Audio Engineering

Valve (Tube-Based) Amplifi ers 357

arrangement, because this simply adds just another increment of waveform distortion, due
toV 2 , to that contributed by V 1.

A much more satisfactory arrangement is that shown in Figure 11.11 , in which V 2 is
operated as an anode follower, which, like the cathode follower, employs 100% negative
feedback, although in this case derived from the anode. This stage contributes very little
waveform distortion. Also, because both valves operate as normal amplifi er stages, the
available voltage from either output point will be largely unaffected by the operation of
the circuit. An additional advantage over the circuit shown in Figure 11.10 is that the two
antiphase output voltages are equal in magnitude, without the need to adjust the preset
gain control, RV1.

Another satisfactory circuit is that based on the long-tailed pair layout, in which,
provided that the tail resistor is large in relation to the cathode source resistance (1/ gm ),
the two antiphase anode currents will be closely similar in magnitude. The advantage
of this circuit is that it can be direct coupled (i.e., without the need for a DC blocking
coupling capacitor) to the output of the preceding stage, which minimizes circuit phase
shifts, especially at the LF end of the passband. By comparison with the two preceding

0V

Input

Output 2

V 2

1/26SN7

V 1

1/26SN7

C 3

0.05μF

C 5

25 μF

C 2

25 μF

Output 1

Vcc

C 1

C 4

C 6

R 4 R 6

220 K 220 K

R 3

27 K

R 7

27 K

R 8

470 R

R 2

470 R R1M0^5

R 1

1M0

0.1μF

0.05μF

0.05μF

0V

Figure 11.11 : A fl oating paraphase circuit.