THEORY CONSTRUCTION "SPLITTERS" Bill Whitlock OVERVIEW Jensen JT-

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THEORY CONSTRUCTION "SPLITTERS" Bill Whitlock OVERVIEW Jensen JT-

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Jensen AN-005
THEORY CONSTRUCTION "SPLITTERS"
Bill Whitlock OVERVIEW Jensen JT-MB-x series transformers, commonly called "mic splitters", designed "bridge" output microphone. "MB" part number stands Microphone Bridging. They available winding versions generally used provide additional, isolated outputs from single microphone. microphone directly connected first preamp input, which provides "phantom power" required, also connected primary winding transformer, which "bridges" first preamp line. direct output ONLY output which will pass phantom power mic. transformer secondary windings then connected additional preamp inputs. Since transformer magnetically couples signal each winding, each preamp "sees" microphone's output signal while having problematic direct connection other preamps. preamp, each isolated transformer output "looks like" normal floating (ungrounded) microphone. THEORY Level Loss input circuitry microphone preamp normally designed recover much available microphone output voltage possible. Since noise floor preamp essentially constant, signal noise performance improved making input voltage large possible. very important understand that fraction available voltage actually delivered preamp depends both output impedance itself input impedance preamp input. voltage divider. voltage lost internal (source) impedance depends (load) impedance preamp input. Loading loss, usually expressed compares output voltage with specified load that under "open circuit" load" conditions. example, (source) will deliver open circuit voltage when loaded preamp with input (load) impedance. loading loss then 0.91 general rule, loading losses negligible load impedance more times source impedance. Therefore, certainly desirable necessary preamp input impedance "match" impedance mic. connect preamp with input impedance, only half voltage loading loss) would appear preamp input, degrading signal noise ratio. Such "impedance matching" transfers maximum power from source load, this what want preamp simplified schematic typical splitter system, Figure accurate losses bandwidth, ignores CMRR balance issues which covered next section. uses Shure SM57 dynamic built-in transformer, which represented equivalent resistance inductance Jensen JT-MB-D transformer winding resistances shown leakage inductances Preamp input impedances shown cable capacitances shown Inspection Figure shows input impedances preamps mixers effectively parallel load microphone. This loading loss decreases "direct" output level small additional ("insertion") losses transformer resistances further reduce "isolated" output levels. These losses shown Table which "worst case" situation where each preamp pessimistic most situations, losses will less than those shown. Frequency Response Inspection Figure will also reveal number low-pass filters. Generally, most dominant filter formed equivalent
Figure Voltage Divider Formed Preamp shown Figure these impedances effectively form
Figure Simplified Schematic Estimating Losses Bandwidth Jensen AN-005
Table Splitter Losses (dB) Loading Loaded Outputs direct only direct direct only direct direct direct only direct direct direct Direct
would damped "flattened" with lower impedance preamp input. maintain good response kHz, it's generally good idea keep total cable length, which includes direct isolated output cables, under 1000 feet standard cable feet "star quad" cable. Generally, this limit extended condenser type with lower, primarily resistive, output impedance.
resistance inductance total effective capacitive load. Since splitter transformer effectively parallels cable capacitances preamp input capacitances, "sees" total this capacitance. most situations, cable capacitance will place upper limit total cable length. Most common cable capacitance about foot (between shielded conductors). So-called "star quad" cable, although amazing freedom from magnetic pickup problems, about twice capacitance foot standard cable. This fact must seriously considered large systems. Anyone familiar with low-pass filters knows that shape rolloff curve strongly affected "damping". energy absorbing resistive components filter control steepness characteristics, especially near "cutoff" frequency, where response most systems, input resistance preamps dominant system damping. Figure shows effect preamp input resistance (and capacitance) frequency response Shure SM57 with feet common cable. upper curves, typical transformer-less preamps while lower curve, typical transformer input preamp. Note ultra-sonic peaks response caused insufficient damping.
