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Interconnection Of Balanced And Unbalanced Equipment
Jensen AN-003

An UNBALANCED input or output connects one of its signal conductors to ground and has a non-zero impedance at the other signal conductor. Unbalanced inputs and outputs are very popular in consumer electronics, electronic musical instruments, and low cost (often called “semi-pro”) audio equipment. Examples of typical circuits are shown below.

Figure 1: Typical Unbalanced Input

Figure 2: Typical Unbalanced Output

The inherent weakness of unbalanced interconnections is that the shield, 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.

A BALANCED input or output uses two signal conductors which have equal impedances to ground. Balanced inputs and outputs 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. Examples of typical circuits are shown below.

Figure 3: Typical Balanced Input

Figure 4: Typical Balanced Output

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 compromise both low frequency response and distortion.

2 - UNBALANCED to BALANCED INTERFACES (cont'd)

Figure 5: "MINIMAL" is an Adapter Cable

Figure 6: “BETTER” uses an Output Transformer to Improve Balance

This interface uses a bifilar type output transformer to improve the impedance balance. This will reduce the degradation of CMRR at low frequencies for the active balanced type input stage. In the example 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.

Figure 7: “BEST” uses an Input Transformer to “Fix” the Imput Stage

Figure 8

Figure 9: Adding a Balanced Output

3 - BALANCED to UNBALANCED INTERFACES

The output impedance of the existing output is defined by the network 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.

This is also a good opportunity to “hot-rod” the output stage, by lowering and tightly matching its output impedances. Lowering RS to 100 S, ±1% and increasing CC to 220 µF, ±20%, works well with any 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, we recommend 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 or better than most current pro gear and, with the exception of the possible “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 at the same time.

Figure 10: “BETTER” uses an Output Transformer and a “Pad”

Figure 11: “BEST” uses a 4:1 Input Transformer to “Do It All”

4 - ABOUT UNBALANCED CONNECTIONS

Please note that, in all the schematic diagrams, connections to the unbalanced 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 of any remaining path used by both signal and ground currents. 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 with high 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 be bypassed altogether. Leave signal LO connected to the IHF plug shield, 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 is almost always room to mount one and then use a mating spade lug or banana plug on the cable’s shield.

5 - ABOUT BALANCED SHIELD CONNECTIONS

In the schematic diagram for the typical “pro” balanced input stage, pin 1 of the input socket is shown connected directly to the equipment 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 or oscillation. I have also proposed that “ground lift” switches be included in all balanced line input circuits, but never on microphone inputs (a microphone “floats” with no ground path of its own) or line outputs.

Probably to avoid adding a new dimension of frustration to unbalanced consumer audio systems, this equipment rarely has a power 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 be over 50 volts. This would tax the common-mode rejection capabilities of any input stage. Obviously, allowing the equipment to float is not a good idea. Unless it is grounded elsewhere, it must be grounded through the cable shield at the balanced input. If you should have some well-designed equipment that actually has a ground lift switch, set it to GND.

Figure 12

REFERENCES

[1]    B. Whitlock, “Balanced Lines in Audio - Fact, Fiction, and Transformers”, Journal of the AES, Vol 43, No 6, June, 1995.

©1995, Jensen Transfomers, Inc.

Figure 13: Contact Information