What does “Constant Voltage” Mean?

Constant voltage is one of the more confusing terms used in the audio industry. Pat Brown brings clarification to this term.

Those new to the audio field are often dismayed by the subjective nature of many audio terms, which may mean different things in different contexts. Here are some examples:

“Bridge”

  • a power amplifier operating mode, where two channels are essentially placed in series to achieve more voltage swing. “Let’s run the amplifier bridged to get more output voltage.”
  • the connection of a high impedance load in parallel across a component output. “Bridge your voltmeter across the loudspeaker line to check the voltage.”
  • a computer network data switch. “We need a new network bridge in the office.”

“Terminate”

  • to solder or crimp a connector onto a cable. “The wiring is pulled, but it still needs to be terminated.”
  • to impedance match a source to a load. “That old passive EQ needs to be terminated to function properly.”
  • to end a computer program or routine. “Terminate the program and let’s try a reboot.”

“Constant Voltage”

One of the more confusing terms is “constant voltage” or CV. What it does not mean is what the name implies, that the voltage, whatever its value, is unchanged over time. So much for a literal interpretation.

The Electrical Engineering Definition

The electrical engineering definition of constant voltage is with regard to how a power source drives a load. If the load has a very high impedance relative to the source impedance, the voltage developed across the load is independent of the load impedance value. For example, if my 100 ohm output mixer is driving a 10 kΩ load, the interface is optimized for voltage transfer, and practically all of the source voltage is delivered to the load. This is the objective of any analog audio interface. If the load impedance were increased to 20 kΩ or 30 kΩ, there would be no change in the signal level (the voltage) at the input. If the source impedance were decreased to 50 ohms or increased to 200 ohms, there would be no change in the signal level to the load. A large impedance mismatch between source and load creates a voltage-optimized signal transfer, that makes the specific source and load impedance values irrelevant with regard to signal transfer.

In contrast, in an impedance-matched interface the source voltage is heavily influenced by the presence of the load, dropping to one-half of its open circuit value when the load is connected.

An Everyday Example

An example of a CV interface is utility power distribution, e.g. a household electrical circuit. You can connect as many devices as allowed by the circuit breaker current rating, but the voltage is unaffected. It’s not CV because the voltage is always 120 VAC, it’s CV because the 120 VAC does not drop as additional appliances are added to the circuit.

Fig. 1 - Household utility power wiring uses a CV interface.

Fig. 1 – Household utility power wiring uses a CV interface.

To generalize, outputs are low impedance and inputs are high impedance, and when this is true the voltage across the load is the same, no matter what the actual impedance values are, hence “constant” voltage. This convention greatly simplifies the analog audio interfaces. In most cases, you just connect an output to an input with the appropriate cable and get on with your life.

I always like to add that this is a low frequency practice (<20 kHz). In high frequency interfaces (video, digital, RF) impedance matching (termination) must be used to maintain the integrity of the signal.

Fig. 2 - A constant voltage interface connects and audio output to an input.

Fig. 2 – A constant voltage interface connects and audio output to an input. Additional loads could be “bridged” across the output to receive the same signal, so long as the total impedance doesn’t excessively load the line (cause the signal voltage to drop).

The Audio Industry Definition

The typical audio industry meaning of constant voltage is with regard to an amplifier driving a loudspeaker or loudspeakers through step-down transformers. The amplifier is operated at its full voltage swing, and a transformer “steps down” this voltage before applying it to the loudspeaker. The amplifier’s output voltage can be anything, but it is usually fixed at a standard value, such as 25 V, 70.7 V, or 100 V. These are RMS voltage for a reference sine wave from the amplifier. Of course the actual VRMS varies wildly depending on the program material, and in most applications it is not like the sine wave that is used to rate the system. This is an effective way to drive multiple loudspeakers from a single amplifier, such as in a meeting room. Someone at some time decided to call this a “constant voltage” distribution system and unfortunately the name stuck.

Fig. 2 - A "constant voltage" distribution system utilizing step-down transformers.

Fig. 2 – A “constant voltage” distribution system utilizing step-down transformers. The output voltage of the step-up transformer is typically 25, 70.7, or 100 Vrms (sine wave rated).

To add to the confusion, a “constant voltage” audio distribution system is “constant voltage” with regard to the amplifier-to-load voltage (the electrical engineering definition given above). In other words, the voltage on the line is independent of the total load impedance. Otherwise the sound level from each loudspeaker would change when others are “bridged” across the line.

At SynAudCon we prefer to call this as a “transformer-distributed loudspeaker system” rather than a constant voltage system, but the latter term will most certainly live on and continue to cause confusion.

Voltage, Not Power…

In both the electrical engineering and the audio industry definitions, the voltage is the parameter of interest. The interface is designed so that the signal voltage driving the amplifier’s input predictably influences the loudspeaker’s frequency response magnitude at the listener. It is the signal voltage that we sculpt with our signal processors. The CV interface is necessary so that audio filters (analog or digital) placed ahead of the power amplifier have a predictable affect on the sound pressure level at the listener. If this were a power-optimized interface (impedance-matched) then this would not be the case, since the voltage to the load (and resultant loudspeaker frequency response) would be influenced by the loudspeaker’s impedance curve. Since the impedance is fixed, and current is drawn, the voltage is the only signal parameter that we can directly change by adjusting a control or applying a filter.

Yes, we are in the “voltage” business, even though the voltage is often hidden inside of a power rating. In conclusion, just remember that in a “constant voltage” system the voltage is anything but constant. More correctly, it is unaffected by changes in the load impedance.    pb