Dynacord Stereo System manual Use of Autotransformers

The use of a 75 watt-per-channel amplifier can also bring added benefits in certain Installations. For instance, there is often a need to drive one or two full-range high-quality loudspeakers — say, for foreground music — in addition to a CV paging line. (One could imagine such a situation in a small airport terminal that included a restaurant or bar, for example.) In such situations, combining music and paging on a single branch may impose a compromise in sound quality that will frustrate the client. The solution here is to use one channel of a high-quality 75-watt amplifier to drive the CV line, and the other to drive the foreground music system.

Use of Autotransformers

Realistically speaking, many installations will present demands, which cannot be satisfied by the techniques that we’ve discussed thus far. One such case would be a moderate-sized 70-volt music-and-paging system which comprises two separate branches, each of which requires approximately 100 watts from the power amplifier.

In a small application such as this, which clearly does not demand anywhere near the power capability of two bridged 150-watt amplifiers, (one per 70-volt branch) must we go back to using dual-winding step-up transformers in order to match a lower-powered amplifier to the CV lines? Not necessarily.

Consider that the DC isolation afforded by a dual-winding transformer could easily be waived if we employ a professional solid-state amplifier, which incorporates protection against output offsets (as most do nowadays). This opens up the possibility of using an autotransformer to boost the output voltage of the amplifier.

Autotransformers offer a significant advantage over dual-winding transformers in that they impose far less insertion loss (chiefly because of tighter magnetic coupling and lower coil resistance). This translates into far more efficient power transfer.

Remember that insertion loss must be factored into all CV design calculations: power transfer is the name of the game here. An insertion loss of only 1dB corresponds to a power loss of over 20%, and an insertion loss of 3 dB cuts the available amplifier power in half!

Such losses directly affect the number of speakers that can be connected to a CV line. The ElectroVoice Model AT100 is a wide-range autotransformer designed specifically for use in high- quality distributed systems. The AT100 features multiple taps at specified impedance ratios, which were chosen, for maximum utility in CV system implementations. Rated at 100 watts long- term average power capacity, the AT100 can operate with very low distortion at peak levels far exceeding 100 watts, and may be used in CV applications either in step-up (amplifier output matching) or step-down (loudspeaker matching) mode.

Applying the AT100 to a typical installation, we could connect each output of a power amplifier with a 4-ohm power rating of 100 watts to the brown and black terminals of an AT100. The 70-volt lines would then be connected across the respective yellow and black terminals (common carries through). Operating the power amplifier at the equivalent of a 4-ohm load utilizes all of its available power, for further cost efficiency. The extremely low insertion loss of the AT100 allows virtually all of that power to be transferred to the line, maximizing the capabilities of the system.

ElectroVoice/Dynacord BGM Guide Page 39