Non-Fixed Current Sources
   Another name for a current source is "current regulator." Current sources provide a constant, fixed current draw. For many applications this is just what is required. But for many audio applications all that is really needed is a very high impedance and not a fixed current draw. In many respects, this is what a choke (inductor) does. As long as it has not saturated and the frequency is high enough to be covered by the inductance working into the load, the inductor presents a high impedance at any amount of current draw. How do we duplicate this function with solid-state circuitry?
   The answer lies in forgoing the voltage reference in the construction of the current source, as without the constant voltage, the current source cannot maintain a constant current flow. AC coupling the output to the input of the current source ensures a high output impedance.

  In practice, the capacitance across the MOSFET will probably be the more detrimental to high rp tube's performance. (The real issue here is current. Low-rp tubes are usually run under higher idle current than high-rp tubes and it takes current to overcome capacitance. Current is to pushing through capacitance as heat is to cutting through cold butter; more is better.)
  A further advantage to this circuit is that it works to maintain a fairly constant output voltage. If we strive to keep down the raw B+ voltage and get away with lower voltage parts, then voltage headroom becomes a real concern. For example, a 170 volt B+ and a 12AX7 with 150 volts on its plate only leaves us with 20 volts of headroom, some of which must be used to encompass the output signal. The voltage across a current source expands and contracts to achieve the desired current flow. Thus loading this 12AX7 with a current source may pull the plate voltage up to 165 volts to make the current source's defined current flow. In contrast, this circuit can absorb a doubling or halving of the idle current with only a +0.5 and -1 volt change in plate voltage. This feature allows using this circuit to load pentodes, which seldom work with current source loading as idle current flow through a pentode is largely independent of its plate voltage.

Current Mirrors
   Solid-state circuits employ current mirrors to maintain equal dynamic in-phase current flows through two separate current paths.

Faux current source that represents a high impedance but not a fixed current flow

   In the circuit above, the MOSFET's gate sees a fixed DC voltage and a fluctuating AC voltage (the audio signal being reproduced). The triode sees its plate working into the ultra high impedance presented by the MOSFET in parallel with the 1 meg resistor via the 0.1 µF capacitor. If the tube has a low rp, such as a 6DJ8, 5687, or a 6BX7, then the 1 meg load is effectively close to infinity. On the other hand, a high rp tube, such as the 12AX7, will be more heavily loaded by the 1 meg impedance.

   The circuitry is wonderfully simple, although puzzling to many. Two transistors and two resistors is all it takes to make a current mirror. As more current flows out of one transistor's collector, the other transistor's collector increases in conduction to match the first.

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