512 Chapter 17 R2 10 K C1 1:1 In - + A1 R1 Ground 2 + - 68 pF 1:1 A1 5532 R3 47 R R4 1K 3 + - Ground 2 R1 Output Male XLR CN1 In 1K NE5532 47 R 3 - + Output Male XLR CN1 1 (a) (b) Figure 17.4: Transformer balanced outputs; (a) standard circuit; (b) zero-impedance drive to reduce LF distortion, with Zobel network across secondary. output is shown in Figure 17.4a; in practice A1 would probably also have some other function, such as providing gain or filtering. In good-quality line transformers there will be an inter-winding screen, which must 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. A small line transformer can easily have 60 per winding, so the output impedance is 120 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 or more, compared with, say, 47 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. 17.6 Output Transformer Frequency Response If you have looked at the section in Chapter 16 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 17.5; 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 this high-Q resonance isn't the sort of lurking horror you want in your circuitry. It could easily cause some nasty EMC problems. 1 CAN CAN C2 15 nF