LEVEL, IMPEDANCE AND COUPLING OF AUDIO EQUIPMENT by Sergio Beristain, AMITRA, Mexico (Presented in Spanish at the SBE National Convention, Miami Beach, Florida, October, 1993) Introduction: Every radio station has both on air and production studios where the equipment is installed that is considered necessary for its normal operation. It is important to obtain the maximum benefit of the equipment in use, especially if one takes into account that the majority of those are not able to satisfy the full range of human hearing, which extends from 20 Hz to 20 kHz in frequency and from 0 to 120 dB in amplitude. With only these two parameters, it is easy to detect many of the errors frequently encountered in professional installations, and to these are added further alterations such as changes of phase, both amplitude and frequency distortion, noise, etc., which complicate the diversity of errors considerably. Also, it is common to acquire equipment of a wide range of brands and specifications, given that it is difficult to find one brand that manufactures all of the equipment that one needs. This demands that one exercise considerable caution when interconnecting this equipment, due to the fact that, in spite of the standards that exist regarding impedance, level, etc., not all the manufacturers use the same references for the input and output impedance specifications of their equipment. Impedance: There are on the market microphones with several different output impedances. (The output impedance is the value employed to couple the signal supply capability of the source to the signal absorption capacity of the connected device (input impedance). It is measured in Ohms and is represented by Z.) It is normal to find microphones with output impedances that vary from 50 to 250 ohms for low impedance systems, and from 10,000 to 50,000 ohms for high impedance systems. High impedance microphones tend to be inexpensive and easy to couple to tube amplifiers that have high input impedances, and which, when used with low impedance microphones, frequently require expensive impedance transformers. However, these impedances are susceptible to the introduction of noise by electrostatic fields, such as those generated by fluorescent lamps and motors, requiring the use of shielded cables. These cables, in turn, have a self-capacitance in parallel with the microphone output, resulting in a loss of high frequencies in longer runs of cable (3 to 5 meters). For this reason, the use of high impedance microphones is limited to short distances. With low impedance microphones (around 50 ohms) the cables are virtually insensitive to the capture of electrostatic fields, however they are more sensitive to electromagnetic fields such as those generated by AC power lines. Through the use of a twisted pair of conductors, the effect of the electromagnetic fields can be eliminated, as these currents circulate in opposite directions and cancel each other in the input transformer of the preamplifier. Cables can be used up to 30 meters in length, but as the length is increased, the resistive impedance of the conductors also increases, attenuating the desired signal. Although these losses have no effect on frequency response, they do reduce the signal to noise ratio, so that the only practical way to increase these cable lengths is through the use of cables that have a lower resistance per unit of length. Microphone lines of 150 to 250 ohms have characteristics that are midway between those previously described as regards the sensitivity to both electrostatic and electromagnetic fields. As a consequence, they require the use of shielded cables with a twisted pair of conductors as well as a balanced input circuit at the preamp to minimize the noise level. These lines can be used for runs of up to several hundred meters. It is common for recording studios to use microphone lines of some 200 ohms with balanced cables and with the shield connected only at the preamplifier to avoid ground loops that can produce a strong hum. When these microphones are connected to a preamp that has a normalized 600 ohms impedance for professional audio systems, approximately 1 dB is lost, which is easily compensated for with the input potentiometer and, as a result, normally no one worries about it. However, microphone input circuit impedances can range from 30 to 1,000 ohms, with 200 ohms being the most common. The 600 ohms impedance was standardized for interconnection of audio circuits to allow the use of VU meters of this same impedance, which became a standard more than 50 years ago. The output impedance of power amplifiers will vary between 2 and 16 ohms. In this case, they function with energy, and for the maximum transfer of energy, they require that the attached devices have the same impedances, otherwise the energy that is not transferred could easily burn up the power amplifier. Operation in low impedance is practical for short distances where the line losses are minimal. To avoid greater losses of energy, amplifiers also can operate with impedances that range from 500 to 2,000 ohms, using Constant Voltage lines operating with long cables and using coupling transformers at each speaker, thus achieving greater efficiency in the distribution of the energy produced. Level: There are several standardized