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High Power Hi-End Hybrid Amplifier End 2023
started on November 20 st , 2022


After checking the enormous potential of this amplifier , see the
Extreme Hi-End Hybrid, I have decided to reengineering the project with new pcbs which allow to use a another driver stage with higher sonic performances and a more compact chassie.

The previous versions use the vacuum tube D3a Siemens which are now very hard to find, here I use the 6Z51P / 6Ж51П like my new Amplifier End 2022, these tubes are very cheap, sound very good, have a higher voltage gain and easy to find in large stock in Ukraina Ebay shop.

The result of listening in the second configuration with the mosfet was higher than expected, deviating a lot from the original version.

Switching the my hybrid amps from regular tubes like the 12ax7 and 6072a (1.6-1.7mA/V) to high transconductance tubes like these triode connected E180F, D3a, 6Z9P and 6Z51P (30-35mA/V) pentodes has dramatically improved the sonic quality.
In the last 2 amplifiers, this one and the Solidstate 2022, the bandwidth widening and the significant increase in slewrate have created a new milestone.

We have an enlargement of the sound scene, an even more extreme micro-detail without adding harshness, an attack and decay of the sounds that involves you.

After the positive results obtain by Ronan Daly about the use of switching power supply tested on the current amplifier in the previous release I also decided to test this but but only after a direct comparison.

This amplifier has regulators on the power supply that filter out any noise like in the my Power Follower so the switching power supply is out of the signal path.

This project collects all the experiences made ​​in these years then it born with all these characteristics:
All the pcb are available on Ebay shop or you can download the Eagle files to produce these in your local area

Here the link to buy the complete pcb set

I don't get money from this Ebay shop.

The output stage pcbs has been design to optimize the signal path with large wire for the high current signal.

You should increase the thickness of the high current pcb tracks with tin (see photos).

If possible add your comment on this amplifier or post photos of your Hybrid Amplifier End 2023 on: / Hybrid Amplifier End 2023


The old configuration
use a single ended vacuum tube stage with anode choke to have a very low distortion.
I am using here the Lundahl
LL1667/15mA instead of LL1668/25mA used in the previous versions but both are valid choices.

●  LL1668/25mA  with windings in series have 100H  680ohm and an air gap for 25mA
●  LL1667/15mA  with windings in parallel have  67.5H (270/4)  600hm  and an air gap for 30mA

For this design will use a tube with high amplification factor and a low internal resistance < 3000ohm.
The low frequency limit can be calculated with this simple method.

Ft(-3dB) = R / ( 2 * pi * L ) = 3000 / ( 2 * 3.14 * 67.5 ) = 7Hz

If we use an anode choke or an inter stage transformer is necessary to consider also the effects of the cathode capacitor because you create a RLC circuit with a specific resonance.

Follows the formula to calculate the resonance frequency:
so we need to keep this value very low to eliminate strange peak on audio low frequency band.

W0 = 1 / radq( L * C ) = 1 / radq( 67 * 470u ) =  1 / radq( 67 * 0.00470 ) = 5.63

F0 = W0 / ( 2 * pi ) = 5.63 / ( 2 * 3.14 ) = 0.9Hz

Here an example of this effect on another project where the Ck was too little.

The new voltage stage use a single ended vacuum tube stage with anode resistor followed by a mosfet
to prevent the voltage amplifier from being affected by the current delivered to the speakers.

The mosfet isolate the 2 stages perfectly and even if the distortion has increased having eliminated the anode inductance we have a constant value.
The frequency response also benefits from the fact that we do not have the parasitic capacitances of the inductance on the signal path.

The moseft used is the IRF820 used also in my last headphone amplifier where it has proven to be extremely transparent on the signal in the configuration as a follower.

Follows a table with the distortion and the frequency response of both voltage amplifiers with different load, the 40Vrms of this test are far above those necessary because they correspond to 200W into 8ohm .

I will repeat soon this comparation with also the output amplifier connected.

