[Boatanchors] Hii FI amp as speech amp

[Brian Clarke]

Let's be a bit more careful here, Bob, W5UQ.

The info Roger, AI4NI gave is only partly correct - it is NOT an excellent primer.

Roger's advice to go look up a transformer book is correct - for both of you.

He also got the Voltage ratio correct. The Voltage ratio is the turns ratio.

However, the impedance ratio is equal to the turns ratio squared. So, a mains transformer designed for 120 Vac input and 12 Vac output has an impedance ratio of 100:1 . Further, feeding the 12 Vac winding as the primary may not work so well, as the 120 Vac winding was designed to cover the copper and iron losses. In a small transformer - up to 100 W, say - this could be 5 to 7% of the input power. It really is far better to get a transformer designed for the job.

Then Roger said:
> The next thing is power 50 watts at 120 volts = 2.4 amps. 
Excuse me? 120 Vac at 2.4 A = 288 W. In the next sentence, he clarifies that the 2.4 A is in a 12 V winding. 12 V at 2.4 A = 29 W. Let us be very clear here: W = V x I. It is not true that 120 V / 50 W = 2.4 A - the arithmetic is correct but the formula is not.

After that, I stopped reading. If a person can get so much wrong in the first couple of paragraphs, what else is wrong?? Perhaps Roger should go back to his flowers?

73 de Brian, VK2GCE.
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