Copyright © 2010 by Wayne Stegall
Updated March 11, 2010. See Document History at end for details.
Line Level Class A Power Supply
This is an example only. It is not intended to be final or
complete design.
Figure
1:
Schematic


Parts List


T_{1P} 
20V transformer (or
somewhat higher if 20V unavailable)

BR_{1P} 
bridge rectifier,
peak current specification
exceeding peak transformer current.

C_{1P}C_{4P}

0.1µF ceramic

C_{5P} 
4700µF, 35V
electrolytic 
Q_{1P}

LM317

D_{1P},D_{2P} 
1N4002 diodes

C_{6P}

10µF, 35V
electrolytic 
R_{1P}

240Ω, 5% 

R_{2P} 
5kΩ or 10kΩ pot 
C_{7P}

100µF, 35V
electrolytic 
Q_{2P},Q_{3P} 
IRF510 nch mosfet

D_{3P}D_{6P} 
1N914 or 1N4148
diodes

R_{3P},R_{5P} 
4.7kΩ, 5% 
R_{4P},R_{6P}

1kΩ, 5%

C_{8P},C_{10P}

220µF, 35V
electrolytic 
C_{9P} 
10,000µF, 35V
electrolytic 
C_{11P} 
3300µF, 35V
electrolytic 

These specific calculations are for a power supply for the circuit of JFET Phono Preamp  Active Inductor Example.
You
may
have
to
change some values for other applications. See
text.for calculations.

Initial Design Decisions
 The power supply is designed to be simple source follower type
for classa friendly output impedance.
 Prefiltering is by LM317 for better line regulation. LM317
circuit design is as recommended by datasheet.
 Unique design element pertains to adding parallel diodes to gate
circuit of source follower. The diodes create a variable RC
constant. This provides for quick charge through the resistor to
nearly the desired gate voltage then a slow increase to an enormous RC
time constant to hold a constant gate voltage.
 Load requirements: 27.9778112mA out of V_{DD1},
19.4563897mA out of V_{DD2}.
Calculations
Some of the component choices are not very critical, but I chose to
calculate these.
 Calculate transformer specification.
v
_{TRANSFORMER} = 0.7071 x (v
_{SUPPLY} + v
_{gsmaxQ2}
+ v
_{DROPOUTLM317} + v
_{DIODEDROP}) = 0.7071 x (18V +
4V + 3V + 1V) = 18.3846V
Round up to standard value of 20V.
 Calculate C_{5P} for ripple of
0.5V.
Refer article:
Power Supply Ripple Calculations
and Capacitor Size, equation (3).
i
_{LM317INPUT} = i
_{DD1} + i
_{DD2} + i
_{R1P}
+ i
_{ADJLM317} = 27.9778112mA + 19.4563897mA + 5mA + 100µA =
52.5342009mA
C_{5P} =

i
fΔv 
=

52.5342009mA
60Hz x 0.5V 
= 1.75114003mF 
Choose instead common larger value of 4700µF
 Choose R_{3P},R_{5P} and C_{8P},C_{10P}
for a time constant of one second.
I chose these values a good while ago to meet this specification.
How I chose the first component in order to calculate the second, I do
not remember. Perhaps it was a noise consideration.
Calculate total noise from capacitor noise equation. (The
capacitor is not the source of the noise itself but interacts with the
resistor in a way to become the sole determining factor.)
v_{NOISEC8PTOTAL} = sqrt


kT
C


= sqrt


1.3806504e23 x 298.15ºK
220µF 

= 4.325615651nV

R
_{3P} = 1/C
_{8P} = 4.545454545kΩ, rounded up to
nearest 5% standard value is 4.7kΩ.
As a matter of curiosity, the 1N914 specification of 5nA reverse
current at 20V suggests an ac impedance of 800MΩ at very low ac
voltages. (This is due to leakage resistance. Ideal
calculated reverse current is very much lower.) 220µF x 400MΩ
gives a settled time constant of 88,000 seconds. Because
this is such a large value, I suspect it to be very imprecise.
 Establish lower boundary for C_{9P},C_{11P} based
on these criteria:
The common source circuit supplied by V
_{DD1} is very sensitive
to its power supply. Indeed, the power supply is in the signal
path for this stage. Maximum output current from V
_{DD1}
is 41.4651312mA. The g
_{fs} of IRF510 is
1.3S @ 3.4A. I want primarily capacitive power supply output
impedance. The
transistor output impedance is nonlinear (although with nice euphonic
2nd order harmonics!) and with the capacitor value determines the pole
above which output impedance becomes capacitive. Initially try
for a pole at 2Hz.
IRF510 specification: g
_{fs}
= 1.3S @ 3.4A.
Calculate MOSFET constant from g
_{fs}@i
_{D}
specification.
_{
}
k_{n}
= 
g_{fs}^{2}
4i_{D} 
=

1.3S^{2}
4(3.4A) 
= 124.2647059mA/V^{2
} 
Recalculate g
_{fs} for specific i
_{D}.
g
_{fs} = 2 x sqrt(k
_{n}i
_{D})
= 2 x sqrt(124.2647059mA/V
^{2} x 27.9778112mA) = 117.9263241mS
R_{outQ2P} =

1
g_{fs}

=

1
2 x sqrt(k_{n}i_{D}) 
=

1
2 x sqrt(117.9263241mS x
27.9778112mA) 
= 8.70477913Ω 
C_{9P} =

1
2πf_{pole}R_{outQ2P} 
=

1
2π x 2Hz x 8.70477913Ω 
= 9.14181398mF

Choose
C_{9P} =
10,000µF and solve for the pole frequency.
f_{pole} =

1
2πC_{9P}R_{outQ2P} 
=

1
2π x 10mF x 8.70477913Ω 
= 1.828362796Hz

Figure 2: Bode plot
representing suppression of MOSFET
nonlinearity by chosen bypass capacitor of 10,000µF 

A more common choice of 1000µF gives a pole frequency of
18.28362796Hz. This choice may give a slight increase in low bass
distortion, and perhaps why some reviewers hear midbass bloom in some
class A equipment.
V
_{DD2} drives a circuit (source follower) which is relatively
insensitive to power supply fluctuations. Therefore you can
choose a more arbitrary value here. Save money here to buy a
larger C
_{9P}.
I choose
C_{11P }=
3300µF.
Document History
January 5, 2010 Created
January 5, 2010 Replace vague load specifications with
exact ones from article JFET Phono Preamp
 Active Inductor Example and update calculations.
March 11, 2010. Corrected for improper g_{fs}
presumptions.