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Hi-end DC coupled Circlotron  Hybrid Amplifier 2021


This amplifier born to try if it is possible increase the sonic performances of the Amplifier End.
t is the result of many months of simulations and instrumental testing.
This design is inspired to the
Circlotron US patent n. 4229706 by James W. Bongiorno and to the Thorens TEM 3200 by Frank Blöhbaum.
I have used the LTspice free simulation tool to design and optimize this circuit.
The DC servo loop does not follow the Frank Blöhbaum European patent n. EP1548934B1.
I thank my friend Marco Ferrari Bortolini who designed with me the servo loop circuit and Michele Branchini who created an incredibly very compact pcb.

ATTENTION:  this is not a project for beginners because for its stability it must be realized following all the my layout, it works at very high frequencies.

all rights reserved @ copyright - free only for DIY

The main characteristics are:

 - dc coupled of all stages
 - no capacitors on the signal path
high slew-rate about 150V/usec
 - high power to drive any louspeakers, 60W on 8ohm and 120W on 4ohm with the mosfet ECW20N20

 - low distortion with very low feedback about 10dB
 - good damping factor near to solid state amplifiers, 90mohm with the mosfet ECW20N20

 - large frequency range about 0Hz to 600KHz at -3dB
 - only the input transformer limit the band to 150KHz to eliminate RF noise

 - no switching output stage like a pure class A, with only 900mA bias current still 17mA with 3A on load
 - this is an amplifier with high efficiency because less than 100W of consumption per channel
 - no matched device are necessary
 - only single ended stages
 - only a single output device

only a transconductance amplifier with I/V
 - only the vaccum tube control the global feedback
 - no high voltage for tubes only  +95V and -155V
 - global feedback easy to modify without high frequency compensation
 - full balanced circuit
 - it need only an input of 0.5Vrms to get 50W
 - the dc servo loop does not change the vacuum tube operating point
 - ultrafast high power discrete Schottky diodes
 - regulated power supply for the driver section
 - high transconductance tube, E180F connected in triode mode




all rights reserved @ copyright - free only for DIY

R1,R31            10Kohm 3W (min)
R3,R30          5600Kohm 1/4W 1%
R4,R29             1Kohm 1/4W 1%
R9,R25            47ohm  1/2W (on pcb bottom side)
R16,R17          900ohm  => 2 x 1800ohm 1/4W 1% in parallel
R10,R23         2350ohm  => 2 x 4700ohm 1/4W 1% in parallel
R8,R24           600ohm  => 2 x 1200ohm 1/2W 1% in parallel
R11,R21          235ohm  => 2 x 470ohm  1/2W 1% in parallel
R6,R27            50ohm  => 2 x 100ohm  1/4W 1% in parallel
R2,R32           100ohm  0.5W 1%
R13,R19          100ohm  1/4W 1% for the 2SK1058 or 82ohm for the Exicon 10N20 or 50ohm for the 20N20
R5,R28            39Kohm SMD 1/4W 1% 1206
R7,R26            10Kohm SMD 1/4W 1% 1206
R15,R18         1000Kohm SMD 1/4W 1% 1206
R12,R14,R20,R22    5Kohm MP925 Caddock
RN1,RN2         1800ohm  1/4W 1% (on pcb bottom side)

C5,C10           100uF   35V electr. bipolar
C2,C3,C6,C7        1uF   SMD Ceramic 50V  (Kemet C0805C104M5RACTU)
C1,C12                   empty
C4,C11                   empty
C8,C9            2.2uF   250V MKP Wima

D1,D2,D3,D4       10V    Zener SMD 3W

T2,T3,T4,T7,T8,T2,T9     MJE15035 PNP transistor
T5,T6                    MJE15034 NPN transistor

IC1,IC2                  OPA2277U SMD SOIC

M1,M2                    MOSFET 2SK1058 or Exicon 10N20 or 20N20

V1,V2                    E180F  6688 Siemens

The parallel of 2 resistors increase the power and the sonic performances, as alternative use Caddock MK132.

RF1             3400ohm  => 2 x 6800ohm 1/4W 1% in parallel
CF1             1000pF   silver mica

RG1,RG2           15ohm  3W
CG1,cG2          220pF   ceramic
DG1,DG2,DG3,DG4          1N5406


To add the two 1800ohm resistors are necessary these modifications on pcb.



The driver stage need a good low ripple regulated power supply so has been used is a modification of the original Michael Maida regulator published on the Texas Instruments application note.

R19,R26,R20,R22           47ohm 3W
R5,R6                     12ohm 3W
R10                       27Kohm 2W + 27K 2W
R12                       68Kohm 3W
R3,R4                    220ohm  SMD 1206
R7,R8                    137Kohm SMD 1206
R9,R11                  1270ohm  SMD 1206
R14,R17                  953ohm  SMD 1206
R15,R18                  100ohm  SMD 1206
R13,R16                    1Kohm trimmer 10 turn

C11,C12,C13,C14          470uF 250V electr.
C5,C6                     10uF  SMD
c7,C8,C15,C16,C17,C18    0.1uF  250V MKP Wima
C3,C4                      1uF  250V MKP Wima
C9,C10                   2.2uF  250V MKP Wima
C1,C2                    empty

B1,B2                    KBP408G diode bridge

D3,D4,D11,D12 ,D9,D10    1N40007

Q1,Q2                    IRFP240 Mosfet

IC1,IC2                  LT3080 SMD SOT-223

KK1,KK2                  SK104 heatsink 63.5mm

Here the pcb created by Michele Branchini. 

