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There's no active stage like almost no active stage at all

Do a search of the tweakers asylum, lots of stuff along these lines.
My ever humble observation is that in DIY land where one does not need to be concerned about interfacing with any equipment other than one's own, one usually does not need *any* active circuitry for i/v and filtering.
Most DAC chips with a bipolar current [ zero at midpoint ] output pin can feed a 50 ohm or so resistor terminating the input of a 1:20 or 1:30 transformer. I've had good results with surplus iron, utc 50 ohm to 50 kohm. There are issues around the higher ouput impedance, but not insurmountable ones.
For the DAC chips that have a positive unipolar current [ one half of peak current at midpoint ] output pin, BB PCM 63, Signetics/Philips 1541 [ and LT 1595/6 with a negative vref ] one can stretch the definition of "in the signal path" slightly and cascode the current output, preferrablly adding a feedback loop to keep the voltage at the current output pin close to zero. Then one can feed an arbitrarily high value load resistor and/or transformer for as much voltage as one wants. The circuit, again, is simple: DAC current out pin to the source pin of an nfet/nmosfet and the minus [ inverting ] input of a fet input opamp, such as BB 604/627; output of the opamp through a 100 ohm padding resistor to the gate of the nfet/nmosfet; plus [ non inverting ] input of the opamp to ground; drain of the nfet/mosfet to through the load resistor and or transformer to a separate well regulated positive supply of +12, +15, +30 volts, or more dedending on ones desired output voltage/impedance. Make sure the nfet/nmosfet can handle twice the supply voltage, as a transformer can swing above it. Optionally, add a small value cap across the resistor and/or transformer for additional high frequency filtering/damping. A reasonably generic single ended tube interstage transformer will work, or if one is iron-phobic, one can capacitor couple, referencing the positive supply as a floating ground. But, the transformer is preferrable, providing ground isolation and high frequency filtering, and is particularly good for 1x non-oversampled implementations using the 1541. Of course, one also needs to pay attention to the clock, clean with registering/reclocking at the DAC chip is a good thing.

Watching the power meter spin backwards now that the morning fog has lifted,


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  • There's no active stage like almost no active stage at all - wildmonkeysects 12:54:46 06/10/01 (0)

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