Cable and Connectors
Cable Connectors Line trannys Impedance Z
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Good quality connectors and cable are essential and worth paying extra for. Wherever possible, use gold plated connectors and cable with tin-plated copper wire.
Mains power cable
for 110V / 240V has 3 cores ( earth active neutral ) and double insulated for safety
has 2 cores. Current varies with power. At 100Watt the current can be 5Ampere at approx 30Volt. Speaker cable is often the same construction and thicknesses as mains cable. It can conduct similar current at lower voltage and does not need to be double insulated, unless specified for high power.
Shielded cable has an earth wire platted around the outside, to act as a shield against external magnetic radiation. Signal within the cable is less than 1Volt and sensitive to interference. Signal cables can pick up magnetic radiation and should be kept clear of mains leads and other electronic equipment. Leads should be short as practical, the excess never coiled, except for transport.
This cable is of excellent quality and can be as thick as your wallet (up to $1000 per meter). Only those gifted with golden ears reading audiophile magazines and chanting superlatives can hear the magical transformation of sound quality.
Hum Buzz Clicks
separate from white noise, is caused by defective electronics, magnetic radiation, poor cables, intermittent connectors and earth loops. Connectors can be loose or corroded (oxidised). Signal cables can pick up magnetic radiation, and should be kept clear of mains leads and other electronic equipment. Leads should be short as practical, the excess never coiled, except for transport.
Earth loop hum is a major problem for professional sound systems, and is often created when 2 or more pieces of electronic equipment are inter-connected. Eddy currents magnetically injected from mains transformers form loops between equipment. Earthing should be at one point only; this involves electrical safety and requires a skilled qualified technician.
- For domestic application Pre-amplifiers (input selectors and tone controls) are combined with Power-amplifiers in one box
- Pre-amplifiers for professional applications are described as signal processing.
Mixers. EQ. Graphics. Processors. Effect systems. etc.
- For professional applications Pre-amplifiers are separated from Power-amplifiers.
Line Level (0VU volume units) or (0dBm) is 775mV. Line level is slightly less than 1Volt, and was adopted as the standard in early audio history for all signal processing, prior to Power-amplifiers.
This standard only applied to past valve technology, in the broadcast industry, where equipment was interconnected with line transformers, referenced to 1 milli-Watt into Z 600ohm. Solid-state technology has no need for this reference, which now only loosely applies.
The Line level standard should revised from 0dBm (775mV) to 6dBV (2V) to solve signal to noise problems.
Signal to Noise
White noise is naturally generated by all electronics throughout the frequency spectrum and there is a limited threshold, which it cannot go below. White Noise has equal energy per cycle. As the frequency (cycles) doubles (for each octave), so does the noise energy (+3dB/octave), resulting in white noise sounding trebley as hiss. White noise is the same in nature as surf or wind rustling through trees. In most cases we cannot hear white noise, because of small inefficient speakers. Electronic white noise from the amplifier can be heard with high quality efficient speakers in a quiet room. To minimise hearing white noise, keep the signal level as high as possible, prior to the power amplifier, and the level control of the power-amplifier as low as practical (signal to noise ratio).
Pink Noise is filtered white noise having equal energy per octave. Pink Noise is similar to energy of music, and used for measurements and research.
Balanced and Unbalanced
Unbalanced is the standard for domestic signal leads, with RCA connectors. Unbalanced cable has a single signal wire with an external wrapped or plated earth shield. Maximum length depending on cable quality before signal loss and interference is 6-12 meters.
Balanced is the standard for professional signal leads, with XLR connectors. Balanced cable has two wires for the signal, with an external wrapped or plated earth shield. The same signal is on both wires, but in opposite phase. Interference that gets through the external earth shield is common (in-phase) to both signal wires. The balance effect of the input circuitry cancels any signal or interference common (in-phase) to both wires. Maximum length depending on cable quality is 30-100 meters.
Impedance is Resistance that varies with frequency. Simple description.
For professional application, Impedance matching is applied to balanced signal cables with isolation transformers. Impedance matching is rarely needed for domestic application.
In the era of Valve technology, accurate impedance matching between equipment was essential. Valve technology functions at high impedances (Z 10K-100K). The capacitance of long interconnecting cables is proportional to length. Cable capacitance interferes with the signal, reducing high frequencies, if the output impedance is high.
The solution was for the output of professional (not domestic) valve equipment to be connected to cables through step-down line transformers, which reduce the impedance to Z 600ohm, so the capacitance of the cable could not affect the high frequencies. Unfortunately reducing impedance through step-down line transformers also reduces signal level. The problem was one of compromise. This is where the Z 600 ohm standard came from.
Solid-state technology is capable of max approx 8V RMS (+20dBm) and with low output impedance of approx Z 10-200ohm.
At low impedance, the cable capacitance has minimal effect. Impedance matching between equipment is rarely needed for domestic application. For professional application, Impedance matching is applied to balanced signal cables, with isolation transformers.
This low output Impedance of solid state circuits is reflected directly through the 1:1 isolation transformer onto the line at the same signal level.
- The lower the Impedance the less the effect of cable capacitance.
- The higher the signal levels the better the signal to noise ratio.
The specifications (Z 150-200-600-10K etc) are sometimes non specific and generally refer to the heaviest load that can be put on a transformer or a circuit, for a specified bandwidth and level.
Large transformers often have higher inductance and are capable of greater bandwidth and level. It is essential for a skilled electronic technician to measure the bandwidth and maximum level the line transformer or circuit is capable of.
Dynamic Microphones often have an internal isolation transformer for balanced line, and have to be correctly loaded for their performance to be accurate, Z 100-600ohm at approx max 100mV.
Phono-cartridges need to be correctly loaded for accurate performance, Z 47K ohm at approx 1-10mV.
Isolation Line Transformers
Isolation Transformers are used to reduce or eliminate noise interference in long cables, between equipment that is powered from different mains circuits. Transformers are balanced so interference common to both signal wires is unable to pass and is cancelled.
Balanced Isolation Transformers will only pass a signal that is in opposite phase on both wires, but will not pass a signal that is in-phase on both wires. A signal that appears on one wire only cannot pass through the transformer, if the centre tap is not grounded. A signal on one wire only can pass, if the centre tap is grounded.
Isolation Transformers are very sensitive to external magnetic interference and must be kept well away from power supplies. Small transformers are often shielded in a mu-metal can for extra protection.
Applications are generalised into 3 categories
(a) Single isolation is the most common. From the sending equipment, the balanced signal is sent direct from an active driver circuit, representing a source impedance Z of approx 10-200ohm. The receiving equipment only has an isolation transformer. The input primary winding may have a centre tap, which is optionally switched to ground, if it is receiving a signal from another transformer.
(b) Double isolation is where both sending and receiving equipment is interfaced with isolation transformers. The centre tap of the receiver input winding normally goes to ground. The centre tap of the sending secondary winding is optionally switched to ground. Double isolation is required when sending and receiving equipment is powered form different mains phases, and where there is potential for voltages differences between the grounds and earth.
(c) Phantom Powered systems are similar to double isolation systems. The centre tap of the receiving transformer sends power through the signal cable to the centre tap of the sending transformer. Some Professional mixing consuls can send phantom power to special microphones e.g. 48Volt.
Note: Some mixing consuls and professional sound equipment have active balanced circuits that partially mimic isolation transformers. In closed environments of recording studios and home etc, these circuits work well, but for professional live application where interconnected equipment is separated by large distances and can be powered from different mains phases, line isolation transformers are essential.
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