by Rob-NYC »
Fri Oct 12, 2012 6:53 am
The use of transformers in speaker systems like this have two purposes.
1) Reduce the effect of line losses by raising the impedance of the audio line so that the line's resistance becomes inconsequential. For instance; consider a long run of very light wire say 24ga has a loop resistance of 4 ohms. If you connect a speaker of 4 ohms to it 50% of the power will be lost in the line. If a 70volt system is used and the transformer is set for 10 watts the line impedance is approx 500ohms. that same 4 ohms loop resistance in the line now has no effect.
2) Using a 70 system and transformers allow the allocation of power levels to be made at each speaker. For instance, main room speakers will be tapped to get 10watts each, speakers in smaller, quieter areas will get lower wattage. Any combination up to the maximum output of the amp may be used.
Both of these advantages are somewhat tempered by the fact that transformers do have slight insertion loss -they are not 100% efficient. Modern trans are often 90- 95%.
Transformers do also affect the frequency response. The typical 10W tran that I have long used is down 2 db @14khz when tapped to full (10W) output. Often lower taps have slightly better response but a 2db attenuation at 14KHZ is negligible for all practical commercial applications.
The Wurlitzer amps have a goofy "fader" that attenuate power to the machine speakers while it allocates more to the externals whether they are line (70v) or the external 8 ohm tap.
So, after all this blab, I suggest that if the speaker is 8 ohms or more just connect it directly to the 8 ohm external tap.
Rob/NYC
"If we believe absurdities, we shall commit atrocities" -- Voltaire