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the my Inpol / Mofo SE class A amplifier - version 3   
started in the Apr 2020



This is a 40-45W Class A Single ended no feedback amplifier.


Since in 1999 I saw the first review of the T.T. of Pathos this intrigued me, I had already made my Power Follower 99 and the sound performances were surprising but with the Inpol it was possible to double the efficiency.  
After 20 year I have created a new verson of the my amplifier the Power Follower 2019 because this has incredible sonic performance but has a big efficiency problem even if this is normal in a true class A single ended amplifier.
However, the idea of ​​using an inductance to double the efficiency was exciting, even if it meant losing the isolation from the power that my Power Follower has.
From the simulations I had verified that an inductance of about 100mH must be used with an air gap capable of manage at least 4A.
In the following years I started a lot of projects with the output transformers for valves and anode load inductors so I got the idea that the Inpol inductance had to be sectioned / stratified to reduce the parasitic capacities so the project has been forgotten.
I recently read Mike Rothacher's article about the Mofo project which got an excellent frequency response using a normal Hammond 193T, cost only 39$.
The Mofo project is very popular on the DIY audio forum (more than 500000 views) [Build This MoFo!].
The Mofo and Inpol are basically the same thing, a mosfet in the common source configuration with an inductance connected between the source and the ground. I don't think you can patent such a circuit because it is equivalent to any tube circuit.
For me it was necessary to check if these performances were confirmed even by using larger inductances with at least 100mH and capable of handling 5A like the Hammond 195T5.  



This are the specifications of the choke used for this project.

Hammond 195T5

Inductance: 100 mH

D.C. Current: 5A

Resistance:  0.64ohm

Weight:  about 5Kg
Dimensions:  95(h) x 117(l) x 120(d) mm



In the Power Follower we have a typical source follower (as an emitter follower but with a Mosfet) working in pure class A with a current generator to set the bias current and to fix the output point to half of power supply voltage (in the following schematic is 20V).
In the Inpol / Mofo there is the same source follower but in this case there is big choke connected to ground and output point is at about 2V.
To have the same output power we need in the first a power supply voltage of 40V and in the second the half only 20V.
The theoretical efficiency in one case is 25% and in the second 50% like a pure class A push-pull.
In reality with a dissipation of 100W the Power Follower have an output power of about 20W and the Inpol / Mofo give 40W.
You must consider that a vacuum tube single ended amplifier with an output power of 20W, for example the my GM70 SE, have the an energy consumption about 120W per channel.


The D1 and D2 are 18V 1W diode zener used to prevent save the mosfet when it receive bad input signal like a vacuum stage startup, in some mosfet like 2SK1058 these zener are integrated.

The phase on output terminals should be inverted because I am using a voltage amplifier that reverses the phase.

In the Power Follower the quiescent current is set  by the resistence on current generator source pin, it can be changed with the simple formula Iq = 0.65 / R where 0.65 is the Q1 transistor Vbe and there is a trimmer to set the source of upper mosfet to half of power supply voltage.

In the Inpol / Mofo there is a single trimmer to set the quiescent current.

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.

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.

For example the IRFP150 have these following differences:

The my voltage stage with the 6072A have an output impedante of 670ohm so the high frequency cut-off can be calculated with:

Ft(-3dB) = 1 / (2 * pi * C * R) = 1 / (2 * 3.14 * 1900pF * 670) = 1 / ( 2 * 3.14 * 1900E-12 * 670) = 125KHz

The my second voltage stage with the 12AX7 have an output impedante of 427ohm so the high frequency cut-off can be calculated with:

Ft(-3dB) = 1 / (2 * pi * C * R) = 1 / (2 * 3.14 * 1900pF * 427) = 1 / ( 2 * 3.14 * 1900E-12 * 427) = 178KHz

In the Power Follower 2019 I have used the IRFP150NPBF by Infineon (RS cod. 541-0856) with only 1900pF.

Many persons will think to drive this current amplifier with E88CC in SRPP (Totem pole)  or D3a/E182CC/5842/6C45 in single ended.

Always consider the high frequency cut-off because a D3a or a 6C45 have an output impedance near to 2Kohm so we will have:

Ft(-3dB) = 1 / (2 * pi * C * R) = 1 / (2 * 3.14 * 1900pF * 2Kohm) = 1 / ( 2 * 3.14 * 1900E-12 * 2000) = 41KHz


Follows the simulation of the current amplifier output impedance that is about 80mohm with IRFP150NPBF and it increase in the low frequency because there is the output capacitor.

