Audio Line Out to Mic Input Adapter

 In this post we'll see how to build a simple audio line out to microphone input adapter.

As you already noticed, portable audio systems, computers and smartphones all seem to be happy to accept a microphone input (MIC IN) for adding audio or voice-overs, but very few have a line input (LINE IN) port to overlay music or any other relatively strong audio signal source instead.

So, if you want to plug something there that is not a microphone, you need to use an adapter to attenuate the signal properly to match the audio signal levels.

This is because the line level signals are typically in volt (V) level and the microphone signals are in millivolt (mV) level. Learn more about Line Level ↗

Luckily, you can buy Audio Attenuator Pads for cheap for this kind of application, but it’s not hard to build one yourself.

The circuit looks like (click image to enlarge):

This little circuit is enough to interface consumer audio signals to a microphone input that is designed for two-wire electret microphone capsules.

Basically, attenuation of the adapter is determined by equation:

attenuation = 20 * log10 ( (10K + 1K) / 1K ) = 20dB

So, our passive line-to-mic adapter circuit provides around 20 dB of signal attenuation, typically enough to make things work.

This is quite accurate especially when the impedance of the microphone input where this adapter is connected has much higher impedance than the resistance of the resistor wired between signal line and ground. A typical microphone input in an audio device has an input impedance of 1.5K or higher.

Values of the components shown in the circuit works best but you can alter them if you know what you're doing. Anyway, it's better to use metal film resistors (MFR) in this circuit as they are less noisy than cheap carbon film resistors.

The 10uF/25V capacitor blocks the DC bias used in the electret microphone input to get to the line signal side, and it must be a bipolar audio capacitor (see below).

The free-form soldered circuit itself can be built to a pretty small metal box.

Note that unbalanced microphone input of an audio device is very sensitive to all noise in the system because it handles low level microphone signals, thus interference from nearby devices can make a significant impact.

Ultimately, what is Balanced and Unbalanced Audio?

In general, one of the main differences is that balanced audio has less risk for unwanted noise, while unbalanced audio can pick up humming (buzzing) sounds in certain environments.

An audio cable carrying an unbalanced signal uses two wires, that is, a signal wire  and a ground wire.

The signal wire carries the audio signal to where it needs to go while the ground wire acts as a reference point for the signal (RCA cable is an example).

The balanced audio cable has a structure similar to an unbalanced audio cable but with one addition. 

A balanced audio cable has a ground wire, but it also carries two copies of the same incoming audio signal, sometimes referred to as a hot (+)  and cold (-) signal (XLR cable is an example).

And, once the hot and cold signal get to the other end of the cable, the polarity of the cold signal is flipped, so both signals are in phase, and perfectly in sync.

If the balanced audio cable picks up noise along the way, the noise added to both of those cables is not reversed in polarity.

Therefore when the cold signal flips in polarity to match the polarity of the hot signal, the noise carried along the cold signal cancels out with noise in the hot signal.

This process is called Common Mode Rejection (CMR), with the noise being the common signal between the two.

Simply put, CMR is a process whereby a signal common to a pair of lines opposite in polarity from one another gets cancelled at its destination

On the other hand, the ground wire in an unbalanced audio cable itself behaves like an antenna as well, picking up undesirable noise along the way.

That's it. Don’t miss the upcoming posts. Stay tuned!