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 (154w):
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 05/300B with 2 heatsink with a coefficient of 0,18 C°/W it is possible dissipate 160W with an ambient temperature of 35 degrees and stay below 70 degrees (which I do not recommend to overcome).
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 (154w):
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 05/300B with 2 heatsink with a coefficient of 0,18 C°/W it is possible dissipate 160W with an ambient temperature of 35 degrees and stay below 70 degrees (which I do not recommend to overcome).
please allow me to hazard a guess, since film type resistors are sort of surface wound, parallelling them in opposite direction lowered the inductance some...
but i doubt that it is needed at all.....
Yes, parallelling them lowerer the inductance but change the direction is not necessary to have this effect.
The Power Follower pcb kit are available on Ebay shop at low cost.
I have provided the files for free so I do not take money from these sales.
I have provided the files for free so I do not take money from these sales.
I would like to have the 20Hz at 0dB or about.
It was just to say that since there are 2 other capacities on the circuit that determine the low cut-off I am not going to reduce this value which is the simplest and least expensive to keep high.
I use the current regulating device. CRD parts from Central Semi.
100v rating and is fast.
The chosen device is picked based on tube type ie low medium hi mu.
Usually ends up being 300ua to 1.5ma.
i wind my own, the choke in the photo has a 1.5inch center leg and stacked to 2 inches, about 500 turns #18 awg wire, dc resistance is around 2.7 ohms and inductance with dc flowing is guaranteed to be more than 100mH...
Here some simulations to compare my Power Follower to the MoFo / Inpol amplifier also using a cheap choke with only 50mH and Rdc=0.7ohm.
The main problem could be the parasite capacity of the inductance that I cannot simulate this without a real model so the high frequency band could be compromised in the reality.
https://www.diyaudio.com/forums/pass-labs/313649-build-mofo-240.html#post6078830
Surely efficiency is obtained but the big 2 advantages already mentioned are lost.
The output MOSFET poses more of a problem in the HF region than the choke . Always best to drive with a low impedence source or use a lateral MOSFET in the output stage . A layer-wound choke , wound similarly to a plate choke is pretty pointless for such low inductance values as those used for choke loaded source followers . I have used chokes up to 320mH with no issues , HF response is typically flat well past 20kHz
316a
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:
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.
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
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.
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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)
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)
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.
The power supply capacitor is not on the signal path so it is not critical.
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:
- Nichicon KG the my first choice
- Jensen Electrolytic these are the my second choice
for voltage < 100 and these are the best for voltage > 100
[*] BHC Slit Foil the my third choice
[*] The Mundorf M-Lytic
[*] Kendeil K01
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.