MICS, PREAMPS and splitters
Apr 1, 1999 12:00 PM, Bill Whitlock
Let me debunk a common myth. Impedance matching is a concept that does not apply to mics and preamps. Because the electrical noise floor, heard as hiss, of a preamp's input stage is constant, S/N ratio increases as the input signal gets larger. Therefore, preamps are normally designed to recover as much of the mic's output voltage as possible. As shown in Figure 1, the mic's output impedance and the preamp's input impedance form a voltage divider.
Loading loss, usually expressed in decibels, compares the output voltage with a specified load to that under open-circuit or no-load conditions. For example, a 150 V mic loaded by a preamp having a 1.5 kV input impedance will deliver 91% of its open-circuit voltage to the preamp (loading loss = 0.8 dB). As a rule of thumb, loading losses become negligible (under 1 dB) when the load impedance is ten or more times that of the source. Connecting a 150 V mic to an impedance-matched preamp with an actual input impedance of 150 V would waste half the mic's output voltage, unnecessarily degrading S/N ratio by some 5 dB.
System frequency response will be affected by the low-pass filter in the Figure 1 equivalent circuit. Generally, the most dominant filter is formed by the mic's internal resistance (RM) and inductance (LM) and the mic's total capacitive load (CC + CL). In most systems, cable capacitance makes up most of this capacitance. Common mic cable has a capacitance of about 25 pF/ft (82 pF/m) between the two shielded conductors. Star quad cable (more on this later) has about twice this capacitance.
Like loudspeakers, real mics do not behave like resistors. Figure 2 shows the frequency response effects of cable length on a simulated Shure SM57 dynamic mic. The response peaks are due to the dynamic mic's inductance, which forms an under-damped low-pass filter when it interacts with cable capacitance and preamp input impedance. Responses for an ideal 150 V source are shown for comparison.
Figure 3 uses the same simulated SM57 mic to show the effect preamp input impedance has on damping the response peaks. The upper curves, 10 kV and 3 kV, are typical of transformerless mic preamps, while the lower curve, 1.5 kV, is typical of a mic preamp using an input transformer. Active or phantom-powered mics, such as condenser types, generally appear less inductive than dynamic types and are less prone to these peaking effects. Their responses will generally be closer to the ideal curves in Figure 2.
Mic splitters Mics, unlike most other audio system devices, have no coupling whatsoever to the power line. This neatly avoids ground loop hum and buzz problems caused by power line currents that might otherwise flow in interconnect cables. Problems are rare because the mic housing and cable shield are grounded only at the preamp input. If a single mic is parallel connected (sometimes called a "hard-wire split") to two or more preamps, however, a myriad of problems crop up.
So-called mic "splitters" are used to provide additional, isolated outputs from a single mic. As shown in the schematic of Figure 4, the mic is directly connected to preamp A via the splitter's direct output. Only this direct output can pass phantom power from the preamp back to the mic. The mic also drives the primary winding of the splitter's transformer, which magnetically couples the signal to the isolated secondary windings, which then drive preamps B and C while avoiding any problematic direct connections between preamps. From its point of view, each preamp is connected to a normal floating mic source. From the mic's point of view, however, the transformer effectively parallels all cable capacitances and preamp input impedances. This produces additional loading loss, which may reduce the direct output level by 1 dB or 2 dB. Loading losses, in practice, limit transformer-type splitters to four-way or less. Transformer winding resistances may cause an additional 1 dB loss at each isolated output. These effects are insignificant in most systems. Because cable capacitances are effectively paralleled, it is generally a good idea to limit total cable length, which includes all the direct and isolated output cables, to about 500 ft (152 m) of standard cable or 250 ft (76 m) of star quad cable when using dynamic mics.
RFI and magnetic fields A mic cable can inadvertently become an RF receiving antenna. Large conductive objects, including steel or concrete slab floors and equipment racks, tend to behave as localized ground planes. Routing cables in or near them can reduce RF pickup. Where fields are especially strong, such as near broadcast transmitter sites, high RF voltages can appear at the ungrounded ends of cables. Although the induced voltage is theoretically equal in all conductors (common-mode), normal impedance imbalances will convert part of the energy into signal. Preamps vary widely in their tolerance of RF signals. Many transformerless designs become radio receivers at rather modest RF levels, but all designs will eventually misbehave if the RF fields are strong enough. See my February 1999 column for more on RFI.
One purpose of a mic splitter is to isolate the shields of the isolated output cables from the input/direct output shields and each other. Although avoiding ground loop problems, this lifting can allow each cable to become a whip antenna. If you are particularly unlucky, the cable length will be tuned to the frequency of a powerful nearby transmitter, producing high RF voltages at the ungrounded or lifted end of the cable. This voltage can be drastically reduced by using an RF terminating network consisting of a series connected 0.01 mF (10 nF) ceramic capacitor and 51 V resistor, at the lifted end. This terminates the line for RF frequencies but looks open at audio frequencies. See Jensen application note AN005 for details on this and other details about mic splitter design and construction.
Hum and buzz can enter the signal path via magnetic induction. Any varying (AC) magnetic field will induce an AC voltage in any conductor exposed to it. Balanced systems will reject such pickup as long as the voltages in the two signal carrying wires is identical. Any difference, by definition, becomes a signal. Induced voltage is proportional to magnetic field strength, which falls rapidly with distance from the field source. Twisting virtually eliminates magnetic pickup because it makes the average distance of each conductor to any external magnetic field source the same. Star quad cable takes this a step further by averaging with four twisted conductors. Note that a mated pair of XLR connectors leaves almost 2 inches (51 mm) of cable untwisted, therefore vulnerable to magnetic pickup. Strong AC magnetic fields are produced by any power cables operating at high current, power transformers, motors, computer CRTs or TV receivers. All mic cabling, and especially connectors, should be routed to avoid such areas. For example, it is a bad idea to place a pair of mated XLR mic cable connectors on top of a power amp.
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