At the Mercy of Cable Physics

The quality of microphone cable and plug design are the often overlooked aspects of a sound system that can come back to bite the user. The bite is in the form of noise, picked up on the cable and amplified by the electronics that come after it.

Fig. 1 shows three typical plug ends for passive microphones: QTS, QTRS, and XLR.

Fig 1a: Single-Ended QTS: Tip - Signal, Ring - Ground

Fig 1b: Differential QTRS: Tip - Pos Signal, Ring - Neg Signal, Sleeve - Ground

Fig1c: Differential with Shield XLR: Pin1 - GND, Pin2 - Pos Signal, Pin3 - Neg Signal, Shield

They all have different characteristics with respect to noise induced in the cable to which they are connected. Electro-Magnetic Interference (EMI) in the form of 50 or 60Hz AC hum, over-air radio and TV station broadcasts, electric motors, etc. have electrical and magnetic waves that can induce noise voltages in these cables. Most of the noise energy is common to all the conductors in the cable. This is called longitudinal noise. However, some of the noise energy presents as a voltage difference between the conductors. This is called metallic induced noise. If the longitudinal noise energy is strong enough, it can exceed the dynamic range of the microphone amplifier input circuitry. This can happen most often with AC hum, and one must rely on the fact that the ground “reference” conductor rides the noise along with the signal conductor(s). This does not always happen, and the signal is then overwhelmed. Addition of a second or third reference connection with less induced noise can bring the circuit noise down within the input’s dynamic range.

EMI generated on the cable that enters the pre-amplifier circuit and potentially affect its performance is measured as EMI susceptibility. In the reverse case, noise generated inside the circuit that couples into the Internal Ground can also couple into the cable shield, and the cable acts like an antenna for high-frequency EMI which can interfere with the operation of other nearby devices. This is called unintentional radiated EMI. Steps may be taken inside the circuit to filter out incoming and outgoing high frequency EMI between the cable shield and the Internal Ground without affecting lower-frequency shielding performance.

The microphone pre-amplifier input stage is usually a differential amplifier, which subtracts the voltage on one input conductor from the voltage on another input conductor.

Single-Ended Connection: QTS

The Quarter-inch Plug with Tip and Sleeve provides for only Ground Reference and Signal conductors. The Signal conductor connects to the Tip of the plug, and Ground connects to the Sleeve. The Ground conductor is connected to the Internal Ground of the pre-amplifier circuit, and in the microphone cable it forms a shield around the Signal conductor, protecting

it from induced noise. Depending on the cable construction, a certain amount of noise gets through to the Signal conductor. As shown in Fig. 2, this results in a significant amount of metallic noise interference. In the pre-amplifier input stage, the Ground voltage is subtracted from the Signal voltage. This process is good at removing most of the longitudinal noise energy, but does nothing to remove metallic noise. This noise starts to rear its ugly head when microphone pre-amplifier or speaker amplifier gain is high.

Differential Connection: QTRS

The Quarter-inch Plug with Tip, Ring, and Sleeve has three conductors: Positive Signal on the Tip, Negative Signal on the Ring, and Ground Reference on the Sleeve. As with the Single-Ended cable, the Ground Reference connects to the Internal Ground of the pre-amplifier circuit, and forms a shield on the cable, but it is around both the Positive and Negative Signal conductors. Depending on cable construction, it can do a poor to reasonable job of shielding the two internal conductors from induced noise, but as seen in Fig. 3, a metallic component still exists. The important distinction here is that the metallic noise component between each of the internal conductors and the shield is almost the same. Because the internal conductors run together, and share roughly the same physical position with respect to the shield along the length of the cable, the metallic noise between these internal conductors will be much less than that between each conductor and the shield. The microphone pre-amplifier will subtract the Negative Signal from the Positive Signal, creating a double-sized signal for amplification. It will also subtract out most of the longitudinal and metallic noise components that are common to both internal conductors, but the pre-amplifier will not remove the very small metallic component of differential noise between them.

Cable Coating (eg: vinyl) The QTRS cable and plug provide a good solution for noise reduction in passive microphones, but the issue of large induced common-mode noise which can swamp the input range is still a problem here.

Differential, Shielded and Referenced: XLR

The X Latching Resilient cable and plug (invented by the Cannon Electric Company) provide the Positive signal, Negative signal, and Ground Reference conductors, as found in the QTRS cable and Plug, but as seen in Fig. 4, all three of those conductors are internal wires, and a fourth conductor forms the shield around them. This shield conductor is typically connected to Frame Ground at the pre-amplifier equipment end of the cable, and left unconnected at the microphone end. Frame Ground is the metal case or metal shield of the pre-amplifier equipment, also referred to as the “quiet” ground that suppresses radiated EMI, and EMI susceptibility.

Fig. 5 shows that the Ground Reference conductor connects to the pre-amplifier Internal Ground, as it does in the QTRS and QTS cables. The connection between the Frame Ground and the Internal Ground on the pre-amplifier equipment may take a number of different forms, primarily to reduce EMI radiation and susceptibility. High frequency EMI suppression on the Frame Ground typically uses an inductive ferrite bead component to connect to the Internal Ground. For suppression of lower frequency interference such as AC hum, a resistor/capacitor (RC) network can reduce hum while providing a low impedance connection over the remaining audio passband. If the microphone is used for voice, the maximum audio passband is roughly 100Hz to 17kHz, and using such a network can be designed to have little affect on audio performance.

Remember that the QTS and QTRS cables connect the Internal Ground to the cable shield, so the Internal Ground Filtering circuit is not present in those configurations.

The shield in the XLR cable is effectively an extension of the Frame, or “quiet” Ground on the equipment. Depending on the cable construction, the XLR shield does a poor to reasonable job of suppressing noise energy induction in the three internal conductors. Metallic noise energy is still induced between the three conductors, but it is of lower energy level between the two signal conductors and the internal ground reference, leading to a quieter pre-amplifier circuit.

Microphone Cable Questions

When using a microphone cable that has an XLR receptacle at the microphone and a single-ended QTS plug at the pre-amplifier, how are the positive signal, negative signal, ground reference, and shield connected to the tip and sleeve conductors on the QTS plug? Are the Negative Signal and Ground Reference conductors tied together at the microphone end (two conductor cable), or the pre-amplifier end (three or four conductor cable)? Is the shield of the cable left unconnected at both ends, or is it connected to the QTS sleeve at the pre-amplifier end?

Is there such a thing as an XLR to QTRS microphone cable? If so, is it three conductor or four conductor cable? If not, is the best solution to terminate an XLR to XLR cable with an XLR to QTRS adapter? How is the shield connected (or not connected) to the pre-amplifier Internal Ground?

Noise in an amplified microphone signal may not always be present, but it can appear at the speaker output at the worst possible times if cable construction and pre-amplifier specifications are not considered. To avoid being at the mercy of cable physics, choose a microphone, cable, and pre-amplifier appropriate to their intended use, evaluate their collective noise performance, and run tests to answer the above questions on cable construction. These steps should guard against pesky noise problems when they crop up.

Ken Buttle