What could be more mundane than the transformers and autoformers that are the backbone of audio distribution systems?
This article will show you that there is a lot more going on with these chunks of iron and copper than you ever suspected.
Learn why transformers are often the power bottleneck in distribution systems, learn how to interpret datasheets, believe or disbelieve manufacturers’ claims, how to specify HV components, and how to set up HV systems to deliver the best possible power, fidelity, and bandwidth.
High Voltage Audio Distribution Systems
Although the term Constant Voltage is still in common use, this article adopts the less confusing High Voltage (HV) terminology.
HV systems are in widespread use for these principal reasons:
* HV systems minimize power losses in low-cost wiring.
* HV systems facilitate connection of multiple loudspeakers without careful consideration of impedance matching.
* Once an individual power adjustment on a loudspeaker has been made, the loudspeaker continually receives the same amount of power even when other loudspeakers are added or removed from the system, resulting in more constant and uniform coverage.
* Volume control by transformer tap at the loudspeaker end is more efficient than resistive pads.
Transformers at the Power Amp End
Boosting the Output Voltage
Solid state power amplifiers usually need a voltage boost to get to the 70.7 volt and 100 volt levels of most HV systems, and a transformer or autoformer will do the job. The differences between transformers and autoformers will be covered later (see the To Isolate or Not to Isolate? section).
In the meantime, the term transformer will be used to refer to both types. The transformer boosts the amplifier output by a fixed ratio, called its turns ratio. The correct transformer will provide the right amount of boost, which is simply the desired HV system voltage, e.g., 70.7 volts, divided by the amplifier full power output voltage.
Here is the basic procedure for selecting an output transformer for an HV system where the amplifier power required P has been determined using suitable methods:
1. Determine TURNS RATIO to get proper HV level.
a) Measure the unclipped rms output voltage available from the power amplifier or calculate if from Ohm’s Law:
VOUT = √P x R
For example, for an amplifier rated 100 watts at 8 Ohms:
VOUT = 28.3 Vrms
b) Calculate desired voltage boost ratio, aka turns ratio,
N = VHV / VOUT, e.g., N = 70.7 / 28.3 = 2.5
Select candidate transformers with turns ratio within 20 percent of calculated N and with datasheet power ratings similar to amplifier output wattage.
2. Determine TRANSFORMER SIZE to prevent saturation.
a) VERY IMPORTANT: Decide on the lowest system frequency fLC for good fidelity and full power delivery.
b) Find datasheet ratings or conduct tests to determine voltage tolerance of candidate transformers at fLC.
c) To qualify, a transformer must not saturate when driven with VOUT at fLC. You may or may not be able to determine this characteristic from datasheets. Read on.
The low frequency voltage capabilities of the transformer will be the primary limiting factor in system power delivery.
TURNS RATIO: Finding It on the datasheet
Turns ratio does not show up on many datasheets, but you can usually calculate it from other specifications. From the information on a transformer datasheet, find any combination of specifications that relates primary voltage, or primary power and impedance, to secondary voltage. Use these equations, based on Ohm’s Law and Joule’s Law, to calculate the missing specification.
For example, from transformer datasheet specs showing “300 watts at 4 Ohms” (amplifier/primary side) and “70.7V output” (secondary), use the first equation to calculate power amp output voltage VPA = 34.6 V. Then, use the third equation to calculate N = 2.04.
Some transformers have what their datasheets call voltage taps. For example, a transformer might have 25 volt, 35 volt, and 45 volt primary taps, along with 70.7 volt and 100 volt secondary taps. For any combination of primary and secondary taps, the effective turns ratio is simply the ratio of the secondary tap value to the primary tap value, as given in the third equation above.
Now that we have the Turns Ratio (Step 1 above), let’s look at the other most important performance determining characteristic, which is voltage capability, usually determined by transformer size and weight.
