by Ray Rayburn
Is the interfaces between a power amp and a loudspeakers is balanced?
Someone recently asked me if the interface between a power amp and a loudspeaker is balanced, and if there was any advantage to using twisted pair wire to connect the power amplifier output to a loudspeaker. What follows was my reply.
“Balanced” at an interface means 4 things:
- 1) The common mode output impedance is balanced (equal with respect to common – usually “ground”)
- 2) The common mode input impedance is balanced (equal with respect to common – usually “ground”)
- 3) The wiring is balanced (the common mode impedance – mostly capacitance – is balanced)
- 4) The input is only or mostly sensitive to the differential mode signal (the difference in signal voltage on the two wires)
Common mode in this context means a signal that is equal on both wires. The reason we have to specify “Common mode impedance” is that it is not the same as the more common “differential or “normal” mode impedance”.
For example consider the loudspeaker. It has a normal mode impedance of around 8 ohms typically. The common mode impedance, however is close to an open circuit, and is extremely well balanced. In other words with the same signal applied to both input terminals with respect to ground, the current drawn by each input is extremely close to zero (close to an open circuit), and the amount of current drawn by each leg of the input is almost precisely the same (extremely well balanced).
Most power amps have single ended outputs. Amps that are run in “bridged mode” have push-pull outputs, as do some amps designed to run on low power supply voltages. Please note that it makes no difference in terms of being balanced if the amp has a single ended or push-pull output.
A single ended power amp does not strictly speaking have a balanced output because the driven output terminal has some finite output impedance while the un-driven output is grounded. However, the amount of unbalance is extremely small, and because of the extreme ratio between the very low common mode output impedance and the extremely high common mode input impedance of the loudspeaker, the result is very good rejection of interference at the interface.
I should note that this is very similar to the power system used here in the USA. Each circuit has a finite but very low source (output) impedance, and the loads have close to open circuit common mode input impedances.
In a balanced interface using twisted wiring provides significant reduction in magnetic coupling to and from the outside world. In other words if the wiring is twisted it will have greatly reduced pickup of external magnetic fields, and also have greatly reduced net radiation of magnetic fields from currents flowing in the wiring. Unlike most audio wiring, loudspeaker wiring can have significant amounts of current flowing in it.
“Twisted pairs are commonly used for balanced line signal and microphone cables, in which the nominal voltages are very low and the input impedance of the load is typically quite high (>10K ohms). Under such conditions, the use of a twisted pair is essential to reduce crosstalk among adjacent cables. … In contrast, however, loudspeakers have input impedances that are quite low and operate on much higher voltages. The potential of inducing an audible signal from adjacent wiring is close to zero”
When multiple loudspeaker circuits are run in a common conduit or tray without twisting each pair it is actually quite common to hear audible cross talk between circuits.
This also confuses Common mode and Normal mode impedances. A typical active balanced line input might have a normal mode input impedance of 10,000 ohms and a common mode input impedance of 5,000 ohms. The desired sensitivity to only normal mode signals with complete rejection of common mode signals is not perfect. The resulting Common Mode Rejection Ratio (CMRR) may be as low as 20 to 30 dB.
By contrast the loudspeaker has a normal mode input impedance of 8 ohms, but a common mode input impedance of 10,000,000 ohms or higher. It also is almost perfectly sensitive to only normal mode signals. As a result the CMRR is extremely high on the order of 100 dB.
Note that even with a well balanced interface such as the typical power amp to loudspeaker, if normal mode coupling happens in the wiring, for example because of not twisting pairs and randomly placing circuits in a conduit, the loudspeaker can still pickup and reproduce audible cross-talk from other circuits.
Another reason for the use of twisted pair loudspeaker wiring is the reduction of the amount of RF picked up by the wiring (the antenna) seen the the power amplifier output (the receiver). rr