I am designing a RCRC filter to go before a Jan Didden Treg for an aikido preamp.
Putting aside such things as cost, space etc.
Am I correct that the aim of such a filter is to get the fc as far below the mains fundamental frequency (50Hz in NZ) within an acceptable voltage drop and/or dissipation across the two resistors?
Is say 20Hz a reasonable target or should I be aiming lower if possible?
Putting aside such things as cost, space etc.
Am I correct that the aim of such a filter is to get the fc as far below the mains fundamental frequency (50Hz in NZ) within an acceptable voltage drop and/or dissipation across the two resistors?
Is say 20Hz a reasonable target or should I be aiming lower if possible?
You are correct in principle, but frequency is 100 Hz after full wave rectification. Low pass RCRC will have 12 dB/octave attenuation. If you pick 25 Hz, there will be 24 dB or 15.85 x ripple voltage attenuation at 100 Hz. More than enough, considering that T-reg has its own unspecified but likely very good PSRR.
Thank you.
I ran some numbers through a calculator.
I already have the 470uF capacitors so that part is fixed. Using 100r resistors I got an fc of 3Hz. -45db at 50Hz.
I wasn't looking at 100Hz so I would have to rerun the calculator but clearly more than sufficient.
With 50r it was -32db at 50Hz which is still more than enough.
Expected current draw is 15mA.
Thus voltage drop would be 1.5v for 100r or .75v for 50r.
Dissipation would be 22mW for 100R or 11mW for 50r.
I have voltage to spare. I'm expecting 325Vdc at the rectifiers and I need 265Vdc at the output of the regulator.
Given all that, it looks to me like there is no reason not to go with 100r since the dissipation is low and I have voltage to drop.
Have I missed anything important?
I ran some numbers through a calculator.
I already have the 470uF capacitors so that part is fixed. Using 100r resistors I got an fc of 3Hz. -45db at 50Hz.
I wasn't looking at 100Hz so I would have to rerun the calculator but clearly more than sufficient.
With 50r it was -32db at 50Hz which is still more than enough.
Expected current draw is 15mA.
Thus voltage drop would be 1.5v for 100r or .75v for 50r.
Dissipation would be 22mW for 100R or 11mW for 50r.
I have voltage to spare. I'm expecting 325Vdc at the rectifiers and I need 265Vdc at the output of the regulator.
Given all that, it looks to me like there is no reason not to go with 100r since the dissipation is low and I have voltage to drop.
Have I missed anything important?
IMO, you didn’t. Under described conditions, you could use even higher resistance and have super low ripple at T-reg input.