4. The Series Balanced Output Stage

The advantages of balanced output stages are well known: two tubes give more more power than one, and because the two output tubes are fed a control signal of inverted phase the even order harmonic distortion cancels out and the sound is superb.

Additional advantages can be had with a different type of balanced output stage, where the tubes are lined up in series for the DC current flowing through the tubes. The principle is called therefore the series balanced output stage and it has the same advantages as the push-pull principle, and a few more.

First, the need for a separate phase inverter triode is eliminated because the lower tube acts as the phase inverter for the upper tube; the cost of a triode is thus saved. Actually, a similar arrangement is also possible for a classical push-pull stage, and was applied in the Philips BX690: one output tube is driven from a tap of the output transformer. But somehow the driverless balanced arrangements are the rule for series-balanced stages and the exception for parallel balanced stages.

Second, the special (low plate voltage) tubes developed for this circuit have a low plate impedance, eliminating the need for an output transformer. This reduces cost and distortion (linear as well as non-linear) and has the possibility for using negative feedback without problems of phase shifted feedback.

Principle of the Circuit

Like a push-pull stage, the two output tubes are controlled with a signal of inverted phase, see the picture. In the absence of a control signal the current through B1 and B2 is the same (it must be because they are in series for DC). With control signals U1 and U2 applied, the DC through the two tubes is still equal, but the two signal voltages at the control grids generate in B1 and B2 two alternating signal currents of inverted phase (because the signals are of opposite phase). This pushes an AC current through the load (the speaker), which is possible because it is connected through a large capacitor.

Practical Implementation

The Diagram shows the output stage of the Philips BX998A. Observe that the triode driver B10 only supplies the contrl grid of B12, while the control grid of B11 is connected to the plate of B12 (through R64), which has the signal in inverted phase. The use of the lower tube as the phase inverter is only possible if the amplifier is driven as a `class A' amplifier. The control voltage for the upper tube is developed over a load resistance R65, reducing the output power slightly. The output stage drives the internal speaker (700 Ohms) directly, but there is an output transformer in parallel so that an external speaker of low impedance can be connected.

The cathode-filament voltage of the upper tube is about the plate voltage of each tube, and can easily be as high as 150V; this excludes the use of output tubes designed for only a small cathode-filament voltage because the insulation would break down. For this reason, Philips has made frequently made use of an EL84/UL84 mixed pair to build the circuit. The UL84 is equivalent to the EL86 (see below) output tube except for the heater, because it is designed for series string heaters and heater-cathode insulation is designed for high voltages. The very exquise Philips BX998 uses a series-balanced output stage consisting of two PL81 tubes (for the bass frequencies). The Philips B4X61A uses an EL84/EL86 pair. The EL86 was designed specifically for this circuit; the EL86 has low plate voltage and low plate impedance, but a very good cathode-heater insulation.

While the classical push-pull arrangement doubles the output impedance of a single tube, the series circuit halves it and has an output impedance that is only a quarter of that of a push-pull arrangement. For the EL86 this can be as low as 800 Ohms, which is low enough to drive one or a few speakers directly. The Philips speakers with 800 Ohm voice coil can be recognized by an A in the type number, e.g., 9710A, and 400 Ohm speakers have a B (and an M means double-conus speaker).

(John Byrns, Tony Duell, and George Gonzalez have commented an earlier version.)


Gerard Tel, gerard@cs.uu.nl.