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 has a limit of about 3 volts output before it starts to clip. Another way to say it is that exceeding 0.03 volts at the mic input will produce clipping.
If you send an average 1 volt of signal into this preamp, it will try to produce a 100-volt signal. Since it is limited to 3 volts, severe clipping is the result, and the noise floor of the output becomes very apparent when there is no signal present. Read that as hums, buzzes, etc.
So, what do we do to solve the problem of overloading a mic input with a line-level signal. The answer is to reduce/attenuate/pad the signal so it arrives at the mic input at its expected average level of 0.003 volts. I will outline a couple of ways I do this successfully, and I like to use a passive DI in my signal chain because I have a ground-lift switch if required. But first a word about typical passive DI’s.
The common step-down transformer in a DI reduces the signal level by about 22dB. (about 1/12 of the incoming voltage) 1/12 of 1 volt is about 8/100 (0.08) volts. This level far exceeds the 0.003-volt input capability of the mic preamp stage. Many people have been led to believe that a DI reduces a line-level signal to mic level. THIS IS NOT TRUE! More attenuation is required.
To accomplish this, I do the attenuation in stages, and I will switch to some dB numbers. The voltage values I used were just good approximations, so I will stick with 0dBu being around 1 volt, and -50dBu is about the 0.003-volt mic level I have used. So, if I am providing a line-level signal of 0dBu, I need to attenuate it by about 50dB, and attenuation is additive, so if I use a DI with a pad switch, I can attenuate the signal by an additional 20dB. So, the usual attenuation of 22dB through the DI transformer added to the 20dB pad switch totals 42dB of attenuation.
Is this enough? For some mic inputs it will be. For others, you may need more. Many mixers have an RCA phono jack in the main output section. If you can use that, you can reduce the signal level to the mic input by an additional 10dB which brings the total to -52dB so you can hit the target this way. If you can’t use the RCA jacks, reducing the master of the line out a bit will get you there with minimal effect on the signal-to-noise ratio.
A POINT TO ADD HERE IS THAT THE ATTENUATION ACTS ON THE NOISE FLOOR BELOW THE SIGNAL EQUALLY. SO, IF YOU ARE SENDING A NICE CLEAN NOISE-FREE SIGNAL INTO THIS ATTENUATION, IT WILL STAY NOISE FREE.
I am currently using my 2 Jensen-loaded passive DI’s made by our own Frank Dewitt. It is a great, simple DI with a ground-lift switch, but NO 20dB pad. So, to get more attenuation I use my little Mackie 1202VLZ mixer. The main XLR has a 30dB pad switch that pretends to be at mic level. It’s still a bit hot, but by using that pad switch + the normal attenuation through the DI, I get the 52dB of attenuation I need to be right on target.
The next consideration is that these mic inputs we are using tend to be an unbalanced 1/8” stereo TRS inputs. So, I use 2 DI’s, or a stereo DI. Now, how do we connect to the stereo inputs? Let me describe a way that uses off-the-shelf cables. First the 2 usual connections from the mixer.
A mono output to a DI, then a short TS-TS ¼” patch to the second DI. For my Mackie mixer I used an XLR-F-to-1/4” phone-plug cable to the first DI.
A stereo feed to the 2 DI’s. From the DI’s use an XLR-F to RCA phono plug adapter. Plug these in a wye cable with 2-RCA phono jacks to a 1/8” TRS plug.
So, you can record a 2-track mono signal, or a stereo signal.
In the presence of a ground loop that will occur with a balanced-to-unbalanced interface, noise may be a problem. That is were you switch the DI’s ground-lift switches in. BTW, these techniques may still be required if a camera has XLR mic inputs that cannot be switched to line level. I hope this rambling message is clear, and helps you in your efforts to stream clean audio.
Using this knowledge and these techniques have resulted in clear, noise free recordings. This is a link to a dual XLR-F to 1/8” TRS plug. BTW, I have been running my mixer’s stereo line-level output through a stereo DI followed by a cable that attenuates the signal an additional 30dB into an unbalanced stereo mic input, and we have UN-distorted audio with no noise issues. I followed my own advice and first matched the console level to the EXPECTED mic input level.