Studio Sound Techniques (Part 3)
More about mixing consoles
Equalisation is used extensively in modern sound recording to accentuate or attenuate various frequencies in order to embellish the sound in some way. Often it is necessary to compare instruments to be mixed to ensure that they blend well, with no unpleasant overtones. Depending on the effect one wants it is possible either to make things sound quite separate or roughly similar. I'll be giving actual figures for you to try on different instruments later on in the series.
Since all sounds are composed of fundamental and harmonic or other frequencies, it is the ratio between these that we are most concerned with. Equalisation (EQ) gives us a degree of control over what is considered to be desirable or otherwise. Because our aural system makes an evaluation on a relative basis, it is important to be able to compare the equalised sound with the straight (unequalised) sound so that a judgment may be on the merits of the proposed EQ. Bear in mind that, when adding an amount of boost at a certain frequency, the overall level will go up. Apart from possibly causing unwanted peaks there is the chance that because it sounds louder it is thought to sound better. To make a like for like comparison adjust the level of the equalised signal to be the same as the straight sound so that a proper evaluation maybe made. In practice most equalisers have in/out switches which should be noiseless in operation. This latter point is sometimes overlooked so do check when buying. Often, only a short section of one track requires EQ, or a change of EQ, so noiseless switches and controls are a must.
There are many types of EQ systems available. One popular type (particularly on hi-fi these days) is the graphic, so called because it has a strong correlation between frequency response and the control settings. Although these are good for gaining an appreciation of frequency response curves they are clumsy if rapid, drastic changes are called for. This is often required when one track has several different instruments or effects alternating between verse, chorus and bridge-with maybe only a beat or two in between each section.
Probably the most common EQ system consists of frequency selector switches, boost/cut controls and an in/out switch. In addition there may be high and low fixed frequency boost and cut from treble and bass controls and high and low pass filters. As the name 'filter' suggests it is often desirable to get rid of parts of the sound which are felt to be cluttering the overall picture. The problem with any fixed frequency system is that invariably a compromise is reached where you trade off the part you want for the part to be rejected.
The most versatile system from a sound engineering point of view then is commonly called Parametric EQ. The main feature of this system is the ability to tune in precisely to the frequency bandwidth one wishes to adjust. The best systems are provided with: a frequency control over less than two decades in one sweep, a bandwidth control which should vary the area covered either side of the selected centre frequency, a boost/cut control with at least +15 dB variation, an in/out switch on each of these sections and a level adjust control to compensate for the gain or loss overall.
If your console doesn't have parametric EQ, don't despair. There are many companies offering these as both 19" rack mounting units or in a small modular format. Prices and specifications vary a lot, so shop around before committing yourself.
There is probably nothing worse for an electro-musician than a noisy control or switch. The causes and cures are many and varied. If you have purchased a secondhand mixing console the chances are it will need some sort of overhaul immediately after installation. Even electronic components are subject to degraded performance through ageing. Capacitors and transistors go leaky and/or noisy and these have to be 'weeded out' as soon as possible.
First try to isolate the faulty amplifier stage by observing which combination of switches and controls allow the noise through to the output. Refer back to Figure 3 given in part two and let's assume we have a noisy channel fader buffer amplifier. It follows from the diagram that nothing ahead of the channel fader is going to affect this noise, but that the pan pot and post-echo-outputs would be noisy. The channel fader might affect the noise output depending on the design of the amplifier and the nature of the fault. In some cases closing the fader may make it worse. Finding the defective component is largely a question of familiarity with the design and experience. However, a 0.22uF capacitor earthed at one end can be useful. By probing around the circuit with the unearthed end it is possible to decouple the noise which may help you find the defective component. The capacitor will introduce a transient current as it charges so care is necessary when using this technique.
The most common cause of noise in potentiometers (pots) and rotary switches is dirt and dust. This is often easily remedied by spraying with contact cleaner. Be sure to use a spray that evaporates completely without leaving an oily residue, otherwise this will act as a dust trap. If the control is still noisy after spraying it may have simply worn out, in which case, as with a sealed pot, replace it. Be sure to replace it with a pot of the same resistance value and characteristic (log, lin etc). If the new replacement still makes noises when operated further investigation is necessary.
A common cause of noise generated by pots, is an errant DC component across the pot often coming from a leaky capacitor.
This can be checked by measuring between the terminals of the pot with a DVM or other sensitive meter, as shown in Figure 5a. One tenth of a volt (100mV) could cause trouble in a sensitive part of a circuit. Ideally there should be no DC across any controls or switches. Figure 5b shows a situation guaranteed to cause 'clicks' or 'pops' when the switch is operated. In both cases the cure is to replace the leaky capacitors, preferably with tantalum types of a suitable voltage and capacitance.
Another source of DC on the output of operational amplifiers is drift in the offset null circuit. This only applies to op-amps fed from dual supply lines (±). Because the output normally sits at 0V, no DC blocking capacitors are required. So if any drift, or inadvertent twiddling of the offset null preset has taken place a troublesome DC level will be present. If it's possible to adjust this out on the offset null preset all well and good, otherwise replace the chip.
A less common cause of noise on pots are radio frequencies (RF); either generated by poorly compensated operational amplifiers, or being picked up in some sensitive part (eg mic amp) of the circuit. This RF component invariably gets filtered out somewhere along the line so don't necessarily expect to see it at the output of the suspect unit. The only way I know of tracing this sort of interference is with an oscilloscope or high frequency millivoltmeter. Because RF interference is associated with instability the very act of delving in to check for its presence often removes the cause. So when you open up the equipment or attach the piece of test gear it miraculously disappear, only to return at some later, inconvenient stage in the proceedings. (See E&MM March '82 for details of curing RF interference).
A good rule therefore whenever fault finding is to check that the fault is still manifesting itself at each stage of disassembly or whenever you move the test gear to a new node in the circuit. The internal impedance of a meteror oscilloscope can be sufficient to 'fix' the fault temporarily. So this information can guide you to the faulty part of the circuit provided you check for the malfunction each time.
If you have a specific problem with your studio equipment that you would like discussed in this series, please write to Paul Becque, c/o E&MM.
Feature by P.A. Becque
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