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Theory And Construction Of MIC 'Spliters'
Jensen AN-005
0 OVERVIEW
The Jensen JT-MB-x series transformers, commonly called "mic splitters", are designed to "bridge" the output of a 150 S to 200 S microphone. The "MB" in the part number stands for Microphone
Bridging. They are available in 2, 3, and 4 winding versions and are
generally used to provide additional, isolated outputs from a single
microphone. The microphone is directly connected to the first
preamp input, which provides "phantom power" if required, and also
connected to the primary winding of the MB transformer, which now
"bridges" the mic to first preamp line. The direct output is the
ONLY output which will pass phantom power to the mic. The
MB transformer secondary windings are then connected to
additional preamp inputs. Since the transformer magnetically
couples the signal to each winding, each preamp now "sees" the
microphone’s output signal while having no problematic direct
connection to the other preamps. To a preamp, each isolated MB
transformer output "looks like" a normal floating (ungrounded)
microphone.
1 THEORY
1.1 Level Loss
The input circuitry of a microphone preamp is normally designed to
recover as much of the available microphone output voltage as
possible. Since the noise floor of the preamp is essentially a constant,
signal to noise performance is improved by making the input voltage
as large as possible. It is very important to understand that the
fraction of available mic voltage actually delivered to the preamp
depends on both the output impedance of the mic itself and the
input impedance of the preamp input.
 Figure 1
Figure 1 - Voltage Divider Formed by Mic and Preamp
As shown in Figure 1, these two impedances effectively form a
voltage divider. The voltage lost in the internal (source) impedance
of the mic depends on the (load) impedance at the preamp input.
Loading loss, usually expressed in dB, compares the output
voltage with a specified load to that under "open circuit" or "no load"
conditions. For example, a 150 S (source) mic will deliver 91% of
its open circuit voltage when loaded by a preamp with 1.5 kS input
(load) impedance. The loading loss is then 20 x log 0.91 or 0.8 dB.
As a general rule, loading losses are negligible if the load impedance
is ten or more times the source impedance.
Therefore, it is certainly not desirable or necessary for the preamp
input impedance to "match" the impedance of the mic. If we did
connect a 150 S mic to a preamp with 150 S input impedance, only
half the mic voltage (6 dB of loading loss) would appear at the
preamp input, degrading signal to noise ratio. Such "impedance
matching" transfers maximum power from source to load, but this is
not what we want our mic preamp to do.
The simplified schematic of a typical splitter system, Figure 2, is
accurate for losses and bandwidth, but ignores CMRR and balance
issues which are covered in next section. It uses a Shure SM57
dynamic mic and its built-in transformer, which are represented by
equivalent resistance RM and inductance LM. The Jensen JT-MB-D
transformer winding resistances are shown as RT and leakage
inductances as LT. Preamp input impedances are shown as RL and
CL, and cable capacitances are shown as CC.
Inspection of Figure 2 shows how the input impedances of the
preamps or mixers effectively parallel to load the microphone. This
loading loss decreases the "direct" mic output level and the small
additional ("insertion") losses due to transformer resistances further
reduce "isolated" output levels. These losses are shown in Table 1,
which is a "worst case" situation where the mic is 150 S and each
preamp is a pessimistic 1 kS. In most situations, losses will be less
than those shown.
1.2 Frequency Response
Inspection of Figure 2 will also reveal a number of low-pass filters.
Generally, the most dominant filter is formed by the equivalent
 Figure 2
Figure 2 - Simplified Schematic for Estimating Losses and Bandwidth
Jensen AN-005 1
Table 1
Splitter Losses (dB) vs Loading  Figure 3
resistance RM and inductance LM of the mic and its total effective
capacitive load. Since the splitter transformer effectively parallels all
the cable capacitances and preamp input capacitances, the mic
"sees" the sum total of all this capacitance. In most situations, cable
capacitance will place an upper limit on total cable length. Most
common mic cable has a capacitance of about 25 pF per foot
(between the two shielded conductors). So-called "star quad" mic
cable, although it has amazing freedom from magnetic pickup
problems, has about twice the capacitance per foot of standard
cable. This fact must be seriously considered in large systems.