Figure Effect Cable Length Response Cable pF/ft preamp Common-Mode RejectionFigure shows more complex equivalent circuit which takes into account impedances, mostly capacitances, which must balanced achieve good (Common-Mode Rejection) ground noise. microphone rather unusual signal source. Unlike most line level sources, electrically "floats" since ground connection own. cable grounded only point preamp cable shield conductor. signal lines have small capacitances this ground, making common-mode source impedances quite high, especially frequencies. have stressed previous papers, these common-mode impedances which must balanced reject common-mode voltages such ground noise buzz [1]. 1974, Jensen introduced individually Faraday shielded "MB" series splitter transformer designs solve tough ground noise problems mobile recording touring concert sound systems. Faraday shields layers thin copper foil placed between transformer windings prevent capacitive coupling ground noise voltages. conventional designs, Faraday shield placed between windings. this single shield connected ground reference signal winding, other winding (which different ground reference) capacitively pickup ground noise. Designs using individual "shield winding" solve this problem, leaving ground noise voltages exist only harmlessly between shields. When splitter transformer primary "bridges" "direct" line from preamp small capacitances depending model) added from each signal line cable shield ground. Jensen "MB" series, these capacitances well matched that they have insignificant effect system CMR. tight match
Figure Damping Effect Preamp Input Impedance Frequency response most strongly influenced total cable length (capacitance). Figure shows response SM57 resistive source, both with cable lengths 1000 feet. course, peaks (some might call desirable "sizzle")
Jensen AN-005
Figure Simplified Schematic Common-Mode Rejection Purposes achieved careful design unit-to-unit consistency maintained through automated winding testing. Likewise, each splitter secondary small matched capacitances depending model) output cable shield ground. Because capacitances comparable those most mics, each "isolated" output "looks" just like preamp, degraded capacitive imbalances. should noted here that cable itself cause capacitive imbalances. Although have never seen manufacturer specify this, have measured capacitance imbalances popular commercial cable. Such imbalance significantly degrade system CMR, especially when used with "transformerless" preamps whose much more sensitive source impedance imbalances than transformer input designs. degraded most high frequencies result usually heard "buzz" with most energy above kilohertz. Issues Audio cables, including cables, effective antennas energy. strong fields, such those near broadcast sites nearby portable transmitters, high voltages appear ends audio cables. Although these voltages essentially equal conductors "common-mode", aforementioned capacitive imbalances will convert portion differential "signal". Preamps vary widely their tolerance signals. Many transformerless designs become radio receivers rather modest levels, will eventually complain enough high performance splitter system designed avoid "ground loop" problems, cable shields "float" end. down side that this also makes each cable effective whip antenna. cable length ambient frequencies just right (Murphy will attend this), whip becomes tuned, producing very high voltages floating ungrounded end. However, this voltage drastically reduced "terminating" floating frequencies. series network consisting 0.01 ceramic capacitor resistor terminates line frequencies above about looks "open" audio frequencies, avoiding "ground loop" [2]. Since multi-channel splitter brings many cables physical location, presents opportunity further reduce antenna effects connecting shields together, above network, frequencies. This tends "average" pickup multiple cables behave local mesh network "ground plane". equivalent such splitter system shown Figure Large conductive objects, including steel reinforced concrete slab floors equipment racks, also tend behave local ground planes with reduced energy near them. this reason, cabling should routed very near such areas. also enter signal path magnetically. Basic physics tells that conductor (wire) exposed varying (AC) magnetic field will have voltage "induced" balanced system signal conductors and, magnetically induced voltages exactly equal conductors, difference voltage will appear signal hum. Since strength magnetic fields falls rapidly with distance from source, signal conductors must have exactly same distance magnetic source avoid pickup. Tight twisting balanced signal conductors helps make average distance each conductor outside magnetic field source same. "Star quad" improves this effectively averaging magnetic pickup four twisted conductors. Bear mind that mated pair connectors leaves almost inches signal conductors untwisted very vulnerable magnetic pickup. would certainly idea such pair mated connectors power amplifier typically strong magnetic field from power transformer.
