The 565 is operable in two modes. In the direct mode, it yields 40 dB of gain with medium impedance performance - similar to the model 566 voltage preamplifier. In the transformer mode, it provides exceptional noise performance (equivalent to the Johnson thermal noise of a 1.4 ohm resistor at 300 degrees K) by switching in a 20 dB impedance matching transformer ahead of the amplifier stage. Its superior magnetic design also yields very wide bandwidth for source impedances of 10 ohm or lower.
Caution: The 565 cannot tolerate dc voltage on its transformer input in excess of ± 10 millivolts. For ac inputs below 50 Hz riding on the dc bias, the allowable dc level for less than 5% gain error will decrease linearly with frequency. Thus for a 5 Hz signal, only 1 mV of dc input bias would be allowable.
The input transformer is torroidally wound to render it insensitive to stray magnetic fields. Additionally, it is completely encased in mu-metal to shield against magnetic pickup. The input transformer also incorporates a conductive shield to prevent capacitive coupling of common mode interference from the input winding to the secondary winding. The aluminum case of the 565 affords a further shielding effect against electric fields.