Audio Engineering

Valve (Tube-Based) Amplifi ers 361

much in the way of exotic components, circuit complexity, or very high-quality output
impedance-matching transformers, and I have sketched out in Figure 11.14 a typical
circuit for a two-valve, 1W headphone amplifi er based on a pair of 6SN7s or equivalents.

In this design the input pair of valves acts as a fl oating paraphase phase-splitter circuit,
which provides the drive for the output valves. Since the cathode currents from the two
input valves are substantially identical, but opposite in phase, it is unnecessary to provide
a cathode bypass capacitor to avoid loss of stage gain. Also, since this cathode resistor
is common to both valves, it assists in reducing any differences between the two output
signals, as the arrangement acts, in part, as a long-tailed pair circuit such as that shown
in Figure 11.12. Since the total harmonic distortion from a push–pull pair of triodes will
probably be less than 0.5% and will decrease as the output power is reduced, provided a
reasonable quality output transformer is used, I have not included any overall NFB, which
avoids any likely instability problems. To match the output impedances of V2A and V2B
to a notional load impedance of 100 Ω , a transformer turns ratio, from total primary to
secondary, of 12:1 is required.

In more ambitious systems, in which NFB is used to improve the performance of the
amplifi er and reduce the distortion introduced by the output transformer, much more
care is needed in the design of the circuit. In particular, the phase shifts in the signal that
are introduced by the output transformer become very important if a voltage is to be
derived from its output and fed back in antiphase to the input of the amplifi er, in that to
avoid instability the total phase angle within the feedback loop must not exceed 180° at
any frequency at which the loop gain is greater than unity. This requirement can be met
by both limiting the amount of NFB that is applied, which would, of course, limit its
effectiveness, and controlling the gain/frequency characteristics of the system.

In

R 1 L 2

C 1 L 1 R 2

Out

Figure 11.15 : Equivalent circuits of idealized coupling transformer.