LC and RC filter, trying to understand them

[pjpoes]

I would love for some background information on these types of power supply filtering techniques. I understand that using an LC or even RC network can improve the specs of a power supply over just a plain capacitor filtered supply. It reduces the impedance, reduces the noise, etc. Can someone point me into the direction of some good information on both RC and LC power supplies. I would like to use RC networks in solidstate power supplies in some devices like preamps and phonostages. Also, do these offer any benefit before a regulator. For instance, you have your transformer, dioces, RC network, and then a pair of monolithic IC regulators, then say a preamp. Would this improve the performance of the regulators compared with just a capacitor filtered network.

 

[wine&dine]

Do you have some EE knowledge? Get SwitcherCAD from www.linear.com and simulate the various options, I find that an invaluable tool to get a qualitative grip on the various tradeoffs.

Note also that adding R or L elements will never give you a lower impedance! What you CAN achieve is lower crest factor (distortion) on the input current, IOW less noise at higher frequencies. Note also that an L-C filter after the rectifier (caka choke input) gives less DC voltage (2*sqrt(2)/Pi)*Vrms as opposed to (sqrt(2) * Vrms), at 12Vrms that is approx. 9V vs. 15V, so "improving" existing circuits with a choke won't work.

The simulator will show you that nicely ...

 

[pjpoes]

Thanks, I knew it reduced the voltage, but my intension was simply to increase the voltage from the transformer to compensate. I have a few trafos around at a high enough voltage to try this with.

I have minimal EE knowledge, in that I went to college for it for 1 year, then changed majors to psychology. However, I have interest, have read many books, was always heavily involved in highschool, and remained involved in college as a hobby as well. I will give it a try, see if I can't understand the program alright.

So how do you lower the impedance on a power supply, I mean, that seems like an important factor. Is that soley the ESR of the capacitors?

 

[jan.didden]

The output Z of raw supplies is not constant but varies with frequency of the load current and also on the point in time (are the diodes just conducting, or are they just switched off? Diodes only conduct about 20% or less of the time, the other 80% or more the transformer is effectively disconnected from the circuit).

For a low output Z you really need to put a good regulator after the bridge and capacitor.

Jan Didden

 

[pjpoes]

Thank you that was very informative. I appreciate the input. Now I understand why the Jung Regulator is so great, too bad it can't be used at higher currents for things like highcurrent preamps or power amps.