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Extreme Hi-End Hybrid Amplifier
started on January 1 st , 2014


This project collects all the experiences made ​​in these years then it born with all these characteristics:

This project is based on the last version of the my 100W hybrid amplifier but here some aspects has been changed to increase the sound quality.
Now the throne of the Amplifier End could collapse because this amplifier should have the same details of the Amplifier End but a better control with complex signals.
All the pcb can be produced at low cost by Quanghao or you can download the Eagle files to produce these in your local area.

Update 17 April 2014

This evening there was the first listening session of this amp and was attended by a friend of mine with a great experience in audio recording and playback.
This friend of mine is also the owner of the Sony VFET I recently updated with new power CLC and pre-ampl bypass.

The impression is listening a pure class A amplifier very refined but without the limitations in the dynamics of these type of amplifiers.
The level of detail and the transients speed are comparable to the Amplifier End but you immediately notice the superiority to control the speakers and and every kind of music is played without problems.
The sound is not soft but it is also not cold, this effect was probably obtain by using MKP on voltage amplifier stage and electrolytics capacitors on current amplifier.
The channels separation is perfect like a pure dual mono operation.
Obviously this amp to achieve its full potential requires a source without compromise like the new ES9018 DAC born with the collaboration with Quanghao.

Update 21 April 2014

At the switch-on there is no noise and no bump on loudspeakers because this amplifier has been design to have a very soft start.
During the switch-off phase there is a little bump but if you switch-off the amp during the music play you will have no noise because the output stage will go down quickly.
Although this amp. is self protected with 2 ways you can add an extra protection that shuts down the amplifier in case of problems.
At this time I still have not done the output short circuit tests but in the diamond buffer design the output current is limited by the driver bias.



The voltage amplifier use a single ended vacuum tube stage with anode choke to have a very low distortion and best frequency band as seen in the first version.
Also in this project I used the unsurpassed D3a connected to triode, just look at the following measures to realize the incredible potential of this tube.

We will use less output voltage because 40Vrms mean 40*40/8ohm = 200W and the target is about 100W.
I am using Lundahl LL1668/50mA anode chokes, these are not the standard, you could use the normal LL1668/25mA but probably it is necessary adjust the cathode capacitor.

If we use an anode choke or an inter stage transformer is necessary consider the effects of the inductance because you create a RLC circuit with a specific resonance.
This is the formula to calculate the resonance frequency


so we need to keep this value very low to eliminate strange peak on frequency band.
Using the Ck=1000uF and L=50H (LL1668/50mA) we get a resonance frequency of only 0.7Hz.
The value of the internal resistance of the tube can modify the peak width generated in the resonance.

The current amplifier is a perfect Diamond Buffer used in some chip like the LH0002 and the BUF634.
This configuration has been analyzed very well in the article Bulding better buffer on Electronics World november 1992.
The same design as been used in many Audio Research amplifiers like the Audio Research D100 and the Audio Research D-400.
Also the Pioneer A-09 and the Pioneer M-6 use a Diamond Buffer on output stage but these implement dynamic current generators with some stability problems.

This current amplifier have an input impedance near to 20Kohm and it absorbs a max peak current of 2mA also when it drive 2 ohm load.

I have add a power supply regulator to eliminate any ripple coming from main capacitors after the diode bridge and it allow a soft start with these advantages:

This regulator integrate a DC loudspeaker protection copied from an old Bartolomeo Aloia design to shunt the regulator capacitors so switch-off the output stage if there are fault on output transistor.

The op-amp in the circuit is a DC servo to eliminate DC offset and it work in very low frequency band.

