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In Reply to: Advice on transformer coupled input / driver posted by martyh on March 3, 2007 at 10:04:52:
Hi Marty,
The designer of this amplifier paid close attention to separating signal reference ground and power ground. The signal is applied between the grid and cathode. The input signal reference is the junction of the two 10-Ohm resistors. The 0.47uF capacitor connects the center tap of the transformer to this reference. The 47K resistor provides a DC reference for the grids. DC current through the 300-Ohm resistor provides bias for the tubes. Any output signal AC current through the 300-Ohm resistor, due to tube mis-matching or circuit imbalances, will not affect the bias because any voltage is common mode injected into the signal reference point.Who is the designer by the way?
Follow Ups:
Here is a link to more information. I’m curious though, will the small cap kill the local feedback? If so, it seems like a neat trick for getting around big bypass capacitors.
Thanks again,
Marty
Hi Marty,Indeed it does kill the feedback and is a way to get around a large electrolytic.
I first saw this on Lynn Olson's site as the Western Electric (repeater amplifier). And I see from the link that you gave, that this is exactly what it is - a Western Electric repeater amplifier, WE-42.
Dave
Cool link, lots to think about. I moved the battery bias on my transformer coupled line driver to the cathode awhile ago. Now I’m thinking of trying this topology on it. Heck, it’s on the bench for PS revisions anyway :)I revised my proposed input to be more in keeping with the spirit of the original. If it’s not too much trouble, what do you think of this?
Hi Marty,I have not used this topology in one of my designs yet and I have not thought it through and done the math to figure what the component values should be. However, since Mr Atarashi has built it, I would advise you to start with his topology and component values with the exception of the ratio of the input transformer. That should be chosen as part of your design of the gains and losses from input to output to give the desired overall gain.
Hi,Just a couple of quick comments. Do you really need so much gain (stepup) at the input? I did not do the math but it seems like you have too much gain (depending on how you hookup the 1660 ofcourse). Speaking of which, I'd take the 1660/5mA or /10mA, it will make your live that much easier trying to balance the 5687s.
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