The Effect of a Step-Up Transformer

Author: N.H. Crowhurst

The audio transformer steps up the noise voltage as well as the audio voltage

When a transformer is used to step up the audio voltages from a microphone or pickup, it steps up the noise voltage that comes with it. Suppose the audio voltage is 1 millivolt, and the resistance of the microphone or pickup is 500 ohms. In a bandwidth of 20,000 cycles (which we expect for high fidelity), the noise voltage from 500 ohms is

If we use a step-up transformer to raise the 1 millivolt to 10 millivolts, the voltage is stepped up 10 times, the current down by 10:1, and total power transferred remains the same. This means the effective resistance (voltage divided by current) is multiplied 100 times. The 0.406-microvolt noise signal will be stepped up with the 1 millivolt audio to become 4.06 microvolts of noise. In this statement, we have not considered any possible noise the transformer may add of its own, due to the resistance of its windings and core magnetization effects.