What Happens When the Power Goes Off?

Theory and Practice – By Dale Shirk

Dale Shirk answers this question, “So what happens to a signal when it is driven into an unpowered input?”

Today’s audio electronics are easy to interface, with one or several high impedance inputs bridging a low impedance output. However those impedance characteristics that we know and love disappear when the power goes off.

For various reasons we sometimes deliberately or inadvertently have signal going to a device which has it’s power turned off. Generally, this does not cause any circuit damage to either unit.

However sometimes we may have a hard-wire split where one output is driving several inputs, one of which is powered off, while another is powered on and passing signal. A few typical scenarios are;

• * The console’s channel insert points are used as access for multitrack recording, either directly into the recorder or to another console. However when recording is not needed the recorder or studio is powered off.
• * Several power amplifiers cover multiple similar areas and are driven by a single processor output. Some amplifiers are switched off for testing or when those areas are not occupied. * A single Aux output of a console drives numerous effects units. The ones not being used are switched off.

In these cases a Distribution Amplifier (DA) really isn’t needed. There are many more cases, such as distributing audio to multiple outlying areas, where a DA would be the preferred way to do it, but a hardwire split is used instead. These can also easily have an unpowered input paralleling active circuitry.

So what happens to a signal when it is driven into an unpowered input? All together now; “It Depends!”

Input circuitry can vary widely, but typically there are a few passive components, then an op-amp or other IC chip. The IC chip will usually have internal input protection which consists of diodes going to the power supply rails to prevent input voltages from exceeding the power supply. Sometimes these are added to the circuit externally. When the power is off, the power supply is at zero volts, so the audio is shunted to ground through a diode. Fig.1 shows the pertinent components of a typical powered off input. There may be other components such as RF filters or resistors to ground that do not adversely effect performance.

Obviously the value of the input resistors has a huge effect on the circuit load it will present. Even with a high value resistor, the load will be non-linear and create some distortion. With a circuit configuration designers call “Instrumentation amp” the resistors can and will be very small. In these cases the power-off load can be very heavy once the voltage exceeds the diode junction voltage of about 0.7 volts. This will allow small signals to pass with little distortion, but larger signals will be heavily distorted. Fig. 2 shows distortion vs. level for a typical console output when signal is driven into several different unpowered units.

The obvious conclusion is that the power switch may not be an acceptable mute switch if other devices are connected to the same signal line.

The second issue we need to look at is what happens at the output end when a unit is turned off and power stays on for other units after it in the signal flow. Obviously there may be switching transients that occur, accompanied by pops, groans, squeaks and even flatulence.

Once the power supply voltage has drained away completely, however, the output circuit looks similar to powered off input circuit, usually with a very small series resistor, 100 Ohms or less. This output impedance is non-linear of course and is very high, nearly open, for small signals below 0.7 volts. For larger signals it effectively becomes diodes to ground.

We do not generally want to drive other signals into an output, so we won’t have the problem of it causing distortion. However some portion of the noise pickup immunity of an interconnect is because of the low output impedance at the driving end. When this unit it powered off it no longer has low output impedance and the interconnect is very susceptible to picking up hum and buzz.

Transformers on inputs and outputs may change the impedances somewhat, but will not make the non-linearity go away.

Although we rarely use them these days, vacuum tube input and output circuits generally go effectively open when powered off. A cold vacuum tube will not conduct normal voltages except that of a small amount of inter-electrode capacitance.

So in summary, we need to be careful about powering off portions of our system. Power sequencers are recommended for proper start up and shut down. If signals are split out to different units with hard-wire splits, all units involved should remain powered. Use gain controls or other means to mute the audio. Muting with the power switch should only be done when audio connections are one-to-one. ds