Tube configuration Va (V) Rk (ohm) Ia (mA) Ck (uF) Vin (Vrms) Vout (Vrms) Load (ohm) Thd (%) Frequency (Hz)
6Z51P LL1667/15mA in parallel 140 68 15 470 0.53 42 100K 0.64 5 - 500K
6Z51P LL1667/15mA in parallel 140 68 15 470 0.53 41 50k 0.78 5 - 500K
6Z51P LL1667/15mA in parallel 140 68 15 470 0.53 39 25k 1.04 7 - 449K
6Z51P Ra=11.75K   IRF820  Rs=6.75K   Id=20mA 320 68 15 470 0.60 40 100K 1.60 0 - 2.4M
6Z51P Ra=11.75K   IRF820  Rs=6.75K   Id=20mA 320 68 15 470 0.60 40 50K 1.60 0 - 2.4M
6Z51P Ra=11.75K   IRF820  Rs=6.75K   Id=20mA 320 68 15 470 0.60 40 25K 1.60 0 - 2.4M
6Z51P Ra=11.75K   IRF820  Rs=6.75K   Id=20mA 320 68 15 470 0.60 40 10K 1.60 0 - 2.4M

How many voltage gain you need on vacuum tube stage ? it is easy to calculate.

For 100W on 8ohm you need this output voltage:  P = V * V / R so V = sqrt(P * R) = sqrt(100 * 8 ) = 28.3Vrms.

The current amplifier loss about -0.65dB correspond to 0.93x so on output stage input you need 28.3 / 0.93 = 30.5Vrms.

I suggest to drive this amplifier directly with your source like DAC, CD and phono pre-ampl so the sensibility should be about 400mV like normal integrated amplifiers.

Now you know the minimun voltage gain necessary on tube stage  Vout / Vin = 30.5V / 400mV = 76x  =>  37dB.

If you think to use only CD or DAC you need less voltage gain and also 32dB are enought.

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.

Advantages of this type of configuration compared to a conventional:

● compensation of Vbe non-linearities between driver and final so very low distortion without feedback
● high slew-rate and large frequency band
● temperature run-away compensation without sensors if driver and final transistors share the same heatsink
● high input impedance if driver stage use current generator (I1 & I2) auto protected by short circuit on output

Disadvantages of this type of configuration compared to a conventional:● 

the bias current in the driver limits the current supplied to the load

It is evident that there are many benefits, but to overcome the problem of the current supplied to the load you need to study well the working points of the driver and final.


There is no trimmer for adjusting the bias so you must use exactly the transistors described in the project.

To use other types you need to verify bias points and in some case set R9-R12 to 1ohm or more.

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 at the max output level (simulated with 40Vrms on 2ohm load so 20A).

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.

The previous pcb was designed to support 8 x 4700uF 63V 25mm LKG (code LKG1J472MESAAK), these are Snap-in for Audio Equipment Grade TYPE-1 so the first level on scale of quality for LKG.
The new pcb has been designed to support 6 capacitors Grade TYPE-2 with a diameter of 30mm like the 2700 63V (code LKG1J272MESBBK) until the 4700uF 63V (code LKG1J472MESBBK).
You can test also some Grade TYPE-3 the highest tone quality grade by the low drag the gold plating terminal adoption.



R1,R25                            47Kohm   1/4W  1%   Vishay Dale RN55
R2,R14                            10ohm   
  2W  1%   Vishay PR02
R3,R36                            280ohm   1/4W  1%   
R26,R27                           2700ohm  
 2W  5%   Vishay PR02
R34,R35                           15ohm    
 2W  5%   Vishay PR02
R23,R33                           270ohm   
 1W  1%
R24,R28                           5600ohm  
 2W  5%   Vishay PR02
R5,R6,R7,R8,R17,R18,R19,R20       0.47ohm 
  5W  5%   Vishay AC03
R15,R16                           220K   
 1/4W  1%
R55,R56                          2200ohm  1/4W  1%   Vishay Dale RN55
R51,R52,R53,R54                    220ohm  1/4W  1%   Vishay Dale RN55  
R4,R13                            6800ohm  
 2W  5%   Vishay PR02
R29,R30,R21,R22                   10Kohm  1/4W  1%
R9,R11,R10,R12                 0ohm            (jump)

