Line Outputs  607

The only advantages that this kind of balanced output has over an unbalanced output is that the total signal level on the interconnection is increased by 6 dB, which if correctly handled can improve the signal-to-noise ratio. It is also less likely to crosstalk to other lines, even if they are unbalanced, as the currents injected via the stray capacitance from each line will tend to cancel; how well this works depends on the physical layout of the conductors. All balanced outputs give the facility of correcting phase errors by swapping hot and cold outputs. This is however a two-edged sword, because it is probably how the phase got wrong in the first place.

There is no need to worry about the exact symmetry of level for the two output signals; ordinary 1% tolerance resistors are fine. Slight gain differences between the two outputs only affect the signalhandling capacity of the interconnection by a very small amount. This simple form of balanced output is the norm in hi-fi balanced interconnection but is less common in professional audio, where the quasi-floating output, which emulates a transformer winding, gives both common-mode rejection and more flexibility in situations where temporary connections are frequently being made.

Transformer Balanced Outputs

If true galvanic isolation between equipment grounds is required, this can only be achieved with a line transformer, sometimes called a line isolating transformer; don’t confuse them with mains isolating transformers.You don’t, as a rule, use line transformers unless you really have to, because the muchdiscussed cost, weight, and performance problems are very real, as you will see shortly. However they are sometimes found in big sound-reinforcement systems and in any environment where high RF field strengths are encountered. They are unlikely to be used in active crossovers for domestic hi-fi.

A basic transformer balanced output is shown in Figure 21.4a; in practice A1 would also have some other function such as providing gain or filtering. In good-quality line transformers there will be an inter-winding screen, which should be earthed to minimise noise pickup and general EMC problems. In most cases this does not ground the external can, and you have to arrange this yourself, possibly by mounting the can in a metal capacitor clip. Make sure the can is earthed, as this definitely does reduce noise pickup.

Be aware that the output impedance will be higher than usual because of the ohmic resistance of the transformer windings. With a 1:1 transformer, as normally used, both the primary and secondary winding resistances are effectively in series with the output.Asmall line transformer can easily have 60

Figure 21.4: Transformer balanced outputs: (a) standard circuit; (b) zero-impedance drive to reduce LF distortion, with Zobel network across secondary.

608  Line Outputs

Ω per winding, so the output impedance is 120 Ω plus the value of the series resistance R1 added to the primary circuit to prevent HF instability due to transformer winding capacitances and line capacitances.

The total can easily be 160 Ω or more, compared with, say, 47 Ω for non-transformer output stages.

This will mean a higher output impedance and greater voltage losses when driving heavy loads.

DC flowing through the primary winding of a transformer is bad for linearity, and if your opamp output has anything more than the usual small offset voltages on it, DC current flow should be stopped by a blocking capacitor.

Output Transformer Frequency Response

If you have looked at the section in Chapter 20 on the frequency response of line input transformers, you will recall that they give a nastily peaking frequency response if the secondary is not loaded properly, due to resonance between the leakage inductance and the stray winding capacitances. Exactly the same problem afflicts output transformers, as shown in Figure 21.5; with no output loading there is a frightening 14 dB peak at 127 kHz. This is high enough in frequency to have very little effect on the response at 20 kHz, but such a high-Q resonance isn’t the sort of horror you want lurking in your circuitry. It could easily cause some nasty EMC problems.

Figure 21.5: Frequency response of a Sowter 3292 output transformer with various loads on the secondary. Zero-impedance drive as in Figure 21.4b.

610  Line Outputs

driven directly from a 5532 output (there were no HF stability problems in this case, but it might be different with cables connected to the secondary), and with 47 and 100 Ω added in series with the primary. The flat part to the right is the noise floor.

Taking 200 Hz as an example, adding 47 Ω in series increases the THD from 0.0045% to 0.0080%, figures which are in exactly the same ratio as the total resistances in the primary circuit in the two cases. It’s very satisfying when a piece of theory slots right home like that. Predictably, a 100 Ω series resistor gives even more distortion, namely 0.013% at 200 Hz, and once more proportional to the total primary circuit resistance.

If you’re used to the near-zero LF distortion of opamps, you may not be too impressed with

Figure 21.6, but this is the reality of output transformers. The results are well within the manufacturer’s specifications for a high-quality part. Note that the distortion rises rapidly to the LF end, roughly tripling as frequency halves. It also increases fast with level, roughly quadrupling as level doubles.

Having gone to some pains to make electronics with very low distortion, this non-linearity at the very end of the signal chain is distinctly irritating.

The situation is somewhat eased in actual use, as signal levels in the bottom octave of audio are normally about 10–12 dB lower than the maximum amplitudes at higher frequencies; see Chapter 17 for more on this.

Figure 21.6: The LF distortion rise for a 3292 Sowter transformer, without (0R) and with (47 Ω and 100 Ω) extra series resistance. Signal level 1 Vrms.