Today, I thought we would talk about AC ripple and Pi section power supplies for as we all know, one of the most important considerations in an amplifier is the minimisation of AC ripple that can be superimposed on the DC voltage supplied by a rectifier.
The value of ripple is dependant for a unit voltage on the reservoir capacitance and the smoothing network - let's take a typical Pi network comprising of C1, a reservoir capacitor, and C2, a smoothing capactor with either an inductance L or a resistance R: -
The ripple voltage is inversely proportional to the values of C1 and C2 - so if you want low ripple then get BIG capacitors. Some people economise in the design of equipment by substituting a resistance in place of a more expensive choke with little appreciable rise in perceived ripple current. Of course in this world 'tha gets nowt for nowt,' and the resistance unfortunately reduces the DC output voltage dependant upon current delivery loading and resistance value. This disadvantage can be minimised by feeding the PA stage output valves from C1 rather than C2 with the ripple being nullified by a push-pull output stage, particularly if this stage is balanced.
Taking the above example: -
For C1 = 8 uF; C2 = 16 uF; L = 10 H then Irip = 210 mV for V0 = 251 V @ C2
For C1 = 50 uF; C2 = 50 uF; R = 500 Ω then Irip = 210 mV for V0 = 221 V @ C2 or 265V @ C1
So there you have it but my take on it is don't be cheap, use a choke and preferably one bought from Mullard Magic!!!!!!