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the Power Follower SW - SE class A amplifier  after 22 years a new test 
original design by Andrea Ciuffoli - 1999 - 2021

This is the new Power Follower with switching power supply, same sound of the normal version but higher efficiency, less cost and less noise.



This is a 20-25W Class A Single ended no feedback amplifier and only 2 active components on the signal path.

If you are looking for an extreme quality amplifier and you don't need high power this is the project for you because nothing can have higher performances.
After 22 year I have rebuild the same project because this is perfect on all aspects except the efficiency but this is normal for a true single ended class A.
This project is an hybrid amplifier composed by a tube voltage amplifier followed by a single ended mosfet current amplifier with an exclusive configuration.
I have published several articles about this my current amplifier and this design was the winner of the award in the Circuit ideas in May 2000 on Electronics World which is the most important electronic magazine in the world.  
This amplifier was built by a lot of people around the world and everyone are enthusiastic about the result, the sound is compared to that of the best single ended vacuum tube amplifiers but this have a higher performances in terms of driving capacity (damping factor) and low distortion.

To read these articles search on the web page:





It seems a silly and common circuit but it has got great differences from any other similar design, that’s why this is unique.
We have a typical source follower (as an emitter follower but with a Mosfet) working in pure class A with a current generator.
Please note that this circuit works only in pure class A, so it requires enough bias current for the requested output power.
There are no particular limits on the power that can be delivered but obviously the efficiency of this amplifier is very low, 25% theoretical and 15% real so you need a big heatsink also for medium power about 20W. 
To drive this current amplifier is necessay a voltage stage
with an output swing not lower than 10Vrms and Rout < 1000ohm.


In my design the first feature making a big difference in the sonic results is the negative power supply and the signal ground tied to the drain of source follower.
With this design we achieve:

The power supply integrate a Virtual Battery to get a reduced ripple and to use lower capacitors.


The 500 Ohm precision trimmer should be set to balance the output fuse voltage to 1/2 of the regulator output.

This single trim should be set after the switch-on, because it just optimizes the output swing to the maximum available.

The 2 x 18V 1W zener are optional and these can be used to prevent crash of the mosfet when it receive bad input signal like a vacuum stage startup (these could decrease the sonic performances).

The phase on output terminals has been inverted because here I will use a voltage amplifier that reverses the phase.

The quiescent current is set  by the resistence on current generator source pin, and can be changed with the simple formula Iq = 0.65 / R where 0.65 is the transistor Vbe.

With a bias current of 3A you can get about 20 Wrms on 4 and 8 ohm load.

The power can be increased with the unique drawback of the dissipated power (heat)!

I have already specified the only problem with this project is efficiency so first of all it is necessary to establish the maximum power that we are able to dissipate.

We will have about 20W with a power to dissipate near to 130W per channel so the efficiency is 15.6% but a single ended with the same output power is very similar, for example the my GM70 SE give this result:

description voltage (V) current (mA) power (W)
anode current GM70 1100 80 88
anode current D3a 180 20 4
filament GM70 20 3000 60
filament D3a 6.3 315 2
    total 154

If we use a Hi-Fi2000 chassie 04/300B with 2 heatsink with a coefficient of 0,31 C/W it is possible dissipate 120W with an ambient temperature of 35 degrees and stay below 70 degrees (which I do not recommend to overcome).

t this point we must consider 2 constraints: the maximum output voltage determined by the power supply voltage and the maximum output current determined by the bias current.

The output power will be limited by both these values so after choosing a power transformer and a bias current we can check the output power for each type of load.

This is my configuration:

Ibias (A) Vdc (V) Power to dissipate (W) Heatsink (C/W) Tamb max (C) Tchassie (C) Vout (Vp) Vout (Vrms) Rload (ohm) Pout max with voltage used (W) Iout (Irms) Pout max with current used (W)
3.0 36.0 108.0 0.31 35 68 17.0 12.1 8.0 18.2 2.1 36.2
3.0 36.0 108.0 0.31 35 68 17.0 12.1 6.0 24.2 2.1 27.2
3.0 36.0 108.0 0.31 35 68 17.0 12.1 4.0 36.3 2.1 18.1

The output device, IRFP150, can be substituted by their TO3 equivalents or by other similar MOSFETs like IRF250, IRFP250, IRF240, IRFP240, with a minimal impact but the IRFP150 has been selected because these have a lower input capacitance and a low Zout.

There are some differences on mosfet specifications from one manufacturer to another, a low input capacity is crucial for having a good high frequency response:

In this new design I have used the IRFP150NPBF by Infineon (RS cod. 541-0856) with only 1900pF.




The presented topology, has no voltage gain (actually it looses something 1.3%) so it should be driven by voltage gain stage, with an output swing not lower than 10Vrms and Rout < 1000ohm. 

