Get To Grips With The Gear

[ii] Mic/Line input characteristics

Input impedance of the console input stage is specified in consideration to the signal source generator output impedance. Most microphones have a typical output impedance of between 150-200 Ohms, but can be as high as 300 to 600 Ohms. Microphones like to "see" minimum impedance loading. This can be 1k Ohms or higher in order to prevent "loading up" of their outputs and to maintain clipping level specifications of the microphone itself. Therefore, the typical input impedance of the console's mic preamplifier input should be somewhere between 1.2K and 3K Ohms.

Microphone output impedances and the console's input impedances can, if not correctly designed, vary over frequency, as a result, the loading of source to input can affect frequency response. Depending upon the type of internal device driver the microphone has, it is possible to see a microphone's output impedance rise at lower frequencies to over 1K Ohms. Remember, we are not looking to impedance match output to input. With a Mic Input impedance similar to this value there can be as much as a 6db reduction in level due to load matching, that is, the signal at that frequency can be reduced by half in voltage terms - a significant hole in your LF response.

Mic Input impedance should be consistant across the audio band 20Khz to 20Khz. Beyond this, better that the input impedance remains high to prevent folded side bands from impacting upon the in-band components as they can cause a significant disruption to the smooth frequency/amplitude response the microphone manufacturer took such care to produce. In reaction you will probably have to apply too much equalisation to counteract the problem - that's if you have time in a fast moving live broadcast!

And before leaving this vast subject, it is important to note that the same microphone can produce different results with different Mic Input preamplifiers (and vice versa). Important to accept that some console manufacturers have spent many years perfecting their particular "sound" of pre amp. Over damping and under damping of the input circuitry when connected to certain microphones give different sounds ranging from dull to bright. It's like flying a helicopter, if you find a particular action gives you the result you desire - do it again.

Line input impedance on professional broadcast mixing consoles is set at 10K Ohms. This is specified for line bridging conditions. In addition, line level sources are often fed from equipment via low impedance line drivers feeding multiple destinations (called a DA or Distribution Amplifier). At 10K Ohms destination impedance, a number of devices can be fed without "sinking" the driver device.

Other features at the Mic Input include a switchable 48 Volt power source for each Mic input to power a capacitor microphone.

Signal inversion is provided, which for a mono input is simply no phase inversion or 180 degree phase inversion. For stereo, the left and right inputs can be inverted independently (phase relative) or both left and right inputs can be inverted together (phase absolute).

The Mic/Line input stage often has a High Pass Filter. Typically this will have a roll-over frequency of about 120Hz with a slope of 12db per octave, though this could be as shallow as 6db per Octave. The feature is similar to the low frequency roll-off switch found on some microphones. Digital consoles will also apply a limiting stage at this point in order not to overload the subsequent Digital-to-analogue converters (D to A converters).

Another feature at the output of the preamplifier stage is a signal feed to LED or LCD bargraph meters. These give an indication of the signal level output from the pre amp. A useful feature to help optimise the operating level of the input stage to the channel faders. The meters can take the form of a full range LCD bargraph located on the meter bridge of the console, in-line with the appropriate channel, or be a small range of indicators positioned alongside the channel fader. A red LED called "Overload" or "Clip" can also be found here which gives a warning to what's called the "onset of clipping". Activation of this indicator is set just below the clipping point of the input stage, typically 1db or 2db below clipping.

It will be useful at this point to talk a little more about Input Headroom and how this can contribute to the overall quality and resilience of signal integrity of the Mic/Line input stage. But first some description of the power supply arrangements and how this can contribute to the overall quality performance of the preamplifier.

Input Headroom is the ability of the preamplifier stage to deal with high levels of input signal. It's the point at which the preamplifier clips the incoming signal due to its inability to handle large signal levels - the better the Input Headroom, the higher level of signal the preamplifier can handle. The principal element that governs Input Headroom is the power rail voltage feeding the preamplifier. This is a plus and minus voltage rail of +/-15volts or higher, in some cases +/-18 volts. With such voltage rails the maximum permissible output voltage swing before clipping is potentially 36 volts peak-to-peak for a +/-18 volts rail. For those who would like to convert this to an RMS figure referred to 0dbu then get out your calculators and do some homework.

It's worth mentioning that high quality preamplifiers are sometimes powered from separate power supplies. These power supplies are linear - they use full wave bridge rectification and RC smoothing circuitry to deliver a clean power rail to the preamplifier. This is often more favourable than the Thyristor switching power supply which can have switching artifacts on the power rail. This can degrade the signal-to-noise performance of the preamplifier. The downside is that linear power supplies are bulky and relatively inefficient compared to their switching counterparts - although switching power supplies are much improved compared to just 5 years ago.

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