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Old 14th February 2020, 01:00 PM   #51
audiodesignguide3 is offline audiodesignguide3
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Join Date: May 2019
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.

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:
  • IRFP150 IRF Ciss=2800pF
  • IRFP150 Fairchild Ciss=2000pF
  • IRFP150 Vishay Cis=2800pF
  • IRFP150NPBF Ciss=1900pF
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 this new design I have used the IRFP150NPBF by Infineon (RS cod. 541-0856) with only 1900pF.


More info are available for this project.

Last edited by audiodesignguide3; 14th February 2020 at 01:08 PM.
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Old 14th February 2020, 01:10 PM   #52
audiodesignguide3 is offline audiodesignguide3
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Join Date: May 2019
This is the complete list of the output stage.


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 1Kohm 1/4W 1% Mouser 594-MBB02070C1001FC1
R15 100Kohm 1/4W 1% Mouser 594-MBB02070C1003FCT
R13 500ohm 1/4W 1%
R8 10Kohm 2W Mouser 660-MO2CT631R103J

Q1,Q2,Q3 IRFP150NPBF RS 541-0856
Q4 MJE340 Mouser 863-MJE340G

D14,D5 zener 18V 1W Mouser 78-1N4746A
F3 fuse 5A FAST with fuse holder Mouser 534-4628

C1,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 Mouser 647-LKG1H472MESCBK
Cpsu min. 4700uF or 10000uF 63V
Dpsu DSA70C200HB schottky diode RS 125-8033
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)
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Old 14th February 2020, 01:11 PM   #53
audiodesignguide3 is offline audiodesignguide3
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Join Date: May 2019
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:

Click the image to open in full size.
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.
The power supply capacitor is not on the signal path so it is not critical.
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