C6,C8,C9,C14                      0,47uF  63V  Wima MKS
C1,C2,C10,C11                     100uF   25V   UFW
C3                                empty
C5                                0.1uF   100V   Vishay MKP1837
C7,C17,C4,C24,C18,C20            2700uF 63V Nichicon KG  type 30x35mm 
C15,C16                           1000uF 63V Nichicon

D1,D2                             Zener 15V  1W
D3,D4                             UF4007
D5,D14                            1N4007
D8,D12,D11,D10,D6,D7              1N4148

F1,F2                             FUSE 5A FAST (with fuse holder)

U$1                                 2N5550 o 2N5551
U$3                                 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
, OPA602 (you can use many others single op-amp like the TL071)

The connections are 63862-1 (CUT STRIP) by TE Connectivity / AMP (cod. Mouser  571-63862-1-CT, cod. RS 718-7987)



R1,R2,R18,R19   220K    1/4W   Vishay Dale RN55
R3,R4,R20,R21     1K    1/4W   Vishay Dale RN55
           68ohm  1/4W   Vishay Dale RN55
R14,R29  47K(conf.1) or 150K(conf.2)     3W   Vishay PR03
R13,R28  22K(conf.1) or  10K(conf.2)     2W  
Vishay PR02
R11,R34        4700ohm  1/4W
R12,R30         470K    1/4W  
R45              68K    1/4W
R46             220ohm  1/4W
R48               1K    1/4W
R27,R39         47ohm   1/2W   Vishay Dale RN60
R47             100ohm  multi-turn trimmer
R5,R6,R8,R9             empty
RF                1ohm  5W   (in series to the secondary filament to reduce the LM317 dissipation)

C1,C3           470uF   6.3V  Nichicon UFG
C2,C4            33uF   400V
C6              100uF   400V
C7             4700uF   16V
C10,C11,C8      220uF   16V
C5,C9,C12,C13   empty

CY5,6,7,8       0.01uF 440VAC 
U$4,U$5         50uF   500V   Vishay MKP 1848H

D1,D2,D3,D4     UF5407
D5,D10          zener 10V 1W
B1              diode bridge 100V 2A  2KBP01
U$19,U$16       1N4007
T3              2N2904

U$6,U$3         IRF840

U$15            LM317

6Z51P + tube socket The connections are 63862-1 (CUT STRIP) by
TE Connectivity / AMP (cod. Mouser  571-63862-1-CT, cod. RS 718-7987)

KK1,KK4  Heatsink type SK104 or EA-T220-38E compatible L=38mm  Thermal resistance = 9.5K/W

KK2 Heatsink type SK104 or EA-T220-38E compatible L=38mm  Thermal resistance = 7.5K/W




R21,R22,R24,R23,R29,R30           82Kohm  2W   Vishay PR02

R25,R26,R28,R27,R33,R34           47Kohm  2W   Vishay PR02
R17,R18,R19,R20                    2Kohm 1/4W  Vishay Dale RN55
U$1,U$2                           IRF820
KK1,KK2                           Heatsink

The connections are 63862-1 (CUT STRIP) by TE Connectivity / AMP (cod. Mouser  571-63862-1-CT, cod. RS 718-7987)

KK1,KK2 Heatsink type SK129 or FA-T220-51E compatible  L=50mm  Thermal resistance = 3.4K/W





If you will use the configuration with anode choke use a transformer like this:

R-CORE model R26-90 with primaries 2 x 115v and the secondaries 0-165v(0.2A) and 2 x 0-9v (1.1A)

available on Ebay or Alixpress shop




If you will use the configuration with IRF820 use a transformer like this:

R80-55 0-9 X2 (3A), 0-250(200mA) 11.5(L)x9(W)x5(H)
Primary voltage: 0-115V-230V  

available in the, Ebay or Alixpress shop.

I have add a 100ohm 25W in series to the primary to reduce a 10% on filament and anodic voltage.


In my last version I have used a custom made by Italtras with these parameters:

















to use with 230VAC in order to keep the flux very low.





