The circuit below eliminates two coupling capacitors. Both the top and bottom triodes directly connect to the phase splitter. How the top triode manages to forgo the need for a coupling capacitor is fairly obvious: just use a low valued plate resistor in the spilt-load phase splitter so as not to provide too-much negative bias for the top triode. But at the onset, the bottom triode sees a positive voltage, making the coupling capacitor seemingly absolutely necessary. The trick lies in the cathode bias resistor's value. As this resistor is bypassed and does not influence the AC functioning of the triode, it little matters to the triode whether the resistor is 200 or 400-ohms in value as long as the correct cathode-to-grid voltage is established to set the desired idle current through the triode. 

      Thus, this amplifier might be an excellent stopping point for those seeking that single-ended sound in a line amplifier that can handle low-impedance loads and patch cords heavily laden with capacitance.

      Lowering both the distortion and the output impedance can easily be accomplished by wrapping a feedback loop from the output to the input. Basically, the degree to which the gain is reduced, is the degree to which the distortion, noise, and output impedance are reduced. In this example the open loop gain is about half of the triode's mu (33), 16 or so. The ratio between the two feedback resistors roughly sets the closed-loop gain to about 3; thus the output impedance and distortion should fall to a fifth of the open values, which might be low enough to successfully drive 300-ohm headphones such as made by Beyer and Sennheiser. Note that the gain comes primarily from the output tubes and the phase splitter tube can lose gain but not provide any gain to this amplifier.
    Adding an additional gain stage to the front of the amplifier will increase the gain, but not lower the output impedance, unless the feedback looped is stretched far enough to encompass the newly added input stage.

     The above circuit's output stage functions as two grounded-cathode amplifiers in parallel with the load impedance. Consequently, this amplifier provides voltage gain and an output impedance equal to r_p divided by 2 in parallel with 10k (1300-ohms in the case of the 6DJ8). The interesting feature of this circuit is that while it has a low distortion output stage, its input stage is not so clean, as it exhibits a good deal of second harmonic distortion, due to its large current swings as it swings just a few volts.

< PREVIOUS

NEXT >