reference levels. Among these, the best known is 0 VU, which is equal, depending on the circumstances, to either +4 or +8 dBm, from which it can be deduced that the only absolute reference is the dBm. 0 dBm in an audio circuit equals 1 milliwatt consumed in a resistance with a nominal impedance of 600 ohms, which develops a voltage in the resistance of 0.775 volts. VU meters are calibrated to this value, given the fact that they are really nothing more than voltage measuring devices. In different audio equipment, one finds nominal levels that range from -40 to +18 dBm. This value is determined normally by the manufacturer, and by employing this nominal value, he establishes the signal to noise ratio of his equipment. For example, let's presume a piece of equipment operates with -10 dBm and has a signal/noise ratio of 45 dB and peak headroom of 10 dB. This means that the device will function optimally with signals that vary from -55 to 0 dBm. If we feed this equipment with a nominal signal level of 0 VU (+4 dBm), we will be producing distortion practically continuously, which will be even more noticeable on the signal peaks, whose levels will typically be from 10-12 dB above the nominal value. In other words, signal levels will be applied as high as +16 dBm, with an apparent signal to noise ratio of 59 dB. On the other hand, if we feed 0 dBm into a piece of equipment that has a nominal input level of +10 dBm, a signal to noise ratio of 50 dB and peak head room of 10 dB, there will be no distortion under normal conditions, but we will only achieve a signal to noise ratio of 40 dB. Because of this, it is always important to connect each piece of equipment one at a time, optimizing each signal level for the best performance, so that the signal in each device is maintained as free of distortion as possible and with the lowest possible noise. Polarity: It is vital to preserve the phase of the audio signals. If the polarity of the connectors is inverted (equaling a phase change of 180 degrees), it is possible to lose up to 100% of the signal (especially with monophonic signals being transmitted through stereo systems). In the majority of the cases in balanced lines, XLR connectors are used with pin 1 to ground, pin 2 low and pin 3 high. However, there are some manufacturers that connect their equipment with pin 1 to ground, pin 2 high and pin 3 low. For this reason it is important to carefully connect each piece of equipment to avoid errors in polarity which can result, at worst, in the complete loss of audio, or, at very least, a portion of the music, and the soloists will disappear. In systems completely monophonic, this detail is irrelevant, except when balanced and unbalanced lines are joined, that is to say where the low pin has been grounded. In such case, the signal will disappear when the high signal line is connected to the grounded low signal line. The levels transmitted in audio cables are not dangerous, and nothing catastrophic will occur, such as a fire or something similar, but it can cancel the signal completely, or could invert the signal to the speakers in such a way that, when one speaker is pushing, the other is pulling, causing acoustic cancellation of the signal. Ground: It is frequently found that audio circuits become noisy when they appear to be connected correctly, except that one cannot forget that we are working with low signal levels and that cables of a certain length or multiple separated contacts to ground will function as excellent voltage dividers, where a voltage developed in one of these lines can be equal in strength to the audio signal voltage itself, covering the desired signal and making it useless for any practical purpose. It is, therefore, important to ground an audio system at only one point to avoid feedback, oscillations and, in general, noises produced by the ground system of the installation. Conclusion: Although interconnecting audio equipment is not a very complicated assignment, and in general free of risks, it is imperative to read the factory manuals for each piece of equipment installed to achieve adequate connections, avoid phase inversions, reduction in the system signal to noise ratio, generation of distortion through excessive signal levels at the entrance to any piece of equipment, etc. With these few efforts, one will reduce considerably the headaches caused by a poorly functioning system, as well as a loss of prestige upon being obligated to make corrections at a time when the system should already be in operation. (This is the translation of a technical article that was given in Spanish at last year's S.B.E. National Convention by AMITRA, the Mexican broadcast engineering society. It is the first of many articles by our international associate organizations that we hope to have translated in the future for the benefit of the membership. This is one way that S.B.E.'s affiliation with other engineering organizations around the globe can benefit every S.B.E. member. There is also interest in translating S.B.E. articles into Spanish for possible publication by AMITRA, so please send me your best technical articles, along with a release by the author allowing publication. Send your articles and comments to: John F. Schneider, c/o RF Specialties, 19237 Aurora Avenue N., Seattle, WA 98133.)