and volume, you might set the amplification a bit lower. Just experiment a bit with this setting.
USB microphones
A USB microphone basically integrates microphone, pre-amplifier and analog-to-digital converter into one device. The advantage is the ease of use. To record a podcast or similar applications, devices like this are quite interesting. However, for music application this can be problematic. The main problem is additional latency that comes form the ADC and USB interface. A latency of <10ms is quite rare and some might even introduce a latency of 20-30ms. Check out our article about latency to learn more about the issues of high latency.
January 14, 2021