R1,R25                            56Kohm  1/4W  1%
R3,R36                            280ohm  1/4W  1%
R26,R27                           2700ohm   2W
R34,R35                           15ohm     2W
R23,R33                           270ohm    1W
R24,R28                           5600ohm   2W
R5,R6,R7,R8,R17,R18,R19,R20       0.47ohm   5W
R15,R16                           220K    1/4W  1%
R55,R56                           2200ohm 1/4W  1%
R51,R52,R53,R54                   470ohm  1/4W  1%
R4,R13                            6800ohm   2W
R49,R50,R21,R22                   10Kohm  1/4W  1%

R11,R10,R12                       0ohm  (jump)

C6,C8,C9,C14                      0,47uF  63V  Wima
C1,C2,C10,C11                     100uF  25V
C3                                empty
C5                                0.1uF   100V   Vishay MKP1837
C7,C17,C4,C24,C18,C20,C25,C19     4700uF 63V Nichicon  KG  type 25x50mm
C15,C16                           1000uF 63V

D1,D2                             Zener 15V  1W
D9,D13                            0ohm  (jump)
D8,D12,D11,D10,D6,D7              1N4148

U1                                2N5550 o 2N5551
U2                                2N5401

Q1,Q3,Q16,Q18                     MJE15033 (PNP)
Q2,Q4,Q14,Q17                     MJE15032 (NPN)
Q9,Q5,Q6,Q7,Q13                   NJW0281G (NPN)
Q12,Q11,Q8,Q10,Q15                NJW0302G (PNP)

IC1                               OPA134 (you can use many others single op-amp)


As mention in the characteristic list at the begin of this page in this project the passive components are without compromise on any section.

On vacuum tube cathode there is an Sanyo OS-CON capacitor, my choice is 2 x 560uF 4V available in the Parts Connexion web store.

The SANYO OS-CON is an aluminum solid capacitor with high conductive polymer or organic semiconductor electrolyte material.

Due to the solid electrolyte used, OS-CON achieves low Equivalent Series Resistance (ESR), excellent noise reduction capability and frequency characteristics.

Normally the capacitors with lower voltage sound better so I prefer to use a 560uF 4V   than a 560uF 16V if there is only 2V on cathode.

The interstage capacitors must be of the highest possible quality because most of the sound quality depends on these.

My choice is the Jantzen Audio Superior Z-Cap Red classified with only 10 by Humble Homemade test but in my opinion these are much more transparent and therefore more valid than others such as Mundorf Supreme MCap.

I use the Jantzen Audio Superior Z-Cap Red in all my loudspeaker crossovers.

As tested in the first version the anode choke give the most low distortion and the best frequency band, also the interstage transformer it more invasive so it should be used only if it is essential in the circuit as a phase shifter, see the Amplifier End.

The LL1668/50mA with an inductance of 50H, an air gap to support a current of 50mA and a static resistance of 680ohm give a sure result.

All the resistors in the current amplifier has been duplicated to use for any resistor a parallel of two components mounted with opposite direction to reduce parasitic inductance and electromagnetic flux.

I learned this method many years ago by the owner and designer of a famous American factory of amplifiers and it use the same also with Solen interstage capacitors.

The alternative to this method is to use Caddock MK132 with advantages.

About the power supply capacitors near the output transistors there are no alternatives to the NIchicon KG capacitors because the my last experience made ​​the Amplifier End has demonstrated that these components without bypass have unsurpassed musical performance.

I have used 8 x 4700uF 63V NIchicon KG capacitors for each channel so we have a great reserve of power available in a very short time.

The 0.47ohm output resistances present on the emitter of final stage transistors are necessary to compensate some differences on output transistors, stabilize operating point and to merge all the outputs.

In order to have no bad effects on the sound the my choice has been the MILLS MRA-5  non-Inductive wirewound resistors with 5 watt and with 1% tolerance.

The output stage pcbs has been design to optimize the signal path with large wire for the output stage and I have increased the thickness of these tracks of the PCB with tin (see photos).

I have some output stage pcbs and I could sell these at 40$ for a pair + shipping.

Using the transistors specified I have on any 0.47ohm a voltage about 0.180V so the bias current is 0.38A so 4 x 0.38A = 1.5A.

If you will use a 2 x 36VAC transformer the power to dissipate is near to 150W for each channel.

