Dynacord Stereo System manual Appendix a Distributed Audio Systems a Primer

Appendix A: Distributed Audio Systems – A Primer

The technique of constant-voltage (CV) loudspeaker distribution has been a standard audio practice since the earliest days of sound contracting.

Why Is It Called “Constant Voltage”?

You may have heard a distributed speaker system referred to as a “70-volt constant voltage system.” Does this mean that there is a constant AC or DC voltage of 70 volts always going through the speaker line? No, it doesn't.

I've heard that the term goes back to early telephone systems. Audio engineers of that time were concerned with how the voltage arriving at the receiving device varied from the voltage sent out by the sending device, and how the voltage transfer would vary in conjunction with changes in the impedance of the receiving device. In distributed speaker systems, where the impedance of the receiving device (in this case, the transformer) is very high relative to the impedance of the sending device (in this case, the power amplifier), then the receiving device receives the same voltage regardless of the impedance of the receiving device (within reason). For example, if the amplifier is putting out a sine wave of 70 volts RMS, then the full 70 volts goes across the primary of the transformer whether you've connected to a 5-watt tap (which is 1,000 ohms) or to a 50-watt tap (which is 100 ohms). So there is “constant voltage” transfer regardless of the impedance.

Does this mean that multiple low-impedance speakers (8 ohms) driven by an amplifier is not a “constant voltage” system? Driving a low-impedance speaker system (16 ohms, 8 ohms, 4 ohms) with a power amplifier is also a “constant voltage” system until you have too low of an impedance for the amplifier to drive. For example, a 10-volt sine wave from the amplifier driving a 16-ohm speaker will continue to be a 10-volt sine wave if you connect an 8-ohm speaker instead. The 8- ohm speaker will simply draw more current from the 10-volt signal, resulting in more power draw. The voltage stays the same but the current draw varies, which results in different power taps.

Developed specifically for distributed paging and public address systems, and adopted by the EIA, the CV method relies upon standardizing the RMS voltage level of distribution lines. The common CV voltage standards are 70-volt (technically 70.7-volt, selected because of electrical, insurance and/or building code regulations limiting the maximum voltage in unprotected speaker lines) and 25-volt.

What Are the Advantages to Constant Voltage Systems?

CV systems permit flexible connection of loudspeakers across the distribution line, much like connecting a light bulb across a power line. Transformers are utilized at each loudspeaker to regulate the proportion of the total amplifier power that the speaker “sees” (which determines the maximum sound pressure level in the corresponding zone). This results in a very reliable system, as long as the net power demanded by each branch does not exceed the rated power of the driving amplifier. Even if one or more speakers become disconnected or fail, the power applied to the others remains the same.

Constant-voltage distribution greatly simplifies the calculations involved in designing a background music or paging system. Within the amplifier’s power limit, speakers can be freely added to (or subtracted from) the system without the relatively tedious recalculation of total load impedance. Moreover, the CV method allows a single amplifier to drive many speakers without resorting to series/parallel connection techniques: all connections are made in parallel with the line.

CV systems require that the output voltage be constant over a relatively wide range of load impedances, to a practical minimum limited by the output capability of the amplifier. Early direct- coupled tube amplifiers rarely met this requirement (or had output voltages that were far too high), and so tube amplifiers for CV systems once employed integral output transformers. Virtually all modem professional transistor amplifiers deliver an output voltage that is essentially independent of load, however, making contemporary CV systems much easier to implement.

ElectroVoice/Dynacord BGM Guide Page 37