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RE: CCS set up for 1620 or 6SJ7

We are used to working with voltage sources. To refresh, a voltage source in an ideal sense has zero output impedance. A real world voltage source can be seen as an ideal voltage source in series with a resistor. Then there is the current source. The ideal current source has infinite impedance. As it happens, if you put an ideal current source in parallel with a resistor in a "black box" you cannot tell the difference between that and a voltage source with series resistance. This is called a Norton equivalent. Why do I bring this up? Turns out you can do something useful by loading a high impedance device like a pentode with a CCS.

Kevin Carter over at K&K Audio uses this idea to good effect with the input stages of his phono preamps. The input stage is a MOSFET/tube cascode. In many ways a cascode is like a pentode. The grid of the upper tube is held at a constant voltage and acts like a screen grid. The gain and plate resistance are both very high. Gain is approximately gm(lower device)*R(load). Plate resistance is in the megohms. What happens when you use a CCS plate load? Gain increases and output resistance which had been ~R(load) goes way up. What we have now is a current source or transconductance amplifier. This amplifier in parallel with a resistor is equivalent to a voltage source amplifier in series with a resistor. By adding a resistor across the output of our current source amp we provide an "escape" for the two current sources so that is not an issue.

Why is this useful for a phono pre? The RIAA filter calls for a series resistance. Putting a large value series resistor in the signal path affects the sound. Could be one reason why LCR filters are popular. With the current source input stage this series resistance becomes a shunt resistance.

Having heard the difference between resistor plate loads and series RIAA resistance and CCS loads with shunt RIAA resistance in the same circuit my ears tell me the one with shunt resistance sounds more dynamic, more lively and with better touch.


"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln


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