Anyone familiar with low-pass filters knows that the shape of the
rolloff curve is strongly affected by "damping". The energy absorbing
resistive components in any filter control its "Q" or the steepness of
it characteristics, especially at or near its "cutoff" frequency, where
response is !3 dB. In most systems, the input resistance of the mic
preamps is the dominant system damping. Figure 3 shows the effect
of preamp input resistance (and capacitance) on frequency response
of a Shure SM57 with 100 feet of common cable. The upper curves,
10 kS and 3 kS, are typical of transformer-less mic preamps while
the lower curve, 1.5 kS, is typical of a transformer input mic
preamp. Note the ultra-sonic peaks in response caused by
insufficient damping.
 Figure 4
Frequency response is most strongly influenced by the total cable
length (capacitance). Figure 4 shows the response of an SM57 mic
and a 150 S resistive source, both with cable lengths of 100 and
1000 feet. Of course, the peaks (some might call it desirable "sizzle")
would be damped or "flattened" with a lower impedance preamp
input. To maintain good response to 15 or 20 kHz, it’s generally a
good idea to keep the total cable length, which includes all the
direct and isolated output cables, to under 500 to 1000 feet of
standard cable or 250 to 500 feet of "star quad" cable. Generally,
this limit can be extended if the mic is a condenser type with a lower,
and primarily resistive, output impedance.
 Figure 5
1.3 Common-Mode RejectionFigure 5 shows the more complex
equivalent circuit which takes into account the impedances, mostly
capacitances, which must be balanced to achieve good CMR
(Common-Mode Rejection) of ground noise.
A microphone is a rather unusual signal source. Unlike most line
level sources, it electrically "floats" since it has no ground connection
of its own. The mic and cable are grounded at only one point ? at
the preamp via the cable shield conductor. The two signal lines have
small capacitances to this ground, making its common-mode source
impedances quite high, especially at low frequencies. As I have
stressed in previous papers, it is these common-mode impedances
which must be balanced to reject common-mode voltages such as
ground noise hum and buzz [1].
In 1974, Jensen introduced the individually Faraday shielded "MB"
series splitter transformer designs to solve the tough ground noise
problems in mobile recording and touring concert sound systems.
Faraday shields are layers of thin copper foil placed between
transformer windings to prevent capacitive coupling of ground noise
voltages. In conventional designs, one Faraday shield is placed
between two windings. But if this single shield is connected to the
ground reference for the signal on one winding, the other winding
(which has a different ground reference) can capacitively pickup the
ground noise. Designs using an individual "shield per winding" solve
this problem, leaving ground noise voltages to exist only harmlessly
between the shields.
When a splitter transformer primary "bridges" the "direct" line from
mic to preamp 1, small capacitances (30 pF to 150 pF, depending
on model) are added from each signal line to cable shield ground.
In the Jensen "MB" series, these capacitances are so well matched
that they have insignificant effect on system CMR. The tight match
Jensen AN-005 2
 Figure 6
is achieved by careful design and unit-to-unit consistency maintained
through automated winding and testing.
Likewise, each splitter secondary has small matched capacitances
(40 pF to 200 pF, depending on model) to the output cable shield
ground. Because the capacitances are comparable to those of most
mics, each "isolated" output "looks" just like a mic to its preamp, and
its CMR is not degraded by capacitive imbalances.
It should be noted here that mic cable itself can cause capacitive
imbalances. Although I have never seen a manufacturer specify this,
I have measured 4% capacitance imbalances in popular commercial
cable. Such an imbalance can significantly degrade system CMR,
especially when used with "transformerless" mic preamps whose
CMR is much more sensitive to source impedance imbalances than
transformer input designs. The CMR is degraded most at high
frequencies and the result is usually heard as a "buzz" with most of
its energy above a few kilohertz.
1.4 RFI and EMI Issues
Audio cables, including mic cables, can act as effective antennas for
RF energy. In strong fields, such as those near broadcast sites or
nearby portable transmitters, high RF voltages can appear at the
ends of audio cables. Although these voltages are essentially equal
in all conductors or "common-mode", the aforementioned capacitive
imbalances will convert a portion to a differential RF "signal".
