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Hi, guys.
I've been toying with the idea of designing a fully balanced tube mic preamp with a transformer input stage. I have seen the schematics on the Jensen website:
http://www.jensentransformers.com/as/as047.pdf (Two JT-115K-E used in symmetrical mic input stage)What I don't understand is why you couldn't use a single input transformer and couple the two outputs to each of the two grids of a differential pair of tubes? The second Jensen schematic shows *two* transformers, one for each tube in a differential pair. Why?
My only suspicion is that the transformer outputs to the grids need to be ground referenced, but that could be taken care of by using 1% matched resistors to ground on each line, right? I've seen it done on non center tapped heater windings of power transformers, why not on input transformers?
Is it simply a case of Jensen wanting to sell twice the # of transformers?
Thanks for any insights you might have.
Sincerely,
-Erik.
Follow Ups:
e.lectronick wrote:What I don't understand is why you couldn't use a single input transformer and couple the two outputs to each of the two grids of a differential pair of tubes? The second Jensen schematic shows *two* transformers, one for each tube in a differential pair. Why?
My only suspicion is that the transformer outputs to the grids need to be ground referenced, but that could be taken care of by using 1% matched resistors to ground on each line, right? I've seen it done on non center tapped heater windings of power transformers, why not on input transformers?
Is it simply a case of Jensen wanting to sell twice the # of transformers?
Thanks for any insights you might have.Well, an audio input transformer and a power transformer are worlds apart. I know that Jensen uses special winding techniques to effectively produce a smooth Bessel high frequency rolloff to avoid phase distortion (or Deviation from Linear Phase as Jensen would say) and that may preclude getting a symmetrical output using a simple resistor voltage divider.
Your best bet would be to ask Bill Whitlock (president of Jensen Transformers) who wrote most of the ASxxx application notes and most likely the one you're referring to. You can reach him via EMail at bill.whitlock@verizon.net .
Whatever the reason I'm pretty sure it's not simply a matter of selling more transformers.
se
Oh I would not be so sure about that last sentence....
Thomas wrote:Oh I would not be so sure about that last sentence....
Well if you've any evidence that the circuit in the application note used two transformers instead of one to sell more transformers, I'm all ears.
And what's with the "Not so hidden advertising" topic? What not so hidden advertising are you referring to?
se
The more transformers he sells, the more money he makes. Thus, lots of iron in his designs. And why not?
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.
-Erik.
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.
se
OK, I get it. So, If I'm not worried about the voltage gain so much, I can make it up in the gain stage and use a single transformer. Cool.
Thanks. I'm starting to grasp it all, now.
Know any good sources of info that deal exclusively with input transformers? Besides the Jensen site, of course.
-Erik.
e.lektronick wrote:OK, I get it. So, If I'm not worried about the voltage gain so much, I can make it up in the gain stage and use a single transformer. Cool.
Yes. Though I'd still talk to Jensen about using the output of that particular transformer to drive both halves of a differential input.
Thanks. I'm starting to grasp it all, now.
Know any good sources of info that deal exclusively with input transformers? Besides the Jensen site, of course.Can't think of any general source offhand. But you might want to check out the web sites for both Lundahl and Sowter .
Good luck!
se
E-mail Bill Whitlock at Jensen.
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