Jensen AN-003

INTERCONNECTION OF BALANCED AND UNBALANCED EQUIPMENT
by Bill Whitlock
Techniques for interconnecting balanced and unbalanced equipmentin audio systems seems to be a murky topic for many users,
technicians, and system designers. It is the subject of numerous calls
to our applications engineers. Confusion, and even controversy, isfurther fueled by some published "guides" whose engineering basis
is dubious, at best. This paper briefly explains the hardware, someunderlying engineering theory, and includes wiring diagrams forinterfaces that solve many typical system problems.

A BALANCED input or output uses two signal conductors
which have equal impedances to ground. Balanced inputs andoutputs are widely used in professional equipment because the input
differential amplifier can, in theory at least, totally null its response
to ground noise which exists equally on both signal lines. Examplesof typical circuits are shown below.

1 - "UNBALANCED" and "BALANCED"
What do these terms really mean? Since I have previously written
about the subject of balanced lines in some detail, I will justsummarize the most important points here. Balance is defined interms of the impedances of the two signal conductors withrespect to a reference, which is usually "ground".
An UNBALANCED input or output connects one of its
signal conductors to ground and has a non-zero impedanceat the other signal conductor. Unbalanced inputs and outputs
are very popular in consumer electronics, electronic musicalinstruments, and low cost (often called "semi-pro") audio equipment.
Examples of typical circuits are shown below.

Typical Balanced Input
Because of its low cost and small size, this "active balanced"
differential input circuit (or some variation of it) is very widely used.
The pairs of 10 kS resistors are trimmed to match within 0.01% inorder to null response to ground noise. When used in real-world
systems, however, a major drawback of this circuit is its exquisite
sensitivity to unbalances in the output (source) impedances of theline driver. This seriously compromises ground noise rejection or
CMRR (Common-Mode Rejection Ratio) of the system.

Typical Unbalanced Input
Usual values for RL are 10 kS to 100 kS, regardless of theequipment type. It is RL that effectively determines the input
impedance of the circuit.

Typical Unbalanced Output
Usual values for RS are 330 S to 1 kS and for CC are 4.7 µF to47 µF in consumer and musical instrument equipment, oftenspecified to drive a 10 kS minimum load. In Semi-pro audio
equipment, often specified to drive a 600 S minimum load, RS is
usually 47 S to 220 S and CC is 47 µF to 220 µF.
The inherent weakness of unbalanced interconnections is that theshield, which is also a signal conductor, is a path for power line
related currents that always flow between equipment grounds. The
voltage drop across the resistance of the shield and connectors adds
directly to the signal, producing the familiar hum and buzz.

Typical Balanced Output
This output circuit provides two equal amplitude but opposite
polarity (symmetrical) signals and equal output (source) impedances
to the lines. Values for RS and CC are generally in the same range as
those for semi-pro equipment and tolerances are typically ±5% for
RS and ±20% for CC. Although the balance of these (source)
impedances strongly control system CMRR performance, especiallywhen used with active balanced inputs, few equipment makers seemto recognize its importance. The signal symmetry of this, or any
other, balanced line driver has nothing to do with ground noise
rejection.
2 - UNBALANCED to BALANCED INTERFACES
The interfaces on the following page do NOT provide the 12 dB gain
necessary to raise the nominal !10 dBV (316 mV RMS) "consumer"
reference level to the nominal +4 dBu (1.23 V RMS) "pro" reference
level. If the pro equipment doesn't have enough gain "reach", an
active interface may be necessary. A step-up transformer, even an
ideal lossless one, is not a viable source of gain in this application.
Reflected impedances cause excess level losses and compromiseboth low frequency response and distortion.

Jensen AN-003

12 - UNBALANCED to BALANCED INTERFACES (cont'd)
For the applications below, cables should be high quality shielded
twisted pair and, to prevent high frequency losses, no more than
1000 pF total capacitance. This is about 20 feet for standard types.

In all cases, unbalanced semi-pro outputs, which generally havelower RS and higher CC values, will typically improve ground noise
rejection (CMRR) by several dB.

2.1 - "MINIMAL" is an Adapter Cable
This interface is sometimes called a "pseudo-balanced" directconnection. Chassis ground currents flow in the shield of the cableand the balanced input senses the signal at the unbalanced output
connector. This theoretically allows the differential input stage toreject the common-mode ground noise.

However, the 470 S source impedance unbalance will seriously
degrade the CMRR of any simple active balanced input stage.
In the example above, CMRR will be only 30 dB at 60 Hz.
If the "pro" input already uses a Jensen line input transformer,CMRR will be about 100 dB at 60 Hz, with only this cable needed.

2.2 - "BETTER" uses an Output Transformer to Improve BalanceThis interface uses a bifilar type output transformer to improve theimpedance balance. This will reduce the degradation of CMRR at
low frequencies for the active balanced type input stage. In theexample above, CMRR will be about 55 dB at 60 Hz, but
because of the interwinding capacitance in an output transformer,

CMRR will gradually fall to about 30 dB for frequencies over 1 kHz.
Compared to the simple adapter cable, it does not improve buzz,
which contains many high frequency components, but it further
reduces 60 Hz hum by about 25 dB. This method can also reduce
hum by over 70 dB when used with an unbalanced input.

2.3 - "BEST" uses an Input Transformer to "Fix" the Input Stage
This interface uses an input transformer to effectively replace the"active balanced" input stage. The input transformer, unlike the input
stage, can tolerate source impedance unbalances with very little
CMRR degradation. In the example above, CMRR will beabout 100 dB at 60 Hz and near 70 dB at 3 kHz, making it very
effective at eliminating both the hum and buzz components.

