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Old 15th July 2017, 06:24 PM   #2601
needtubes is offline needtubes  United States
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For the lower FET, I'd think you would want to vary Vgs for minimal DC offset on each output terminal. How much you need will depend on the current you chose through the CCS.
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Old 16th July 2017, 05:40 PM   #2602
metalman is offline metalman  Canada
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Originally Posted by Rodeodave View Post
Also, about the McMillan resistors (R46/R47), is there any sense in trying to omit them? Or are they most definitely needed to insure low and steady DC offset at the outputs?
The McMillan resistors are there to reduce (not eliminate) absolute offset. It will have no impact on relative offset. How much they are needed depends on how closely matched everything is, including resistors etc... Better matching yields reduced offset, both relative and absolute. I went to great pains to match everything, and it had real benefits. At turn on, without the McMillan resistors I get an absolute offset of 7 volts that quickly drops below 1 volts after a few minutes, and stabilizes less than 0.1V afte 30 minutes. With the 10k McMillan resistors the absolute offset starts at 3-4 volts, drops below 0.7 volts within a minute, and stabilizes at less tha 0.1V after 30 minutes. With 10k resistors the audible difference is pretty negligible, if you really listen carefully you can make out a slight difference. In the end I took them out and run without the, as I have my mosfets heavily heatsinked and don't have to worry about thermally overloading them during warm up.
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