A Tip When Designing Power Supplies

[steve]

As a tip, try to eliminate chokes in the B+ power supply filtering circuits. The reason is a choke is non linear VS frequency and causes distortion in multiple ways. For one, as the frequency varies, the isolation between two adjacent filter capacitors varies.

Besides the inductive reactance, which varies with frequency, the inductor also has dc resistance of the wire which does Not vary VS frequency. Below is a photograph taken, showing what happens to a signal of varying frequency across a typical choke. X horizontal line represents increasing frequency, Y vertical line represents increasing amplitude.

In the photo below, the schematic shows a very basic thevenin equivalent circuit of a choke with 200 ohm dc winding resistance. The 200 ohm resistor connected directly to ground is the load. With a perfect choke with zero dc resistance, the line would be angled but straight. However, even with a straight, angled line, isolation varies between two adjacent filter capacitors, causing distortion. (In reality, a choke also has distributed interwindingcapacitance etc etc, but not necessary to our discussion.)

With no inductance, only dc resistance, the straight line would be horizontal, so a constant isolation between filter capacitance. With both inductance and dc resistance, the line is curved. Of course some circuits need a choke to minimize hum. But not all circuits.

It is best to eliminate chokes completely. However, if one cannot, try to eliminate the choke nearest to the gain stage and use a non-inductive power resistor instead.

cheers

steve

[Folsom]

Resistance and/or chokes mostly work for tubes though... A little voltage point is not a big deal if you have 400vdc, but if you have 26vdc then it can be a potentially large amount of output wattage lost.

Anything with an inductor is complicated and tends to run the risk of sounding life sucking... There are ways and not ways to use them.

[steve]

Yep, chokes mess up the sound.

For general public, a choke connected to the decoupling capacitor in the power supply (in this case with tubes) causes a change in RL vs frequency in a non-linear way (similar to the decoupling capacitor changing RL).

The equation for gain for a common cathode triode stage is: -mu x RL/rp + RL

Another equation for the gain is: -gm x rpRL/rp + RL

mu = amplification factor, often called just u.
gm = transconductance
rp  = plate resistance of the tube
RL  = plate load resistance (from plate to B+ voltage)

As RL changes vs frequency, the gain of the stage changes vs frequency.

Books for reference.

"Semiconductor and Tube Electronics An Introduction" by James G Brazee (One of my college textbooks.)
RCA "Radiotron Designers Handbook" edited by Langford and Smith, by over 2 dozen engineers, 1960.

I have not seen any SS amps using chokes, although it is certainly possible in pre output stages drawing minimal
drain/collector current.

cheers

steve