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Original Message

Re: Not so hidden advertising

Posted by Steve Eddy on January 25, 2001 at 18:24:40:

e.lektronick wrote:

Well, actually, I really don't have any evidence that points to that. But I am curious why Jensen didn't wind the transformer for balanced secondary operation as an option. A center tap would have been welcomed by lots of DIY'ers with a bent for balancing the whole shebang from input to output.

Ok, I looked at the application note a bit more closely and realized why the circuit was done with two transformers instead of one. It wasn't for "effective" center tapped output, it was for the VOLTAGE GAIN.

The 115K-E is a 1:10 step up transformer. The circuit in the application note results in a 1:20 step up. So you get a voltage gain of 20 while retaining the performance of the 115K-E. All else being equal, when you increase the voltage gain of the transformer, you limit its low frequency performance with respect to maximum input level.

For example, compare the 115K-E to the 13K7-A, which is a 1:5 step up. The maximum 20 Hz input level for the 115K-E (the point at which THD reaches 1%) is -2.5dBu. The maximum 20 Hz input level for the 13K7-A is +7dBu. So the point of the circuit in the application note is to end up with a 1:20 step up transformer with the performance of their 1:10 step up.