Any heatsink used have a Rt C/W = 0,48 so 0.24 for each channel so the temperature will be     0.24 * 150W + Tenv = 36C + 30C = 66C.





Here all the files to simulate this circuit with LTSpice software.

Add these lines in the file C:\Program Files\LTC\lib\cmp\standard.bjt to have the transistors models:

.MODEL Qnjw0302g pnp IS=5.16751e-16 BF=114.657 NF=0.895716 VAF=50.2189 IKF=6.409 ISE=3.9641e-15 NE=4 BR=1.47167 NR=0.923324 VAR=255.567 IKR=6.34299 ISC=3.96408e-15 NC=2.82194 RB=2.66347 IRB=0.1 RBM=2.0828 RE=0.0001 RC=0.0652395 XTB=1.45322 XTI=1.08126 EG=1.05 CJE=2.14504e-09 VJE=0.4 MJE=0.376227 TF=2.16864e-09 XTF=1000 VTF=843.737 ITF=501.348 CJC=5e-10 VJC=0.95 MJC=0.251547 XCJC=1 FC=0.8 CJS=0 VJS=0.75 MJS=0.5 TR=1e-07 PTF=0 KF=0 AF=1

.MODEL Qnjw0281g npn IS=4.36849e-12 BF=98.1488 NF=1.01332 VAF=37.9046 IKF=9.71849 ISE=1e-16 NE=1.8326 BR=0.79921 NR=1.09994 VAR=339.743 IKR=5.77305 ISC=1e-16 NC=2.71592 RB=2.74892 IRB=0.33289 RBM=2.74892 RE=0.000344671 RC=0.03203 XTB=1.7742 XTI=1.12262 EG=1.20598 CJE=3.66793e-09 VJE=0.74806 MJE=0.85 TF=2.27115e-09 XTF=1000 VTF=912.955 ITF=296.602 CJC=5e-10 VJC=0.95 MJC=0.270858 XCJC=0.98254 FC=0.8 CJS=0 VJS=0.75 MJS=0.5 TR=1e-07 PTF=0 KF=0 AF=1

.MODEL Qmje15032 npn IS=3.7344e-10 BF=86.8313 NF=1.23974 VAF=31.5491 IKF=9.1678 ISE=9.2499e-12 NE=3.28127 BR=5.59346 NR=1.33161 VAR=2.1791 IKR=5.15023 ISC=4e-13 NC=4 RB=9.54492 IRB=0.1 RBM=0.1 RE=0.000568481 RC=0.0931741 XTB=0.737036 XTI=1.04983 EG=1.206 CJE=3.05969e-09 VJE=0.648491 MJE=0.352663 TF=4.94819e-09 XTF=1.50001 VTF=1.0001 ITF=0.999982 CJC=3.00108e-10 VJC=0.600021 MJC=0.40991 XCJC=0.8 FC=0.534651 CJS=0 VJS=0.75 MJS=0.5 TR=1e-07 PTF=0 KF=0 AF=1

.MODEL Qmje15033 pnp IS=7.51228e-10 BF=134.35 NF=1.25737 VAF=12.5778 IKF=1.88497 ISE=7.74267e-12 NE=3.34528 BR=5.14173 NR=1.47488 VAR=1.4505 IKR=7.47186 ISC=3.25e-13 NC=4 RB=4.37743 IRB=0.1 RBM=0.1 RE=0.000332989 RC=0.381218 XTB=0.223027 XTI=1 EG=1.05 CJE=3.06005e-09 VJE=0.64838 MJE=0.352991 TF=4.78203e-09 XTF=1.50001 VTF=1.00006 ITF=0.999988 CJC=3.00101e-10 VJC=0.600019 MJC=0.409916 XCJC=0.8 FC=0.534975 CJS=0 VJS=0.75 MJS=0.5 TR=1e-07 PTF=0 KF=0 AF=1


Here the output of the power supply simulation, dual 33VAC, load 4ohm, 20Hz sine wave output 30Vp => 113W and only 10000uF after diode bridge.