Preamps vary widely in their tolerance of RF signals. Many
transformerless designs can become radio receivers at rather modest
RF levels, but all will eventually complain if fed enough RF.
In a high performance splitter system designed to avoid "ground
loop" problems, cable shields "float" at one end. The down side is
that this also makes each cable an effective whip antenna. If the
cable length and ambient RF frequencies are just right (Murphy will
attend to this), the whip becomes tuned, producing very high RF
voltages at its floating or ungrounded end. However, this voltage can
be drastically reduced by "terminating" the floating end at RF
frequencies. A series network consisting of a 0.01 µF (10 nF) ceramic
capacitor and 51 S resistor terminates the line at frequencies above
about 300 kHz but looks “open” at audio frequencies, avoiding a
"ground loop" [2].
Since a multi-channel splitter box brings many cables to one physical
location, it presents an opportunity to further reduce antenna effects
by connecting all shields together, via the above network, at RF
frequencies. This tends to “average” the pickup of the multiple cables
and behave as a local mesh network or “ground plane”. The RF
equivalent of such a splitter system is shown in Figure 6. Large
conductive objects, including steel reinforced concrete slab floors
and equipment racks, also tend to behave as local ground planes
with reduced RF energy near them. For this reason, cabling should
be routed in or very near such areas.
Hum can also enter the signal path magnetically. Basic physics tells
us that any conductor (wire) exposed to a varying (AC) magnetic
field will have an AC voltage “induced” in it. A balanced system has
two signal conductors and, if the magnetically induced voltages are
not exactly equal in the two conductors, the difference voltage will
appear as a signal &8212; hum.
Since the strength of magnetic fields falls rapidly with distance from
the source, the two signal conductors must have exactly the same
distance to the magnetic source to avoid hum pickup. Tight twisting of balanced signal conductors helps to make the average distance of
each conductor to any outside magnetic field source the same. “Star
quad” improves on this by effectively averaging the magnetic pickup
of four twisted conductors. Bear in mind that a mated pair of XLR
connectors leaves almost 2 inches of signal conductors untwisted
and very vulnerable to magnetic hum pickup. It would certainly be
a bad idea to lay such a pair of mated connectors on top of a power
amplifier and the typically strong magnetic field from its power
transformer.
Jensen AN-005 3
 Figure 7
2.1 The Schematic Diagram
Figure 7 is a complete schematic of a 3 channel, 3-way mic splitter.
It’s important to understand that the ground symbols used in
schematics are not pictorial representations of actual wire paths, but
merely a convenience to simplify the drawing. All similar symbols are
understood to be somehow connected to each other via wires or
circuit board traces, or via the chassis in the case of the chassis
ground symbol.
2.2 Basic Construction Decisions
Basic specifications like number of channels, number of splits per
channel, type of connectors, and mounting configuration will largely
define any given splitter box design. Although most splitters designed
by large, experienced sound companies share a number of features,
there is no real standardized design [3].
The next decision might be whether to use conventional "point-topoint"
internal wiring or a printed circuit board. Both the JT-MB-C
and JT-MB-D are available from Jensen in PC mounting versions.
If you opt for point-to-point construction, you may want to use
solder lug type terminal strips. The Cinch "50 series", having from 1
to 8 insulated terminals on 0.375" centers, is still available through
Newark and other electronics distributors. These were very widely
used in electronics prior to the advent of PC boards.
2.3 Wiring, Grounding, and Options
The “typical application” schematic on the “MB” series data sheets
shows a single channel as it would be constructed in its own
individual chassis. If you intend to build more than one channel in
a chassis, follow these guidelines:
- Mount the “input” and “direct output” connectors close to each
other so their pins 1 can be connected with a short length of
heavy gauge solid tinned copper wire. This bare jumper
becomes a tie point for the primary shield (GRY for MB-C and
MB-D, WHI/ORG for MB-E) and the 51 S + 10 nF RF network.