The input impedance of the current amplifier is 110KOhm - 1800-2800pF and its resistive value can be adjusted by a pretty wide range, just using a different input resistance (max 220Kohm). 

The current amplifier do not introduce any alteration on the signal so is very important take care of driver stage/voltage amplifier.






PCB available in this Ebay shop

I advise to build this project using a pcb instead of an air wiring because it is certainly more stable.

PCB available in this Ebay shop

This is a single channel pcb so duplicate the components list.

PCB files available for DIY.

Mount the resistances in parallel configuration in opposite phase or direction.

R1=Rbias     0.2ohm 5W                     Mouser 71-LVR5-0.2
R3            1Kohm 2W                     Mouser 660-MO2CT631R102J
R9            2Kohm 2W                     Mouser 660-MO2CT631R202J
R11/R10      500ohm multi turns trimmer    Mouser 81-PV36W501C01B00
R4,R5        470ohm 1/4W 1%
R6,R7       220Kohm 1/4W 1%                Mouser 594-MBB02070C2203FCT
R12         2700ohm 2W                     Mouser 660-MO2CT631R272J
R16          220ohm 1/4W 1%
R14             -                          NOT USED
R15             -                          NOT USED
R13             -                          NOT USED
R8              -                          NOT USED

IRFP150NPBF                   RS     541-0856
Q3              -                          NOT USED see jump

Q4           MJE340                        Mouser 863-MJE340G

D14,D5       zener 18V 1W                  Mouser 78-1N4746A

F3           fuse 5A FAST with fuse holder Mouser 534-4628

C1              -                          NOT USED
C4           470uF 63V                     Mouser 647-UPW1J471MHD3
C2           0.47-1.0uF 400V MKP
C3           100uF 50V                     Mouser 647-UPW1J101MPD
C5           1000uF 50V                    Mouser 647-LKG1J102MESZCK

Cout         min. 4700uF 50V Nichicon KG
Rout         1Kohm 2W                      Mouser 660-MO2CT631R102J

Isolators    Bergquist SP400-0.007-00-104  RS RS541-0856

The connections are 63862-1 (CUT STRIP) by TE Connectivity / AMP (cod. Mouser  571-63862-1-CT, cod. RS 718-7987)


PCB available in this Ebay shop

This is a two channels pcb so the components list is complete.

PCB files available for DIY.

Mount the resistances in parallel configuration in opposite phase or direction.

R1,R2,R18,R19      22Kohm 1/2W 1%                    Mouser 71-RN60D-F-22K
R3,R4,R20,R21     2200ohm 1/4W 1%                    Mouser 71-RN60D2201F

R10,R26,R22,R23    39Kohm   2W                       Mouser 71-CCF02-J-39K
R7,R24             100ohm 1/4W 1%                    Mouser 71-RN60D1000F/R
R37,R39             47ohm 1/4W 1%     
R11,R34           4700ohm 1/4W 1%
R12,R30       2 x 100Kohm 1/4W 1% to have 200K
R13,R28           4700ohm 1/4W 1%
R14,R29           150Kohm 3W
R17,R33            3.3ohm 2W
R25,R35              0ohm
R15,R16,R31,R32    220ohm 1/4W 1%

C1.C3             220uF 6.3V   OS-CON                Mouser 667-6SEPC220M+TSS ??
C2,C4              33uF 400V                         Mouser 647-UVY2G330MHD
C5,C6             100uF 400V                         Mouser 647-LGU2G101MELZ
CY1-CY8            10nF 440VAC                       Mouser 80-R474I210050A1K

D1,D2,D3,D4,D6,D7,D8,D9  UF5406                      Mouser 625-UF5408-E3
D5,D10                   zener 10V 1W                Mouser 78-1N4740A-TAP

U$3,U$6           IRF840                             Mouser 844-IRF840APBF
U$11,U$7          33uF 400V Solen MKP or Vishay MKP 50uF 500V 

KK1,KK4           Extruded Style Heatsink for TO-220 Mouser: 532-513102B25

Bergquist SP400-0.007-00-54        RS 169-2177

TO-220 nylon platstic Insulator hole size M3

The connections are 63862-1 (CUT STRIP) by TE Connectivity / AMP (cod. Mouser  571-63862-1-CT, cod. RS 718-7987)

The heat sinks have been grounded to avoid receiving radio frequency so these must be isolated from the transistor.



Obviously it is fundamental to use a high quality interstage capacitor and for this project I will test 4 different types of a good UK company.
This test was necessary to decide which capacitor to use because it is not true that it is enough to buy the most expensive one to be sure of having the maximum sonic performance.
It happened to me to discard many high-level interstage capacitors, also very well evaluated.
In all my sonic performance tests I always search to be sure of an objective result using more persons and these with different experiences.
The ClarityCap has been manufacturing high quality audio grade capacitors for over 30 years.

The low  frequency cut-off is determined by the value of this capacitor and input resistance of the current amplifier (110Kohm).

