Postby Al » Mon Aug 04, 2014 7:14 pm
The main thing with audio transformers is the turns ratio - impedance ratio is the turns ratio squared, so a 1200 ohm to 600 ohm transformer would have a turns ratio of (sqrt (1200/600)) or 1.414 to 1. And the voltage ratio is the same as the turns ratio. Now, a transformer will reflect an impedance from primary to secondary or secondary to primary that is proportional to the ratio of the rated impedance to the connected impedance. So, for example, if you connect a 1200 ohm load to the secondary of a 1200 to 600 ohm transformer, the impedance reflected to the primary will be 2400 ohms. Low frequency response(below 100 hz or so) is determined by the inductance of the windings(which is determined by the core material and number of laminations). High frequency response is determined by stray capacitive coupling between windings, core material, and distributed capacitance across the turns of each winding. Power handling capacity is determined by core size and material and I squared R losses(copper losses) in the windings. And distortion is largely determined by core characteristics(saturation flux density levels).
So you see that transformer design is largely empirical.
In the type of application that you're using them for, the isolation provided by a transformer is a more important consideration than impedance matching. You need to be concerned primarily with impedance matching where you're interested in maximum power transfer from one device to another. The devices that you're using as source and load devices are generally high impedance - voltage responding(if you're using line inputs on your 70V mixer-amp), so input and output voltages and meeting the devices' requirements are more important than trying to match impedances. As long as you don't go to extremes in selecting an [absurdly wild] turns ratio the result will sound good. And the isolation provided by the transformer will eliminate any ground loop noise entering the system.
