An audio transformer transfers a signal between two windings while changing its voltage and impedance by a fixed ratio. Engineers use transformers to match a source impedance to a load, balance an unbalanced signal, provide galvanic isolation that breaks ground loops, and step microphone or instrument levels up or down. This calculator works out the turns ratio you need between two impedances, and the impedance a primary winding presents when the secondary is loaded.
How it works
The behaviour of an ideal transformer is governed by a single relationship between turns and impedance:
Z1 / Z2 = (N1 / N2)²
Here Z1 is the primary impedance, Z2 the secondary impedance, and N1/N2 the primary-to-secondary turns ratio. Rearranging gives the turns ratio directly:
N1 / N2 = sqrt(Z1 / Z2)
The voltage ratio of an ideal transformer equals the turns ratio, V1/V2 = N1/N2, while the current ratio is its inverse so that power is conserved. When the secondary is loaded with a resistance R_load, the impedance reflected back to the primary is:
Z_primary = (N1 / N2)² × R_load
Example
To match a 600 Ω line output to a 150 Ω input, the impedance ratio is 600/150 = 4, so the turns ratio is sqrt(4) = 2, i.e. a 2:1 step-down transformer. A 4 V signal on the primary appears as 2 V on the secondary, and the 150 Ω load looks like 600 Ω to the source — a textbook impedance match.
Notes
Real transformers depart from the ideal: winding resistance, leakage inductance, and core saturation limit bandwidth and add a little insertion loss, so a “1:2” transformer rarely gives exactly double the voltage at the extremes of the audio band. Choose a part rated for the signal level and frequency range you need, and remember that the impedance figures quoted on transformer datasheets are design impedances, not fixed resistances. Everything here is computed locally in your browser.