The ESL impedance is simulated with LTspice vers IV with the following circuit diagram:
- The step up trafo is an important component, but LTspice does not have a trafo model, so to keep things simple we have made a simulation, where all component values are transformed to the secondary side.
- All resistors are assigned the values that you will find in Schematics and Specifications.
- R15 is the 1 Ohm resistor on the primary side that is transformed to the secondary side as explained i Specifications.
- All capacitors are calculated with the standard formula for a plate capacitor:
c = k*e0*A/d
where k=relative dielectricity constant (1.0), e0=8.854 E-12 F/m, A=area in m2, d=stator to stator distance in m.
- The step up ratio is set to 1:200 for this simulation. Actual values depend on the specific trafo. John uses four mains trafos (6V:230V) giving a total step up of 153. Steen uses two trafos salvaged from a Martin Logan CLS II, they have a number of taps with different step up ratios with a maximum of 1:240 for two trafos in series.
- In the simulation, we want to know the impedance on the primary side as a function of frequency. This is achieved by stimulating the circuit with an AC current generator I1 and measure the resulting voltage on top of the AC source. To compensate for the step up we have reduced the AC signal from 1 A to 1 A/(200*200) = 0.000025 A. Now we can directly measure the impedance as the voltage on top of the AC source.
It can be seen that the impedance (solid green) falls from 20 Ohm to 7 Ohm at 20 kHz. This should be an easy load but the load is very dependent on the step up ratio, which should not exceed 1:200.
Real measurements shows impedances down to 2 Ohm, so all in all you need an amplifier that can tolerate capacitive load and low impedance.