All the pcb will be available at low cost on Ebay shop or you can download the Eagle files to produce these in your local area.

Current Amplifier pcb
Voltage Amplifier for both configurations pcb
Voltage Amplifer for IRF820 configuration pcb
Soft start module pcb

Protection module pcb
Diode bridge

See Ebay shop to buy pcbs

Here the link to buy the complete pcb set

I don't get money from this Ebay shop.


For this design is necessary a tube with high amplification factor > 60x and low internal resistance < 3000ohm.

In the driver section you can decide to use the Siemens D3a (expensive pentode to use in triode connection) or the Russian 6Z51P / 6Ж51П (cheap pentode tested on Bartola article).

I have used these russian pentode also in the my last Power Folllower, in the Inpol/Mofo and in my last Amplifier End.

These vacuum tubes sounded more transparent than the NOS Mullard and Siemens. 

The socket pins are not the same for both the tubes, the pcb is for 6Z51 so it need a little modification to be used with D3a.


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


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

Download triode.asy from Ducan's Amp Pages.

Add these following ines in the file
C:\Program Files\LTC\lib\cmp\standard.bjt or in C:\Users\[Windows user]\Documents\LTspiceXVII\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



The following is an example of normal power supply, there are 2 toroidal transformers with about 300VA.

You can use min 4 x10000uF and max 4 x 22000uF.

Following the experince of my last amplifiers I have used Schottky Fast Soft recovery diodes in the power supply but you can use normal 36A diode bridge.

To create a fast diode bridge with these TO220 I have created a simple pcb.


It is possible use switching power supply modules, one per channel to to avoid ground loops.

There is a little difference in sound by hot switching between conventional power supply made of transformer, diode bridge, capacitors and these switching power supplies.

These modules are availbale at low cost on Alixpress shop.

Input voltage: AC200-240V
Output: ;
     Main voltage +-55V/6.5A;
     Independent 12V/0.5A;
     Auxiliary +-12V/0.5A (with the main voltage for ground)
Voltage regulation accuracy: main no-load 3%, with load 3%; (when AC220V input)
Continuous power: 350W (can work continuously for long-term 350W work at 25C ambient temperature, fan cooling is required)
Rated power: 700W (can work continuously for about 5 minutes at 25C ambient temperature)
Instantaneous power: 900W (power generated during dynamic signal impact, less than 100 milliseconds)
Conversion efficiency: Max 98%
Dimensions: length, width and height = 96*96*38mm
Weight: 310g



It is possible use also a pair of MEAN WELL LRS-150-36 connected in series to obtain a dual voltage.

It is a 150W Single Output Switching Power Supply with a variable DC output in the range 32.4 ~ 39.6V.

There are integrated protections for short circuit, overload, over volatge, over temperature.

This model has been choised because it is cooling by free air convection and give until 4.3A.

There are also more expensive models with PFC like the UHP series.





The soft start is necessary only you are using normal power supply with transformer and capacitor

B1          diode bridge 100V 2A  2KBP01
IC1               4013 or HCF40138
IC2               7805 in TO220 case
IC3               NE555
T1,T3             BC337

C1               100uF  16V
C3               100uF  16V
C4               470uF  16V
C5                 1uF  63V  MKT

R2                22Kohm 1/4W
R1,R3,R6,R8        1Kohm 1/4W
R13              8200OHM 1/4w
R5,R18           4700ohm 1/4W
R4,R7,R9          4.7ohm 5W 

U1,U2          Relay Fider mod. 41.61  12V

The connections are 63862-1 (CUT STRIP) by TE Connectivity / AMP (cod. Mouser  571-63862-1-CT, cod. RS 718-7987)

JP1 led power-on
JP2 led dc detected so fault

and use the following module to add an extra dc protection to switch off th amplifier in case of problems

R17,R14                         empty (not used)
R16,R15                         10Kohm 1/4W
C6,C7,C12,C13                   empty (not used)
C8,C9,C10,C11                   220uF 35V
D8,D9,D11,D12,D13,D14,D15,D16   1N4148
D17,D18,D19,D20,D2,D3,D5,D10    empty (not used)
OK2,OK3                         empty (not used)
OK1,OK4                         SFH610

The connections are 63862-1 (CUT STRIP) by TE Connectivity / AMP (cod. Mouser  571-63862-1-CT, cod. RS 718-7987)

This is the 1NO1NC switch available in 2 configuration Momentary or Latching and you can buy from Alixpress.