2The input stage is operated unbalanced by grounding its ! input anddriving its + input with the transformer's output. If the transformer
can be installed inside the "pro" equipment, the resulting
balanced input will give outstanding CMRR performancefrom either balanced or unbalanced (using the adaptercable) sources.

Jensen AN-003

2.4 - A Simple Alternative
The output impedance of the existing output is defined by thenetwork between the op-amp output (whose closed loop output
impedance is negligible) and the output connector. An identical
network to ground is then added as shown.

Adding a Balanced Output
A simple modification to equipment with unbalanced outputs canconvert it to have true balanced outputs. Get (or trace the circuit tomake) a schematic of the equipment's output circuitry. Depending
on available panel space, the new 3-conductor output connector canbe added or used to replace the existing connector. This
modification uses the existing unbalanced output as the + outputand adds an impedance matched passive network to ground for the! output. In most cases, it is as simple as shown above.

This is also a good opportunity to "hot-rod" the output stage, bylowering and tightly matching its output impedances. Lowering RS
to 100 S, ±1% and increasing CC to 220 µF, ±20%, works well withany popular op-amp known to the author, except for the TL06x,TL07x, or TL08x series (their high open loop output impedance
makes them unstable with capacitive loads such as cables). For opamps operating from symmetrical supplies up to ±18 volts, werecommend Panasonic 16 volt bi-polar electrolytics, part number
ECE-A1CN221S, available from Digi-Key or other Panasonic
distributors. These parts have the lowest distortion characteristic of
any we've tested. The modified output will have balance as good orbetter than most current pro gear and, with the exception of thepossible "gain reach" problem mentioned earlier, will produce
excellent results in a professional environment. If the unbalanced
output is retained, do not use (or connect cables to) both outputs atthe same time.

3 - BALANCED to UNBALANCED INTERFACES

3.1 - "BETTER" uses an Output Transformer and a "Pad"
If the balanced output uses a transformer, omit the one shown andconnect HI and LO directly to ORG and YEL respectively. In theexample above, CMRR will be about 60 dB at 60 Hz, but
decrease at 6 dB per octave, making it effective for hum but

not for buzz. The same transformer capacitance will unbalance thebalanced line at high frequencies, which won't affect the interface
itself, but may affect the CMRR of other inputs "bridging" the line.
The 12 dB signal attenuation is provided by the two resistor "pad".

3.2 - "BEST" uses a 4:1 Input Transformer to "Do It All"
This interface uses a 4:1 step-down input transformer to provide
12 dB of signal attenuation, excellent ground noise rejection, very
little line loading, and full preservation of the line's balance.

In the example above, CMRR will be about 120 dB at 60 Hz
and 85 dB at 3 kHz, effectively eliminating hum and buzz. The
transformer must be located as shown to reduce capacitive loading.

Jensen AN-003

34 - ABOUT UNBALANCED CONNECTIONS
Please note that, in all the schematic diagrams, connections to theunbalanced IHF plug (formerly the "RCA plug" or "phono plug") are
shown as the single point to join SH (shield) and LO (signal low or !).
This is very important to absolutely minimize the resistance ofany remaining path used by both signal and groundcurrents. If connections are made as shown, the only remaining
common path is the contact resistance of the shield connection
between IHF plug and IHF socket. Use a good quality IHF plug withhigh spring force contacts and make sure the contact points are clean
and free of oxides.
With a little extra effort, this small remaining shared path can bebypassed altogether. Leave signal LO connected to the IHF plugshield, but connect the cable shield directly to the equipment chassis.
If the equipment manufacturer has thoughtfully provided a binding
post or banana jack marked GND, this will be easy. If not, there isalmost always room to mount one and then use a mating spade lugor banana plug on the cable's shield.
5 - ABOUT BALANCED SHIELD CONNECTIONSIn the schematic diagram for the typical "pro" balanced input stage,pin 1 of the input socket is shown connected directly to theequipment chassis, based on widely used conventional practice.
However, in previous writings, I have proposed that in large, very
high performance audio systems using balanced interconnections,this practice can degrade CMRR and introduce system crosstalk oroscillation. I have also proposed that "ground lift" switches beincluded in all balanced line input circuits, but never on microphone
inputs (a microphone "floats" with no ground path of its own) or lineoutputs.
Probably to avoid adding a new dimension of frustration tounbalanced consumer audio systems, this equipment rarely has apower connector that includes a safety ground. Its chassis floats
above the environmental ground, sometimes over 50 volts above it.
If connected to ground, or other equipment that is grounded, a small
current (generally under 1 mA) will flow through the connection. If
this floating equipment were connected to a balanced input with pin
1 lifted, the common-mode voltage presented to the input could beover 50 volts. This would tax the common-mode rejection
capabilities of any input stage. Obviously, allowing the equipment tofloat is not a good idea. Unless it is grounded elsewhere, it must begrounded through the cable shield at the balanced input. If you
should have some well-designed equipment that actually has aground lift switch, set it to GND.

REFERENCES[1]

B. Whitlock, "Balanced Lines in Audio - Fact, Fiction, andTransformers", Journal of the AES, Vol 43, No 6, June,
1995.

[2]

B. Whitlock, "Answers to Common Questions about Audio
Transformers", Jensen Application Note AN002© 1995, Jensen Transformers, Inc.

Jensen Transformers, Inc.
7135 Hayvenhurst Avenue
Van Nuys, California 91406
Tel (818) 374-5857 Fax (818) 374-5856

4Jensen AN-003