The value of the choke is important to keep a good low frequency band, using 10mH instead of 100mH mean -0.5dB at 20Hz.

but the main problem with little choke is the distortion at low frequency, folllows the simulation with 50mH and 100mH.


Also the value of the output capacitor must be enough to have no loss at low frequency so the 10000uF until 4ohm load,
folllows the simulation with 3300uF and 10


Follows a table to calculate the transformer voltage (Vac), bias current (Ibias) and power to dissipate (Pd).

Load Vrms Pout Vp Irms Ap delta Vdc Ibias Vac Pd
8 11 15.1 15.5 1.38 1.94 1.15 17.8 2.2 15.9 39.8
6 11 20.2 15.5 1.83 2.59 1.15 17.8 3.0 15.9 53.0
4 11 30.3 15.5 2.75 3.88 1.15 17.8 4.5 15.9 79.5
8 12 18.0 16.9 1.50 2.12 1.15 19.5 2.4 15.9 47.3
6 12 24.0 16.9 2.00 2.82 1.15 19.5 3.2 15.9 63.1
4 12 36.0 16.9 3.00 4.23 1.15 19.5 4.9 15.9 94.7
8 16 32.0 22.6 2.00 2.82 1.15 25.9 3.2 22.6 84.1
6 16 42.7 22.6 2.67 3.76 1.15 25.9 4.3 22.6 112.2
4 16 64.0 22.6 4.00 5.64 1.15 25.9 6.5 22.6 168.3
8 17 36.1 24.0 2.13 3.00 1.15 27.6 3.4 23.9 95.0
6 17 48.2 24.0 2.83 4.00 1.15 27.6 4.6 23.9 126.6
4 17 72.3 24.0 4.25 5.99 1.15 27.6 6.9 23.9 190.0

Obviously for a 5A bias current the transformer must be min 10A.




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.

I thought of this voltage amplifier looking for something that would sound great without using anodic inductors and interstage transformers.

Compared to the 100W hybrid amplifier, in this case we don't need an extreme dynamic to drive the current amplifier, 17-18Vrms are enough.

This new release can use D3a, 6Z51P and 6Z9P all are pentodes used in the triode mode configuration.

There is a good article about these Russian pentodes on BartolaŽ Valves.

The socket is not the same for both the tubes so it is necessary modify the pcb to use the 6Z51P.

The D3a is almost impossible to find and very expensive, the 6Z51P and 6Z9P are very cheap because there are large stock on russian market.


 I suggest to buy a large stock of 6Z51P to make a selection on voltage gain.

  • 6Z9P

  • 6Z9P-E

  • 6Z51P (in my case too much voltage gain)

  • D3a (in my case too much voltage gain)


This is the same schematic used in the Power Follower with E180F but here it is necessary another tube to get enough gain and  dynamic range.



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

To support the high dissipation has been used 2 mosfet in parallel.


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

R1,R2,R4,R5  1800ohm 1/4W 1%
R3           10Kohm  2W
R6,R7        220K    1/4W 1%
R9,R11       0.22ohm 5W       Mills RMA5 

C1           4700uF 16V   Nichicon  model UKA1C472MHD
U$1          0.68uF 400V  Clarity Cap CMR or other audio grade MKP

R8           10Kohm multi turn

IRFP240PBF (it would be desirable to make a selection based on current)

D1,D2        zener 15V 1W                 
D3           zener 10V 1W

F1           fuse 10A FAST with fuse holder Mouser 534-4628

Cout         min. 10000uF 50V Nichicon KG   Mouser

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)


This is the complete schematic of the voltage amplifier including the power supply section allocated in the same pcb.


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

Mount the resistances in parallel configuration in opposite phase / 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     33Kohm   2W                      
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  
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   

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.

  • Clarity Cap CMR MKP 630VDC  
    This is the most expensive capacitor of the set with a really detailed sound and a good sound stage.
    After the test I chosen to use this but I recommend it only to those who have a really good source because otherwise it could sound too tiring. 
    This is definitely a totally transparent component that adds nothing to the signal.
    The micro details that can be discovered in already known tracks are impressive.

  • Clarity Cap CSA MKP 630VDC 
    This is the second choice, it is an incredible value for money,
     in some case it could be preferable to the first because it give a good detailed sound, of course not like the first, but it is more soft so this effect helps in the case of digital sources a bit harsh like CD player.

