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What I have experiemented with

Posted by Gordon Rankin on July 11, 2006 at 07:03:36:
Gang,
There are basically 2 types of current dacs. The ones with +/- supplies where the iout = 0v in steady state or the single supply dacs (i.e. TDA1543, 1545) that have the iout = some voltage depending on VCC and their internal current sources.
I have been looking at the +/- ones because some of the better 24 bit dacs use these. I looked at this schematic and don't really see much difference between that one and just connecting a 100 ohm to ground then adding gain & output. The battery is used to bias the 100 ohms negative to the dac which should put the cathode at like 0v. The 100 ohms would also bias the tube depending on the plate voltage and current. The change in current both throught the 100 would change the bias and therefore the signal at the plate. Problem is these dacs cannot drive the 100 ohm resistor. Most of the output impedances are like 500 to 1000 ohms. So driving the 100 would be an issue.
Remember these dacs where created to drive the - input of an opamp. Which by itself is a high impedance input. BUT when adding the feedback resistor from the output of the opamp back to the - input (i.e the output looks like ground), thereby setting the imput impedance the dac is seeing is basically the feedback resistor to ground.
BUT... remember this is an active part. So preserving that can help with dynamics and such.
My ideas have run that the cathode should equal 0 volts. Therefore the grid will have to be sourced from the negative supply. A single pot from - suppky to gnd with the wiper bypassed with a cap to the grid should work fine for experimentation. I then placed a low noise current source from - to the cathod and used either a choke on the plate or a resistor to B+.
Remember common grid retains phase so this stage will be out of phase coming off the plate.
Thanks
Gordon

J. Gordon Rankin