@CantoFan123:
Filter caluculation
Exact filter calculation is difficult due to the characteristic of the transformer. But because the proposed filter is very smooth and frequencies of a few hundred Hz aren't dangerous an approximation is sufficient.
Problem is the impedance of the transformer winding X_T which is the sum of the DC resistance of the winding plus the load on the secondary side divided by the winding ratio in square plus a frequency dependent part. The frequency dependent part can be ignored if the transformer is driven within the frequency range from the datasheet. Without frequency dependency of X_T the transmittance factor of the filter becomes A=(X_T+R)/(X_T+R+X_C), where R is the resistance of the series resistor and X_C is the impedance of the capacitor: X_C=1/(2*π*f*C).
With high winding ratios (as higher as more current controlled -- which is what we want) the load dependent part of X_T is very small compared to R and the DC resistance of the winding, i.e. it has only a small effect in the formula for A. I estimate the output load with a few hundred Ohm, lets say less than 300 Ω. (I never measured it. Resistance measurement with a multimeter does not work because a small current between the electrodes due to electrochemical effects. Measurement with a larger DC current is not recommended because DC currents are dangerous. It would have to be measured with AC current.)
Example 1 (Reichelt part, my configuration):
Winding ration is 10, i.e. load dependent part of X_T is the output load divided by 100.
R = 10 Ω
X_T = 6 Ω
C = 100 µF
f = 500 Hz
==> X_C = 3.18 Ω
==> Transmittance factor A=0.83
Example 2 (Digikey part):
Part number 237-1146-ND is the only transformer I found on Digikey that may work. The 2.9 Ω side should be connected to amplifier with an series resistor. I probably will try out this part because it has a higher winding ratio compared to my part. But it is possible the this part becomes to inefficient due to the high output resistance.
Winding ration is 25, i.e. load dependent part of X_T is the output load divided by 625
R = 10 Ω
X_T = 3 Ω
C = 100 µF
f = 500 Hz
==> X_C = 3.18 Ω
==> Transmittance factor A=0.80
You see for feasible transformers R is the dominating part in the formula for A. A simple rule is therefore C=100µF for R=10Ω and C=47µF for R=20Ω.
Digikey part numbers for capacitors: 493-17272-ND and 493-6080-ND
Another issue: the Capacitor is also required to protect the transformer against damage from low frequencies, especially DC current.
Electrodes:
For placement see
https://www.sexmachinereviews.co.uk/adv ... .html#male
Butt electrode (prostate) should be purchased. With a current controlled device it feels better as larger the electrode and as lower the contact resistance. The conductive rubber pads I tested have a to large resistance, i.e. the current is located to the place where the current is connected to a metal cable. Electrodes from stainless steel sheet (or washers) work better for me. That is something you have to try out.
Note that is is possible to solder stainless steel with good old Sn63Pb37 solder and special flux.
