2. The Input Channel Section

This comprises the input preamplifier stages with gain control and trim (an explanation of the difference between "gain" and "trim" is provided later). The input channel section also comprises the Equaliser and Filters, and the Dynamic Processing, that is, processes for Compression and Expansion, Limiting and Gating functions. It's worth mentioning that Equalisation and Dynamics are not exclusive to the input channel section, they can also be found in the Groups, Auxiliaries, and Mains sections - each manufacturer provides different levels of flexibility and additional features. It's quite common in the broadcast application to include a Limiter in the Mains output section of the audio console as the subsequent output is often the transmission feed. Having what is called a "brick wall" limiter (a ratio of 100:1) gives the transmitter overload protection during times of high transient peaks.

With the introduction of digital consoles, users will find that input and output sections will have a delay feature provided, this is typically +/- 1ms of delay to a maximum of about 5 to 10 seconds. This is often a pool-able resource, that is, available to allocate across the entire console - useful in the input sections to offset propagation issues due to reflections and spatial delays related to microphone placement, and in the output sections to compensate for video timing delays due to video effects and source-feed timing issues.

[i] The Mic/Line Pre-amp stages:

Figure 2 shows a typical Mic/Line input stage of an audio console's input channel. It comprises Mic/Line switched inputs, Phantom Power, Phase Invert, Preamplifier and High-Pass Filter. It's common to find two Mic/Line inputs per channel input, although this will often be limited to one each if stereo Mic/Line pairs are used.

The input arrangement of the Mic/Line input stage is not the same for all consoles. Some manufacturers use only a low level (high sensitivity) Mic input and "pad" or attenuate the input to create a high level (low sensitivity) line input. Padding is done using resistor configurations in T or Pi arrangement. There are intrinsic consequences to such an arrangement however it reduces the cost of manufacture compared to providing dedicated Mic/Line inputs.

Typical low level signals to the Mic input could be as low as -70dbV (0db=1V) from a ribbon microphone, to about -50dbV to -40dbV from the output of a dynamic microphone. Signal levels are dependent upon the sound field in which the microphone is placed, and the microphone's sensitivity specification (Microphone sensitivity specifications are given in db where 0db=1V/pa). At the other extreme, capacitor microphones in high level sound fields (SPL) can deliver an extremely high level to the Mic Input measured in volts rather than in millivolts. Therefore, it's often required to know what is the Maximum Input Headroom to the Mic Input (as well as the console's Operating Headroom up to the channel fader - but more on this later).

You would expect a well specified mixing console to be able to handle an input signal of at least +24dbu, even better still to levels of +26dbu and +28dbu before the onset of clipping (0dbu=0.775V RMS). Preferable for the microphone to clip due to sound field extremes rather than the Mic input stage to clip first. There is nothing worse than a console's front end unable to deal with a high dynamic range.

In the example shown in Figure 2 above we show dedicated Mic/line inputs with the Preamplifier stage applying negative gain (attenuation) of -15db, and positive gain (amplification) of up to 72db. Course gain can be applied using continuous knob rotation or using stepped or pre selected changes of 5db, with a separate fine gain adjustment of +/- 0.25db or lower depending upon the manufacturer's design.

Mic and Line inputs in a professional mixing console will be balanced. Unbalanced arrangements can be used though these should match unbalanced sources. In such cases it's often advisable to use an unbalanced to balanced device for correct operation (called a balun). Using a balanced source to a balanced input provides reductions in noise and hum when the signal source is fed over distance (the specification is called Common Mode Rejection, where an unwanted signal appearing on both inputs is cancelled out by a differential pair - more on this later). The differential inputs (plus and minus) can either be electronically balanced to discrete transistors or an Integrated Circuit, or be connected to transformer coupled inputs. Improvements can also be seen in HF response due to a reduced cable capacitance effect - improvements in slew rate and transient response.

The transformer-coupled input is often found in audio consoles used in trucks as they often connect to external feeds tied to a different earth (a venue or concert hall for example). The transformer provides electrical isolation to the input stage of the mixer giving added protection from erroneous voltages on the signal feed.

Over-driving a transformer coupled input at low frequencies affects the low end frequency response due to transformer core saturation. But with good quality transformers this has reduced impact, and there are not many who can truly claim to hear the difference between -6db from norm at 10hz and -1db at 15hz such is the non-directional and psycho acoustic nature at such a low frequency. However, there is no denying that low frequency roll off does occur in transformer coupled inputs. And let us not forget the differences induced by variations in input impedance. But given that lightening (man-made or divine) could arrive at the input, many truck operators opt for the transformer coupled alternative if the added weight in the truck is not going to be an issue. If so, then we suggest that the project builder does without the extra video monitor for the Video Effects operator!