- If the chassis will not be intentionally grounded by rack
mounting or otherwise, connect the tie point mentioned above
directly to the chassis and omit the RF network, but only at
channel 1. This allows the chassis to be grounded through the
shield of direct output 1. This scheme assumes that, regardless
of the actual number of channels used, channel 1 would always
be connected to a grounded mixer or preamp input.
- Do not wire mating (cable end) connectors to connect the metal
shell to pin 1. If plugged into the splitter, connectors so wired
will connect pin 1 to the chassis ? defeating the float of the
"input" to "direct out" shield grounds. The only way to deal with
connectors wired this way is to insulate the chassis mounted
XLR shells from the chassis.
- Keep the leads on the RF network 51 S resistor and 10 nF
capacitor as short as possible. It is desirable for this network to
have a flat impedance up to several hundred MHz. To do this,
the parasitic inductance of the leads must be minimized by
keeping them very short and grounding the capacitor to a
solder lug at the connector.
- Mount the transformers securely, especially if the splitter will be
used "on the road". Do not rely on the mounting bracket alone
to support the MB-C or MB-D in "road" applications. The WHI
wire of the transformer is internally connected to its case. If the
transformer mounting insulates it from the chassis, connect the
WHI wire to the chassis. If the mounting connects it to the
chassis, the WHI wire may be clipped off.
- Tightly twist (3 to 5 twists per inch) the wire pairs for each
transformer winding. For example, twist BRN and RED together,
twist ORG and YEL together, and twist GRN and BLU together.
Note that a twisted pair should be just that ? do not include
shield or case leads with the signal pairs. Do not twist all the
transformer leads together in one bundle ? this is guaranteed
to degrade CMR.
- Each isolated output should have its signal wires tightly twisted
Jensen AN-005 5
Figure 7 - Complete 3-Channel, 3-Way Splitter
Jensen Transformers, Inc.
7135 Hayvenhurst Avenue
Van Nuys, California 91406
Tel (818) 374-5857 Fax (818) 374-5856
and the RF network installed (as in 4 above).
Jensen AN-005 4
- "Ground Lift" switches are optional. There are many
opinions regarding to the need for them:
If all the isolated outputs feed mixers or preamps which
are adequately grounded to a system ground, it should
never be necessary to close a "lift" switch.
If however, there are large ground voltages between
the "direct" and "isolated" outputs, closing a "lift" switch
may reduce the voltage. There is a risk that, if this is
done, heavy currents will flow in shields/pins 1 and,
under these conditions, some equipment will produce
hum through an unrelated internal mechanism [4].
If an isolated output feeds battery powered equipment
which floats (without a ground reference of its own),
closing the "lift" switch will eliminate possibly large
ground voltage differences.
- If the splitter will not always be used in a fixed
configuration, where any of isolated outputs may not be
used, it is recommended to install 2.7 kS damping resistors
across HI and LO (pins 2 and 3 for XLRs) output pins. If an
output is unused, and especially if it has a cable attached
which is open at the far end, the unloaded secondary of the
"MB" transformer will exhibit an undamped resonance
which can adversely affect frequency response of the
system.
3 INSTALLATION
To prevent magnetic hum pickup, locate the splitter away from
strong AC magnetic fields, such as those produced by lighting or
other cabling operating at high current, power transformers,
motors, computer CRTs, or TV receivers. All mic cabling,
especially at the connectors, should be routed to avoid the areas.
 Figure 8
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REFERENCES:
[1] B. Whitlock, "Balanced Lines in Audio - Fact, Fiction, and
Transformers", Journal of the AES, Vol 43, No 6, June,
1995.
[2] R. Morrison, "Grounding and Shielding Techniques in
Instrumentation", Third Edition, Wiley & Sons, 1986, p. 86.
[3] C. Michie, "A Standardized Mobile Snake Splitter System",
Recording Engineer/Producer, October, 1980, pp. 124-128.
[4] N. Muncy, "Noise Susceptibility in Analog and Digital Signal
Processing Systems", Journal of the AES, Vol 43, No 6, June,
1995.
 Figure 9Copyright 1996, Jensen Transformers, Inc.
 Figure 10: Contact Information
Jensen AN-005 5
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