SOME CONSIDERATIONS WHEN CONNECTING A LINE-LEVEL OUTPUT TO A MIC-LEVEL INPUT. By Michael David Miller Ever since Covid-19 came along and caused the shutdown of churches across the country, I’ve noticed a common problem brought up in multiple audio forums. Suddenly everyone is trying to find a way to stream their service by assembling systems with whatever they have to work with to do so. Many church tech volunteers have reported anywhere from unsatisfactory to unusable results. Some of the usual complaints are excessive hum and buzz, distortion, muffled sound, and crackling sounds. In many cases the techs are trying to connect the output(s) of their audio console directly to a mic input, either stereo or mono, on a camera, computer, or other recording device. The problems I speak of when connecting a line output to a mic input are severe distortion, and noises such as buzz and hum. While sometimes a part of the problem has to do with noises associated with ground loops, the main problem is usually a severe mismatch in the audio signal voltage level of the mixer output, and the expected average signal voltage level of a microphone input. In other words, the line -level signal voltage is far too high for the microphone pre-amplification stage you are plugging into. Let’s start with analog inputs and outputs because even with digital consoles, most inputs and outputs we deal with are analog. Let’s put aside balanced vs. unbalanced and focus on voltage levels. 1) Microphones in normal use produce a voltage range of about .001 – .01 volts average. (-6o to -40dBu) 2) Line outputs usually range from about .3 – 1.25 volts average. (-10 to +4dBu) 3) The job of a mic input is first to increase or amplify the relatively weak mic signal to around line level. This means that it amplifies/multiplies the signal voltage by 100-1,000 times. 4) Input clipping occurs when the amplifier, or in this example, the mic preamp, gets to a voltage limit that it cannot exceed. This results in squaring off the top of the signal which produces a series of odd harmonics that sound very harsh. Once this occurs, you cannot fix it further down the signal path by reducing the signal. All you do is reduce a DISTORTED signal. An example is if you raise the gain too high on a mixer channel gain control causing the channel clip light to get very flashy indeed. The channel fader that follows can only reduce the distorted audio signal. 5) All active analog audio amps have a noise floor which is a fairly constant layer of hisses, buzzes, and hums that is much lower that the audio signal at a relatively unnoticed level. Turning down the master output level does not change it. It tends to produce a poor signal-to-noise ratio (SNR). The microphone preamplifier stage we find in computers, cameras, etc. are expecting around .003 volts of signal on average. It will increase/amplify/multiply this signal by a factor of 100 or more. So, the signal level of 0.003 volts is raised to 0.3 volts. This microphone preamplifier stage […]
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