As component design improved, 70V systems began to achieve high-fidelity status, but there were two weak links in the chain: the step-up and step-down transformers. Good broadband transformers that could resist core saturation and distortion were expensive.
Half of this problem was solved in 1967 When Crown International introduced the DC-300. It was most likely the first high-powered low-distortion solid-state power amplifier capable of directly driving a 70V line without a step-up transformer. And in June 1987, the Macro- Tech 2400 was introduced with the capability of directly driving a 100V line. Thus, today only loudspeaker needs a transformer to step down the voltage.
APPENDIX: TRANSFORMER INSERTION LOSS
Transformers have insertion loss (power loss due mainly to resistance). This loss should be included in system calculations for precision.
Converted to a power ratio, insertion loss can be expressed as
PR = 10 (L/10)
where
PR = power ratio
L = insertion loss in dB (always a positive number).
Some transformer manufacturers compensate for insertion loss by adding extra windings. In that case, the power delivered to the loudspeaker is the rated value of the tap. The primary draws the rated power times the power ratio of the insertion loss.
In this case, you can calculate the primary impedance as follows:
Pt = Ps + L
where
Pt = total power in dBm
Ps = power to the loudspeaker in dBm
L = insertion loss in dB
or
Pt = Ps * L
where
Pt = total power in watts
Ps = power to loudspeaker in watts
L = insertion loss (as a ratio).
Then the primary impedance is calculated as follows:
Z = (70.7)2/Pt = 5000/(Ps * 10(L/10))
where
Z = primary impedance in ohms
Pt = total power in watts
Ps = power to loudspeaker in watts
L = insertion loss in dB.
Other transformer manufacturers do not compensate for insertion loss. In this case, the primary impedance matches its rating. However, the power delivered to the loudspeaker is less than the power applied, due to the insertion loss.
Ps = Ptr/L
where
Ps = power to loudspeaker in watts
Ptr = power drawn by transformer in watts
L = insertion loss (as a ratio)
To determine whether a transformer is compensated, measure the power (E2/Z) delivered to the loudspeaker when connected to 70.7 volts. If it is less than the rated power, the transformer is not compensated for insertion loss.
When making loudspeaker SPL calculations based on sensitivity ratings, subtract the insertion loss in dB from the loudspeaker sensitivity rating (if the transformer is not compensated for insertion loss). In transformers that compensate for insertion loss, the speaker receives the power indicated. Consequently, each transformer draws a little more power from the line than is indicated. The final impedance will be too low if you add power equal to the amplifier power.
With non-compensated transformers, the labeled power is not the power received, so the loudspeaker SPL will be lower than calculated. The impedance will read correctly, but the acoustic output will be lower than expected.
APPENDIX: LINE LOSS
See the line loss calculator here.
REFERENCES
Daniels, Drew. Notes on 70-Volt and Distributed System Presentation, db, March/April 1988.
Davis, Don. Sound System Engineering, 2nd Ed., Indianapolis, Howard W. Sams Co., 1987, pp. 85-87, 402- 405. 138905-1 10-05
This article provided by Crown Audio.