3. Transformation Ratio
This one is quite simple. The transformation ratio is the ratio between the number of turns in primary and secondary windings. The signal voltage multiplies proportionally to the transformation ratio. So, if the transformer has a ratio of 1:35, the signal will be amplified by 35 times. Correspondingly, such ribbon mic transformer will provide around 30 dB of voltage gain. The most widespread ratios among the ribbon mic transformers range from 1:28 to 1:40.
Now, the logical question pops up – if the transformers provide extra gain without adding the noise, why not increase the ratio further? The answer is simple. Transformers increase the output impedance together with the gain. Moreover, while the gain increases proportionally to the ratio, the output impedance increases by the square of the ratio.
It means, that there must be a balance between the signal gain and the reasonable output impedance. For microphones it is 250 – 300 ohms by modern standards. Such impedance corresponds to 1:40 ratio with a typical big ribbon. For thinner and narrower ribbons with higher output impedance the ratio is lower, 1:28 is a typical value. Such ribbon mic transformers also normally have higher primary winding inductance to maintain the even bass response.