Analog simulation of magnetic analog recording exoeriments part 3

Starting with 70μs.


What inductance is needed shunt to a given output impedance to create a 70μs premphasis?


And what frequency is this?


Using the definition of period being the inverse of angular frequency we just 1/70μs to find 14,286 radians per second. (We have motion around a circle and a sinewave is defined as a projection to the sin axis in trigonometry). To find the frequency in Hz we devide by 2 and then by π=3.14...


So the turnover frequency is 2.275KHz.


We can find the infuctsnce needed from this but it's easier to go to first principles.


Capacitance x resistance gives time (hence time constant). The higher the series resistance charging a high capacitance capacitor, the more time it takes. In fact 1 Farad x 1 Ohm makes 1 second. This is also called exponential time constant or half time (time for a quantity to become half).


Similarly with inductors T=L/R. In mechanical systems the higher the mass of a car and the less the resistance or friction the more time time it will take for the car to stop.


Rearranging L=TR (T times R)


Assuming a microphone of Z out 200Ω for example the Pleiades filter or inductir for 70μs premphasis will have to be 70μs x 200Ω = 14KμsΩ = 14msΩ = 14μΗ (since seconds times ohms makes Henries).


For the portable Sony CD player used in some of these experiments which has a line out impedance of 370Ω the Pleiades inductance would have to be about 25mH. Fortunately such Pleiades filters have already been made inside an XLR female to male and they will be tried.


To summarize: By connecting an inductor to a given output impedance a variable with respect to frequency potential devider is made. The higher the frequency the higher the impedance of the inductor. The less loading to the output impedance. The less voltage drop. So we have a frequency rising characteristic frequency as when feeding a tape recording head from a high output impedance. This archives a constant current condition wrt frequency.


For a line out impedance of 370Ω and a needed premphasis of 70μs, calculating as above, we need an inductance of 25mH.


Next comes the question at what voltage level the magnetic core will start saturating at 2.28KHz.


This will be done by connecting a high voltage electron tube sinewave generator.


To get the core to start softly clipping at the CD player -10dBm line out level we would need to reduce the iron, or other magnetic material the core uses.


So as to get it to overload or saturate at the required operating point.



See also next euroelectron post on magnetic clipping.