For the filaments has been used a LT1083 Postive Adjustable Regulated Power Supply Module available on Alixpress shop.

or the better LM317 module with soft-start on Ebay


For the filaments has been used a LM317 - LM337 Regulated Voltage Power Supply module available on Alixpress shop.

or this


For the output stage you can  use a simple power supply with only a diode bridge and a capacitor or a CRC or also a CLC like the Amplifier End.
There are some choices for the power supply capacitors:

Here you can add more capacity using a higher chassie.

I have design a pcb to create a very compact diode bridge (see also the photos).



This amplifier need a balanced / differential input created with an input transformers like these.

  • Taobao
  • AliExpress     45$ (two pieces)
  • Alixpress       44$ (two pieces)

    From the input transformer to the amplifier module has been used a CORDIAL CPK 220 Microphone cable, 2 x 0,20 mmq, diam 4,7 mm.




    It use a pair of good E180F Siemens for each channel, these are penthode used in triode connecction (g2 connected to anode).

    • S = 18mV /V
    • Ri = 2.7Kohm
    • u  = 50
    • Vf = 6.3V    If = 300mA
    • Vg2(max) = 175V
    • Ik(max) = 25mA



    The first mosfet tested in this project is the 2SK1058 Hitachi Renesas, please keep attenction to fake.

    The second mosfet tested is the ECX10N20 Exicon, lower output impedance but also lower dynamic range so to have the same output power of 2SK1058 you need to increase the power supply voltage of the output stage.

    The last mosfet tested is the ECW20N20 Exicon, lower output impedance and same dynamic range of the 2SK1058.



    On paper each lateral heat sink of this chassie have a capacity of 0.25 °C/W so it will keep the output devices in a good safe area also when the environment condition are terrible 35 °C.
    It is possible use 4 units container but you should use normal power supply instead of CLC for the output stage because there is no enough space for all the parts.

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

    Dissipante 04/300B 4U 10mm SILVER  
    Product Code: 1NPD04300B

    temperature coefficient 0,31 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.

    If you think to use the last Exicon mosfet enlange the size 75.7mm to about 80mm otherwise it is necesary bend the pins like this image.



    Any serious solid state amplifier need a protection circuit because a fault on output transistors or mosfet can destroy the loudspeakers.

    This design need also a relay on output terminals to keep disconnected both the output pins (+ and -) during the start-up phase at the switch-on for 1min.

    I have decided to use 2 x AIYIMA 2.0 Digital Power Amplifier Speaker Protection Board Delay Relay Speaker Protection available on Alixpress online shop. 

    This module use 2 optoisolator
    PC817 for each input and are necessary only some little changes to increase the start-up time, to increase the accepted input voltage and to obtain a faster reset of timer.




    this use the NE555 module from Alixpress



    Follows some models necessary for the simulations.

    .MODEL mje15035 pnp IS=5.81508e-15 BF=313.373 NF=0.85 VAF=40.5017 IKF=0.897023 ISE=6.74258e-16 NE=1.04249 BR=0.958017 NR=0.894461 VAR=148.639 IKR=7.05393 ISC=6.74258e-16 NC=2.84461 RB=3.62039 IRB=0.1 RBM=0.1 RE=0.000923293 RC=0.233799 XTB=2.92628 XTI=1.01325 EG=1.17461 CJE=1.5597e-09 VJE=0.99 MJE=0.554057 TF=1.35882e-09 XTF=1000 VTF=467.207 ITF=58.3338 CJC=1.58888e-10 VJC=0.4 MJC=0.23 XCJC=0.786287 FC=0.8 CJS=0 VJS=0.75 MJS=0.5 TR=1e-07 PTF=0 KF=0 AF=1

    .MODEL mje15034 npn IS=3.92866e-12 BF=260.938 NF=1.02215 VAF=15.3399 IKF=0.160087 ISE=1e-08 NE=2.54491 BR=26.0938 NR=1.10885 VAR=153.399 IKR=1.60087 ISC=1e-08 NC=1.89024 RB=0.41209 IRB=0.1 RBM=0.41209 RE=0.0001 RC=0.208002 XTB=0.897431 XTI=1.39234 EG=1.206 CJE=1.61534e-09 VJE=0.698417 MJE=0.382854 TF=1.03079e-09 XTF=1000 VTF=100000 ITF=42.9041 CJC=1.04458e-10 VJC=0.441587 MJC=0.23 XCJC=1 FC=0.8 CJS=0 VJS=0.75 MJS=0.5 TR=1e-07 PTF=0 KF=0 AF=1

