What About Insertion Loss?
Insertion loss seems to be a mandatory specification for distribution transformers, although we find it difficult to understand why. Losses in loudspeakers themselves totally dominate most audio systems, and audio power is easy to find and cheap to buy.
It is almost impossible to overheat even very lossy distribution transformers with audio program material: there is just too much iron, copper, and surface area in a transformer that is large enough to couple at low frequencies, and average audio power is so much lower than the peaks that the transformer must accommodate.
Finally, the winding resistance that causes insertion loss can be a desirable feature, since it improves the transformer’s tolerance for DC offsets.
NOTE: Insertion loss is not useful as distribution transformer specification. Instead, focus on the a transformer’s ability to deliver power at the highest and lowest required frequencies.
Transformers at the Loudspeaker End
On the secondary side of the power amplifier distribution transformer, we connect HV loudspeakers in parallel (each with its own step-down transformer), and adjust power taps as required to achieve required loudness in each area.
System designers usually specify HV loudspeakers with integral transformers, so you might suppose that the drivers and transformers are well-matched, so why worry? It turns out that ten little 10 watt transformers in parallel can be just as troublesome as one big 100 watt transformer.
The transformer characteristics discussed above apply equally at the loudspeaker end.
Loudspeaker Step-down Transformers and Saturation
The transformers at the loudspeaker end are prone to saturation, just like their big cousins at the amplifier end. Imagine the effect of 10 or more transformers all reaching voltage saturation in parallel and at the same time.
Moreover, since saturation depends only on voltage, each loudspeaker transformer should be able to withstand full HV voltage levels at the lowest system frequency, regardless of the amount of power allocated to it. This is why most HV loudspeaker modules include a series capacitor to reduce the likelihood of transformer saturation.
Note, however, that these capacitors are perfectly suited to carrying saturation current spikes once those spikes begin to flow.
Is Impedance Matching Important?
For best results, adjust loudspeaker taps so that they sum to either the power amplifier output rating or to the power amplifier distribution transformer rating, whichever is smaller. This will present the proper rated load impedance to the power amplifier, allowing it to develop rated power, while minimizing saturation effects in the transformer at low frequencies.
If additional loudspeakers are added or power tap settings are increased beyond the amplifier or transformer rating, load impedance for the power amplifier will go down proportionately. However, a smaller but still linear load impedance is usually the preferred alternative to a highly nonlinear saturating transformer.
To Isolate or Not to Isolate?
Distribution transformers are available in both autoformer (non-isolating) and transformer (isolating configurations), so you have the option of isolating the loudspeaker wiring from the amplifier wiring. Here’s a couple of reasons why you might want to.
Safety Isolation
Distribution transformers can provide an additional barrier between lethal mains power potentials and accessible loudspeaker terminals. HV lines from isolating transformers have no potential relationship with respect to earth ground, so that shocks are unlikely except in the event of contact with both lines at once. Some electrical inspectors require that HV lines be isolated from power amplifier outputs.
Isolation to Break Ground Loops
For the many types of audio equipment that have shielding potentials connected to safety ground, non-isolated HV loudspeaker lines occasionally provide a return path for ground loops with enormous extents. In this era of plastic loudspeaker enclosures, this is becoming more rare.
Summary
1. Selecting a Distribution Transformer at the Power Amp End:
a) Select proper turns ratio to boost to HV system level and to present an acceptable impedance to the power amplifier. You may have to derive turns ratio from other specs.
b) Select for adequate low frequency voltage capability to prevent saturation for good low frequency power transfer. You may have to make your own measurements.
c) Verify sufficiently low leakage inductance and low core losses for good high frequency power transfer. It’s usually OK to use datasheet information.
2. Transformers at the Loudspeaker End:
a) Suffer from the same problems and can equally become the weak link in a system.
b) Are often “matched” to and included with loudspeaker modules, so be aware of inherent characteristics.
3. Toroids versus E-cores: details are more important than shape.
4. Transformer isolation: can improve safety and eliminate ground loops.
Provided by Rane.
