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This is a 40-45W Class A Single ended no feedback amplifier.
This are the specifications of the choke used for this project.
Inductance: 100 mH
D.C. Current: 5A
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.
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
At 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:
IRFP150 IRF Ciss=2800pF
IRFP150 Fairchild Ciss=2000pF
IRFP150 Vishay Cis=2800pF
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 10000uF.
Follows a table to calculate the transformer voltage
(Vac), bias current (Ibias) and power to dissipate (Pd).
Obviously for a 5A bias current the transformer must be min 10A.
The presented topology, hasNO 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.
Both the following designs give good performances (voltage gain and output impedance), the tubes used have a good reputation for sound quality and the vacuum tubes are in the current production.
Obviously we must not forget the connection of the reference of the ground filaments because each valve has its maximum Vkf (cathode - filament voltage) and in this circuit the second stage has the cathode at about half of the supply voltage therefore a reference will be chosen between ground and this cathode (so about 80V for the 6072A and 90V for the 12AX7).
This 6072A driver powered at 280V and with an
input signal of 0.58Vrms give about
18Vrms with 1% thd.
This 12AX7 driver powered at 280V and with an input
signal of 0.33Vrms give about
19Vrms with 0.56% thd.
Here the distortion is lower but has been apply a local feedback on first stage becasue there is no cathode capacitor.
FINAL SCHEMATIC AND PCB
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
Q1,Q2 IRFP240PBF (it would be desirable to make a selection based on current)
D1,D2 zener 15V 1W
D3 zener 10V 1W
fuse 10A FAST with fuse holder Mouser
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.
Ebay shop for the PCB same of the PF2019
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 100Kohm 1/2W 1% Mouser
R5,R6,R22,R23 220Kohm 1/2W 1% Mouser 594-MBB02070C2203FCT
R3,R4,R20,R21, 1000ohm 1/4W 1% Mouser 594-MBB02070C1001FC1
R7,R8,R24,R25 5600ohm 1/4W 1% <<< 5600ohm for 6072 and 4000ohm for the 12AX7
R15,R16,R31,E32 220ohm 1/4W 1%
R17,R33 3.3ohm 2W Mouser 660-MOSX2CT52R3R3J
R12,R30 470Kohm 1/4W 1%
R14,R29 150Kohm 3W
R13,R28 27Kohm 1W
220uF 6.3V OS-CON
<<< not use for 12AX7
C2,C4 33uF 400V Mouser 647-UVY2G330MHD
C5,C6 100uF 400V Mouser 647-LGU2G101MELZ
CY1-CY8 10nF 440VAC RS 335-066 o 80-R474I210050A1K
U$3,U$6 IRF840 Mouser 844-IRF840APBF
U$11,U$7 33uF 400V Solen MKP
D1,D2,D3,D4,D6,D7,D8,D9 UF5406 Mouser 625-UF5408-E3
D5,D10 zener 10V 1W
KK1,KK4 Extruded Style Heatsink for TO-220 Mouser: 532-513102B25
Isolator Bergquist SP400-0.007-00-54 RS 169-2177
Isolatos TO-220 nylon platstic Insulator hole size M3
63862-1 (CUT STRIP) by
/ 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.
In order to keep the Vkf under the max value it is necessary add 4 resistors 2 x 100K 2W and 2 x 39K 1W (6072A) or 2 x 47K 1W (12AX7), see photo1 and photo2.
12AX7 - 6072A socket.
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.
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.
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) =
I suggest to use values in the range 0.47uF to 2uF.
Here 3 different 6072A tubes of current production and of course each has a different sound characteristics.
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.
OUTPUT AND POWER CAPACITORS
You must consider the power supply capacitor on the signal path like the output capacitor so both must be Audio grade.for voltage < 100 and these are the best for voltage > 100
BHC Slit Foil the my third choice
The Mundorf M-Lytic
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
SOFT START AND TEMPERATURE PROTECTION
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
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)
Thermally Conductive Insulator Aavid 4180G +
Thermal Interface Products Accessory / Grease Aavid 101800F00000G
time (min) degrees (°C)
time (min) degrees (°C)
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:
IRFP150 in case of a single mosfet
Output power until 25-30W
Rout=85mohm (no Rs)
Ft=200KHz (Rg=470ohm - driven by my 6072A)
IRFP240 for the parallel
Output power until 40-50W
Ft=220KHz (Rg=900ohm - driven by my 6072A)
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 - 220KHz at -3dB on 8ohm with Cout=10000uF
Frequency response 7Hz - 220KHz at -3dB on 4ohm with Cout=10000uF
24 Oct 2020
Last evening the first listening session of the new version of my Inpol / Mofo with the 195T5 chokes and the MeanWell LRS-150-24 switching power supplies. The result was surprising because with these switching power supplies improve a lot the result. Probably the 500VA was not enough big for the both channels with bias 30VDC 4A but I have no enough space to use two 500VA transformers. Now the Inpol / Mofo it has almost reached the levels of my reference the PF2019, however if the Inpol / Mofo has a lower depth of scene and some less micro details. Obviously now we have to ask ourselves whether if is better to build the PF2019 or the Inpol / Mofo. The Inpol / Mofo with its doubled efficiency represents an obligatory choice if you need 40-50W but I think that if 20-30W are enough for your use the PF2019 is the right choice even if it consumes twice as much. In the driver stage has been tested 6072A of two different companies JJ and EH. The source used was HQPlayer + Network Audio DSC pure DSD DAC with permalloy output transformers + DACT 10Kohm.