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RE: BDT Preamp Schematics

Posted by John Swenson on May 17, 2008 at 00:57:11:
You can certainly try this with the Sowter, nothing stopping you from doing so. Just carefully work out the load impedance based on the gain you need and the ratio.

Since the circuits impedance is very high there is no specific impedance you have to aim for, use whatever gives you the gain you want given the transconductance of 1mA/V.

Say you want a gain of 5, that means a load of 5K if it was a 1:1, but with the 6:1 you want it 6 times that or 30K. Thus the load impedance is 30K/36 = 833 ohms, pretty close to what the transformer was designed for. If you want more gain increase the load resistor, for less decrease the load resistor. Just make SURE you put an explicit load resistor across the secondary, that is what determines the impedance the circuit sees. If you don't put one in there the load will be the 50K impedance of the power amp which will be X 36 from the transformer which is 1.8 megaohm, which is a gain of 1800 for the tube / 6 = 300 for the whole shebang. I think that would kind of overload things! (and probably oscillate all over the place)

The only issue with a 6:1 is to make sure you have enough voltage for the PP swing you are going to need. With the voltages as speced that comes out to about 100V PP maximum across th primary. With a 30X gain thats a maximum input signal of 3.3V PP for full output. That might be fine, just be aware of it.

John S.