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

Use Momentary model if you think to use the Soft start from Alixpress otherwise if you use my Soft start use the Latching type.



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

I suggest to use the CUSTOMIZATION service by HiFi 2000 for the M3 threading on the heatsinks.

File dxf to use

If you will use the switching power supply for the current amplifier use this model:

Dissipante 04/300B 4U 10mm
Product Code: 1NPD04300B or 1NPD04300N
temperature coefficient 0,31 C/W per each side
Internal height 165mm

If you will use the normal power supply for the current amplifier probably you should use this model:

Dissipante 05/300B 5U 10mm SILVER or BLACK
Product Code: 1NPD04300B or 1NPD04300N
temperature coefficient 0,18 C/W per each side
Internal height 210mm

Inner baseplate for Dissipante 300mm
Product Code: 1BASEPD300

Add an internal aluminium panel 3mm size 20 x 29cm



This is a very hi-end amplifier with a incredible sonic performances but reach this level it is necessary use a very good interstage capacitor.

I suggest these models:

See some test results on Humble Homemade Hifi .


Here a first measurement with 2 x 37VAC r-core transformer.

Here the final measurements with 2 x 55VDC switching power supply.





This project does not have a bias adjustment so you have to use the same transistors I used.

In my version with 2 x 36VAC  400VA toroidal transformers I have this bias.

2 x 22000uF 63V on each channel

2 x 46.5VDC on amplifier input

2 x 42VDC after regulators

from 65mV to  69mV on 0.47ohm output resistors so 0.147A for each output transistor

about 800-900mV on the 10ohm resistor so 2 x 90mA on driver stage

4 x 0.147A = 0.59A

2 x 90mA = 0.18A

= 0.76A    =>     2 x 46.5VDC x 0.76A = 71W on each channel

Using a 2 x 55VDC on amplifier input you will arrive to about 95W




1) set the variac to about + 2v and -2V dc to check that the heatsink is not connected to these tensions
2) control the voltage after the regulators these should rise slowly in a few seconds
3) set the variac to + 15V and -15V to verify that the output voltage goes to zero volts dc
4) set the variac and -20V to + 20c to verify that the voltage across the resistance from 0.47ohm is almost equal among all the transistors (0.05V - 0,06V)
5) set the variac at the final voltage



Here a test environment with a single high power r-core for both channels.




Here the version of a my friend with separated power supply using 2 x 42VAC 420VA toroidal transformers and 4 x 19000V 75VDC capacitors

124W on 8ohm with a perfect decay 1.54% thd

150W on 8ohm with about 2.54%

1 x HiFi2000 Dissipante 04/300N 4U 10mm BLACK​
1 x HiFi2000 Pesante 02PN 2U 10mm BLACK​


Here the final version in a 5 unit chassie.




Configuration with IRF820 and switching power supply

description unit price quantity total (euro)
HI-Fi2000 chassie with machining (about) 300 1 300
80VA transfomer for volatge amplifier  70 1 70
Swicthing modules 35 2 70
LKG capacitors 7 12 84
Various components 150 1 150
6Z51P (10 items to make a selection) 4 10 40
output transistor 4 20 80
Interstage capacitor 40 2 80
 rca, iec, output terminal 50 1 50
total   924

Configuration with IRF820 and normal power supply

description unit price quantity total (euro)
HI-Fi2000 chassie with machining (about) 300 1 300
80VA transfomer for volatge amplifier  70 1 70
Toroidal transformer   300VA 70 2 140
LKG capacitors 15000uF 63V 25 4 100
LKG capacitors 7 12 84
Various components 150 1 150
6Z51P (10 items to make a selection) 4 10 40
output transistor 4 20 80
Interstagr capacitor 40 2 80
 rca, iec, output terminal 50 1 50
total     1100