The active input amplifier circuit was used, rather than an input transformer.

The input amplifier accepts balanced signals which is efficient in significantly attenuating common-mode signals (ie signals of the same amplitude and phase on both the hot and cold signal lines). In order to maintain a high CMRR (common-mode rejection ratio) it is necessary to accurately match the input impedances for the hot and cold signal lines. This is critical, since a mismatch of only a few ohms can severely degrade the balanced amplifier's performance which usually results in mains buzz and hum appearing on the input signal.

It's important to match the 10K resistors (R1A, B, C, D, E) as closely as possible. Start with a bunch of 1% resistors and match them to within 10 ohms or better. A 10K 1% resistor can be between 9900 ohms and 10100 ohms. Ideally we should match them to within 1 ohm (you'll need a good 4 1/2 digit meter to do this) however 10 ohms (0.1%) should give satisfactory results.

To demonstrate the CMRR differences I simulated the input circuit with different tolerance mismatches between the 10K resistors. The results shown here show that an ideally matched set of resistors will give a CMRR of about 94dB (using a good NE5532 model on hand). This drops to 68dB when a 1 ohm mismatch occurs and further down to 48dB for a 10 ohm mismatch. Using a 1% spread (possible with no matching using 1% resistors) the CMRR drops to about 28dB. So by matching to within 1 ohm you can pick up an extra 40dB of rejection. That's 100 times less common-mode mains noise.

CMRR Simulation Results

These results are for the combination of tolerance spread that gives the worst case. However it demonstrates the significance of matching these resistors. All the simulations were done at a frequency of 100Hz with a constant 20 ohm source impedance. It should be noted that any mismatch in the source impedance (output drivers and cable resistance) will contribute to a reduction in CMRR. However there is usually a possiblilty of a much larger variation in the higher input impedance of the input amplifier than the much lower impedances from the cables and signal source.

 R1-A R1-B R1-C R1-D R1-E Vin (V) Vout (V) CMRR (dB) Tolerance (%) 10100 10100 9900 10100 9900 2.1867 87.223m -28 1 10010 10010 9990 10010 9990 2.1857 8.7586m -48 0.1 10001 10001 9999 10001 9999 2.1866 917.2u -67.5 0.01 10000 10000 10000 10000 10000 2.1866 45.51u -93.6 0

Input amplifier circuit