Jensen AN-005
Figure Simplified System show "Antenna" Effects CONSTRUCTION Schematic Diagram Figure complete schematic channel, 3-way splitter. It's important understand that ground symbols used schematics pictorial representations actual wire paths, merely convenience simplify drawing. similar symbols understood somehow connected each other wires circuit board traces, chassis case chassis ground symbol. Basic Construction Decisions Basic specifications like number channels, number splits channel, type connectors, mounting configuration will largely define given splitter design. Although most splitters designed large, experienced sound companies share number features, there real standardized design [3]. next decision might whether conventional "point-topoint" internal wiring printed circuit board. Both JT-MB-C JT-MB-D available from Jensen mounting versions. point-to-point construction, want solder type terminal strips. Cinch series", having from insulated terminals 0.375" centers, still available through Newark other electronics distributors. These were very widely used electronics prior advent boards. Wiring, Grounding, Options "typical application" schematic "MB" series data sheets shows single channel would constructed individual chassis. intend build more than channel chassis, follow these guidelines: Mount "input" "direct output" connectors close each other their pins connected with short length heavy gauge solid tinned copper wire. This bare jumper becomes point primary shield (GRY MB-C MB-D, WHI/ORG MB-E) network. chassis will intentionally grounded rack mounting otherwise, connect point mentioned above directly chassis omit network, only channel This allows chassis grounded through shield direct output This scheme assumes that, regardless actual number channels used, channel would always connected grounded mixer preamp input. wire mating (cable end) connectors connect metal shell plugged into splitter, connectors wired will connect chassis defeating float "input" "direct out" shield grounds. only deal with connectors wired this insulate chassis mounted shells from chassis. Keep leads network resistor capacitor short possible. desirable this network have flat impedance several hundred MHz. this, parasitic inductance leads must minimized keeping them very short grounding capacitor solder connector. Mount transformers securely, especially splitter will used road". rely mounting bracket alone support MB-C MB-D "road" applications. wire transformer internally connected case. transformer mounting insulates from chassis, connect wire chassis. mounting connects chassis, wire clipped off. Tightly twist twists inch) wire pairs each transformer winding. example, twist together, twist together, twist together. Note that twisted pair should just that include shield case leads with signal pairs. twist transformer leads together bundle this guaranteed degrade CMR. Each isolated output should have signal wires tightly twisted
Jensen AN-005
network installed above). "Ground Lift" switches optional. There many opinions regarding need them: isolated outputs feed mixers preamps which adequately grounded system ground, should never necessary close "lift" switch. however, there large ground voltages between "direct" "isolated" outputs, closing "lift" switch reduce voltage. There risk that, this done, heavy currents will flow shields/pins and, under these conditions, some equipment will produce through unrelated internal mechanism [4]. isolated output feeds battery powered equipment which floats (without ground reference own), closing "lift" switch will eliminate possibly large ground voltage differences. splitter will always used fixed configuration, where isolated outputs used, recommended install damping resistors across (pins XLRs) output pins. output unused, especially cable attached which open end, unloaded secondary "MB" transformer will exhibit undamped resonance which adversely affect frequency response system.
INSTALLATION prevent magnetic pickup, locate splitter away from strong magnetic fields, such those produced lighting other cabling operating high current, power transformers, motors, computer CRTs, receivers. cabling, especially connectors, should routed avoid areas.
REFERENCES: Whitlock, "Balanced Lines Audio Fact, Fiction, Transformers", Journal AES, June, 1995. Morrison, "Grounding Shielding Techniques Instrumentation", Third Edition, Wiley Sons, 1986, Michie, Standardized Mobile Snake Splitter System", Recording Engineer/Producer, October, 1980, 124-128. Muncy, "Noise Susceptibility Analog Digital Signal Processing Systems", Journal AES, June, 1995. Copyright 1996, Jensen Transformers, Inc. Figure Complete 3-Channel, 3-Way Splitter
Jensen Transformers, Inc.
7135 Hayvenhurst Avenue Nuys, California 91406 (818) 374-5857 (818) 374-5856
Jensen AN-005

THEORY CONSTRUCTION "SPLITTERS" Bill Whitlock OVERVIEW Jensen JT-

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