  • Clarity Cap MR MKP 
    A good result but some details are lost compared to the first two.
    Here the evaluation expressed is in contrast with the verdict expressed by Humble Homemade Hifi.

  • Clarity Cap ESA MKP 
    This capacitor has a very convincing value for money, there is a big difference from the MR and CSA.

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.



This is the power supply design for the current amplifier, it use two MeanWell LRS-150-24.


I have add a capacitor on DC output the type Nichicon 4700uF 50V KG.

As output capacitor has been used a Nichicon 10000uF 35V LKG1V103MESBBK 30x50mm.

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.




You must consider the power supply capacitor on the signal path like the output capacitor so both must be Audio grade.

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 output capacitor the value 10000uF 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 * 10000uF * 4ohm) = 1 / ( 2 * 3.14 * 10000E-6 * 4) = 4Hz

Using this value the output impedance give an acceptable 0.15ohm at 100Hz and 0.8ohm at 20Hz, this is much better than any SE tube amplifier but if you want to get a best damping factor use 33000uF to have 0.25ohm at 20Hz.




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

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



I am using this module got on Alixpress because it incluse a soft-start and a temperature protection at 75°.



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

Dissipante 05/300B 5U 10mm SILVER  
Product Code: 1NPD05300B

temperature coefficient 0,18 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.

In a second phase I have add another hole on the heatsinks for the temperature sensor (see below in the photos section)

Here the cost of the chassie

1 x Dissipante 05/300B 5U 10mm SILVER (1NPD05300B) €187.00
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

Totals Sub-Total: € 276.15
BRT for Italia (Weight: 12.00kg): €15.00
IVA 22%: €56.83
Total: €355.20


Vandal Resistant  Push Button Switches

Manufacturer Apem Manufacturers 
Part No. AV021003C900
RS  No. 174-6381

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.



I have tested 3 different types of insulators for the assembly of mosfets.

Environment 2 x IRFP240 total 30.9VDC 3.66A 113W to dissipate on Hi-Fi2000 heatsink 3 units H120 (smaller than what will used).

The senson was in the center of heatsink.

Sil-Pad Bergquist SP400-0.007-00-104 (RS 707-3367)

time (min)  degrees (°C)
   0          27
   5          42
  10          52
  15          58
  20          62

 Thermally Conductive Insulator Aavid 4180G + Thermal Interface Products Accessory / Grease Aavid 101800F00000G

time (min)   degrees (°C)
      0          27
   5          42
  10          53
  15          57
  20          60

Kapton film + Thermal Interface Products Accessory / Grease Aavid 101800F00000G

time (min)  degrees (°C)
   0         27
   5         42
  10         52
  15         57
  20         60

So the Sil-Pad it has been confirmed as valid and easier to use.



Obviously before the first start up the bias trimmers must be set for the minimum current so turn the trimmer like show.


I think the best mosfet for this project are:

I have found this values of voltage on Rs (source resistor) on my IRFP240 so I have selected the first for one channel:




2 x IRFP240 with Rg=900ohm and Rs=0.22ohm

total bias 4A 30VDC.

about 40W on 8ohm thd about 2% with perfect decay
about 40W on 4ohm thd about 2% with perfect decay

Rout about 0.2ohm

Frequency response 3.5Hz - 200KHz at -3dB on 8ohm with Cout=10000uF
Frequency response    7Hz - 200KHz at -3dB on 4ohm with Cout=10000uF

Here some plot with the tube 6Z51P

Ra=8.25Kohm (4 x 33Kohm)


Ra=8.25Kohm (4 x 33Kohm)


Here some plot with the tube 6Z9PE

Vin = 0.6Vrms
Vout = 17Vrms
Ft(-3dB) = 142Khz
Ra=8.25Kohm (4 x 33Kohm)

Vin = 0.66Vrms
Vout = 19Vrms
Ft(-3dB) = 142Khz
Ra=9.4Kohm (4 x 39Kohm)







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

My configuration
description unit price quantity total (euro)
Vacuum tubes    2  
Solen MKP 33uF 400V 10 2 20
Components + pcb 100 1 100
Mosfet 4 4 16
Interstage capacitors 40 2 80
HI-Fi 2000 chassie 300 1 300
Vandal Resistant  Push Button 20 1 20
Soft-start + termal protection 24 1 24
LRS-150-24 22 2 44
Hammond 195T5 100 2 200
4700uF 50V 7 2 14
10000uF 35V 8 2 16
Connectors 26 1 26