Here the output of the power supply simulation, dual 33VAC, load 4ohm, 20Hz sine wave output 30Vp => 113W and 22000uF after diode bridge.


The power supply for the current amplifier use two 500VA toroidal transformers.

The good power supply transformers for audio should have an electromagnetic flux 20% lower than normal (14000 gauss instead of 17000).

I have used a rectifier module based on four ON Semi MBR40250 Schottky diode to create a full wave bridge rectifier.

Here a good description of the benefits derived from the use of Schottky diodes in audio amplifier.

The same diodes as been used in my Amplifier End but you can start to assemble this project also with a common diode bridge like the IRF 36MB60.  

After the diode bridge there are a pair of 10000uF 63V (size: 50x80mm) capacitors to create a first soft rectification.

As explained in the simulation before using larger capacitors does not bring us benefits.

About the capacitors I don't leave many choices because only with these types you can be sure to get the best result and no bypass will be necessary.

  • Nichicon KG
  • Jensen  electrolytic

In this project I have used 8 x 4700uF 63V KG (size 25x50mm) on any channel and these are very near to the output transistors.





To get the max sonic performances the my choice is a slow turn on power supply for the filaments using a common LM317 and a virtual battery operation power supply for the anodic with a power mosfet.

For this power supply I have used the R-CORE model R26-90 by DiyClub.Biz with primaries 2 x 115v and the secondaries 0-165v(0.2A) and 2 x 0-9v (1.1A).

R6,R8                             1ohm     2W
R74                               47Kohm   1/4W 
R7                                1Kohm    1/4W
R9                                220ohm   1/4W
R12                               100ohm   multi-turn trimmer
R4,R5                             10Kohm   1/4W  1%
R3                                470Kohm  1/4W  1%
R1                                39K      2W
R2                                100K     3W

CY1,CY2,CY3,CY4                  10nF   400V
C2                               150uF  400V
C1                               33uF   400V
US10,US11                        56uF   400V Solen MKP
C3,C7                            2200uF 25V
C4,C5,C6                         220uF  16V

D4,D6,D7,D8                       UF5408

D1,D2,D3                          1N4007
B1                                diode bridge  3A 100V
IC1                               LM350
Q1,Q3                             IRF740




The cabinet is a Hi-Fi 2000 model  04-400B Silver 10 (1NPD04400B) buy on the Audiokit webshop.

     Formato: 4 Units
     1x Frontal Panel in aluminium 10mm oxidised silver 450 mm
     1x Back in metal painted black anti-scratch
     4 Lateral Heatsinks 200mm oxidised black  (each with Rt C/W = 0,48)
     2x Covers in metal painted black anti-scratch
width: 450mm internal 360mm
depth: 400mm Internal
height: 165 mm

weight:: 11,5 KG

To add a 1BASEPD400 Inner pierced Base for Pesante Dissipante 400mm.

To create a base to fix the transistors on heatsink  I have used a pair of 3PDA0302 Rear Panel of Dissipante 3U aluminium cut on both sides at 28cm to have a pair of 3mm panels 12 x 28 cm.






Vandal Resistant  Push Button Switches

RS Stock No. 174-6381

Manufacturer Apem Manufacturers

Part No. AV021003C900

The panel cut out  is 22mm, the same of noval socket, so this is the ideal choice for the 10mm front panel because you can use the same cutter.


Assembled Soft Starter Start-up For Power Amplifier,W/ Thermal Dectector,Protect buy on Ebay shop at 30$.
Attention there are some similar product and not all work with a push-button, see these images to compare the products.


It is possible to find some components at lower cost, in red the most important for the final result

description unit price quantity total (euro)
D3a 25 2 50
Solen 56uF 250V 20 2 40
HI-Fi2000 chassie 200 1 200
500VA transformers 100 2 200
30VA transfomers  40 1 40
10000uF 63V 15 4 60
4700uF 63V KG 6 16 96
transistor 36 2 72
LL1668 90 2 180
Z-Cap 15 2 30
other 100 1 100
DACT 150 1 150