Ft(-3dB) = 1 / (2 * pi * C * R) = 1 / (2 * 3.14 * 0.68uF * 110Kohm) = 1 / ( 2 * 3.14 * 0.68E-6 * 110000) = 2.13Hz

I suggest to use values in the range 0.47uF to 2uF.



E180F - CV3898 old Mullard

    E180F - 6688 Siemens Gold Pins New Old Stock

  6Z9P - 6ZH9P - E180F



After the good result obtain in the last Inpol / Mofo project I have decided to test the switching power supply also in my best project.

I have got the same sound of the normal version but higher efficiency (about 20W saved for each channel), less cost (200 euro saved) and less noise (at 100-120Hz).

For each channel I have a MEAN WELL LRS-150-36.

It is a 150W Single Output Switching Power Supply with a variable DC output in the range 32.4 ~ 39.6V.

There are integrated protections for short circuit, overload, over volatge, over temperature.

This model has been choised because it is cooling by free air convection and give until 4.3A.

The power supply transformer for the driver stage is an 40W R-core model R26-09.

You can buy on Alixpress online shop.

The secondaries have 2 x 220V  50mA and 2 x 6.3V 0.8A.


I'm sure someone won't like the output capacitor but this is essential because on the output of the mosfet there is half the power supply voltage.

Any change to the design to eliminate this component, such as dual power supply or virtual ground, compromises the main characteristic of this circuit: the total decoupling from the power supply..
You must consider the power supply capacitor on the signal path also on any traditional amplifiers with dual power supply..
Obviously this capacitor must be of the highest possible quality and these are my choice::

The Nichicon KG capacitors has been used on all my last hybrid amplifiers and in the my Amplifier End I decided to eliminate the bypass capacitors 47uF Solen MKP originally used because the sound is much better without these.

I have chosen for the outptu capacitor the value 4700uF because this give a low frequency cut-off very low also on 4ohm load.

Ft(-3dB) = 1 / (2 * pi * C * R) = 1 / (2 * 3.14 * 4700uF * 4ohm) = 1 / ( 2 * 3.14 * 4700E-6 * 4) = 8.5Hz

Using this value the output impedance give an acceptable 0.2ohm at 100Hz and 0.8ohm at 20Hz (see simulation below), this is much better than any SE tube amplifier but if you want to get a best damping factor use 10000uF.




All the resistors on the signal path have 2 positions on pcb because ...

otherwise to get a slightly better result using the MK132 Caddock.




In order to dissipate all the heat generated by this amplifier in my case I chose this container by HiFi 2000.

Dissipante 04/300B 4U 10mm SILVER  
Product Code: 1NPD05300B

temperature coefficient 0,31 C/W per each side

Inner baseplate for Dissipante 300mm
Product Code: 1BASEPD300

 I used the HiFi 2000 company for almost all the mechanical processes and here there are some specifications used for this phase.

Here the cost of the chassie

1 x Dissipante 04/300B 4U 10mm SILVER (1NPD04300N) €149.00
1 x B) Drilling front panel 10mm (LAV10MM) €25.00
2 x A) Drilling panel 2/3/4mm (LAV4MM) €50.00
1 x Inner baseplate for Dissipante 300mm   €14.15


  Vandal Resistant  Push Button Switches


The panel cut out for thid button is 22mm, this is the same size of noval socket so this is the ideal choice for the 10mm front panel.




Here follows the final measurements on the complete PF2019 in this condition:

  • Voltage 36VDC 

  • Rbias 0.2ohm 5W  0.6V  = 3.0A


Here the frequency response on 4 and 8ohm load measured with ADALM2000 USB and Scopy software.












It is possible to find some components at lower cost but the interstage and output capacitors are important for the final result.

The cost of the chassie could be reduced using the HiFi 2000 Dissipante 04/300B 4U with 4mm front panel and doing mechanical work at home.

My configuration
description unit price quantity total (euro)
Vacuum tubes with shipping 16 2 32
Vishay MKP 50uF 500V  10 2 20
Components + pcb 110 1 110
Mosfet 3 12 
Interstage capacitors 40 2 80
HI-Fi 2000 chassie 10mm front panel + processing 250 1 250
Vandal Resistant  Push Button + relay 20 1 20
10000uF 63V 8 2 16
LRS-150-36   25  2 50 
Connectors 26 1 26

Cheap configuration
description unit price quantity total (euro)
Vacuum tubes with shipping 16 2 32
Vishay MKP 50uF 500V  10 2 20
Components + pcb 110 1 110
Mosfet 3 12 
Interstage capacitors (min acceptable) 10 2 20
HI-Fi 2000 chassie 4mm front panel + home made processing 150 1 150
Normal switch 6 1
10000uF 63V 8 2 16
LRS-150-36     25 50
Connectors 26 1 26