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Re: Selecting Output Transformers for WOT Line Preamp

Posted by KevinC (M) on June 28, 2005 at 20:26:23

In Reply to: Selecting Output Transformers for WOT Line Preamp posted by Snoopyma on June 27, 2005 at 11:35:37:

Any of the choices you name have enough primary inductance to make a reasonable low frequency limit and the 437A has low enough Rp to also provide a reasonable high frequency limit, so the real quesiton is how much gain do you want. The 437A has a pretty high mu, so unless you really need the gain, the LL1689 at 18:2 is probably the best choice. You will get low output impedance and good frequency response performance.

As far as the gap to specify, of course it depends on the operating point and maximum output level you choose. Many transformer users are very careful to get a transformer gapped for exactly the current that they plan to use, however, that is only necessary if you are planning to use the full output level swing. For example, if you chose the LL1660/18mA in connection alternative Q and operated your tube at 18mA of bias, you would have a gain stage capable of putting out 57V RMS at 30Hz. I have never needed that output level from a line stage, but maybe someone would. So if I operated the tube at 25mA with this transformer I would reduce my maximum output to about 30-35V RMS at 30Hz. Put that way, it doesn't seem to be such a big deal! With your tube (low Rp) it probably makes almost no real difference whether you choose 25 or 30mA appropriate gaps, unless you want really high output levels.

Kevin Carter
K&K Audio
www.kandkaudio.com

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