    .MODEL mje350 pnp IS=6.01619e-15 BF=157.387 NF=0.910131 VAF=23.273 IKF=0.0564808 ISE=4.48479e-12 NE=1.58557 BR=0.1 NR=1.03823 VAR=4.14543 IKR=0.0999978 ISC=1.00199e-13 NC=1.98851 RB=0.1 IRB=0.202965 RBM=0.1 RE=0.0710678 RC=0.355339 XTB=1.03638 XTI=3.8424 EG=1.206 CJE=1e-11 VJE=0.75 MJE=0.33 TF=1e-09 XTF=1 VTF=10 ITF=0.01 CJC=1e-11 VJC=0.75 MJC=0.33 XCJC=0.9 FC=0.5 CJS=0 VJS=0.75 MJS=0.5 TR=1e-07 PTF=0 KF=0 AF=1

    .SUBCKT 2SK1058 D G S B
    M1 D G S B 2SK1058 L=2U W=29.7482M
    .MODEL 2SK1058 NMOS (VTO=403.969M KP=20U L=2U W=29.7482M GAMMA=0 PHI=600M LAMBDA=184.988F RD=60.8251M CBD=2.56138N IS=10F CGSO=1.13517N CGDO=1.13517N TOX=0 NSUB=0 TPG=1 UO=600 RG=50 RDS=1MEG )

    .SUBCKT ECX10N20 1 2 3 3
    * Model Generated by PEDC *
    *Copyright(c) Power Electronics Design Centre*
    * All Rights Reserved *
    * Power Electronics Design Centre *
    * Dept of Elec & Electronic Engineering *
    * University of Wales Swansea *
    * Singleton Park *
    * Swansea SA2 8PP *
    * Tel : +44 (0)1792 295420 *
    * Fax : +44 (0)1792 295686 *
    * E-mail : *
    * Model generated on Dec 6 1999
    * External Node Designations
    * Node 1 -> Drain
    * Node 2 -> Gate
    * Node 3 -> Source
    M1 9 7 8 8 MM L=1 W=1
    * Default values used in MM:
    * The capacitances are added externally
    * Other default values are:
    * RS=0 RD=0 LD=0 CBD=0 CBS=0 CGBO=0
    +VTO=0.473 LAMBDA=0.092 KP=1.585
    RS 8 3 0.41
    D1 8 9 MD
    .MODEL MD D IS=1.0e-32 N=50 BV=250
    +CJO=1.0e-9 VJ=0.7 M=0.5
    RDS 8 9 1e+06
    RD 9 1 0.58
    RG 2 7 80
    * Gate Source capacitance Cgs0
    CAP1 7 8 400e-12
    * Gate Drain capacitance Cdg0
    CAP 7 4 10.5e-12
    * Gate Drain Capacitance Cdgj0
    * Modelled as a diode
    D2 4 9 MDD
    .MODEL MDD D IS=1e-32 N=50
    +CJO=94.8e-12 VJ=0.3 M=1
    .ENDS ECX10N20



    2SK1058 with Rbias = 100ohm and current about 0.85A thd 0.7% at 21Vrms on 8ohm.

    2SK1058 with Rbias = 82ohm and current about 0.6A.

    Follow the measurement of distortion decay on 8ohm load, thd of 0.046% at 3Vrms about 1w. 

    Follow the measurement of distortion decay on 8ohm load, thd of 0.56% at 21Vrms about 55w.

    Follow the measurement of distortion decay on 4ohm load, thd of 1.3% at 17Vrms about 72w.




    10N20 with Rbias = 82ohm and current about 0.9A.

    The Exicon mosfet are more easy to find in the marker but these lost more voltage, here a test of some years ago.

    Follow the measurement of distortion decay on 8ohm load, thd of 0.045% at 3Vrms about 1w.

    Follow the measurement of distortion decay on 8ohm load, thd of 0.56% at 16Vrms about 32w.

    Follow the measurement of distortion decay on 8ohm load, thd of 1.12% at 18Vrms about 40w.

    Follow the measurement of distortion decay on 4ohm load, thd of 2.35% at 12Vrms about 36w.



    ECW20N20 with Rbias = 50ohm and current about 1.-A.

    Follow the measurement of distortion decay on 8ohm load, thd of 0.08% at 5Vrms about 3w.

    Follow the measurement of distortion decay on 8ohm load, thd of 0.5% at 23Vrms about 66w.

    Follow the measurement of distortion decay on 4ohm load, thd of 2.6% at 22Vrms about 121w.



    Follow the measurement of frequency response on 8ohm load,

    the peak on high frequency is generated by the input transformer and it can be eliminated with a simple RC filter on the secondary.

    Here follows the frequency response with 2.2nF after the grid resistors 1Kohm (not used this filter)

    Here follows the frequency response with a RC cell after the input transformer 3k4ohm (6800//6800) + 1000pF silver mica

    This is the real frequency responce of Thorenz TEM 3200.






    Estimated costs
    description unit price quantity total (euro)
    Vacuum tubes E180F 10 4 40
    10000uF 35V 6 2 12
    Components + pcb 150 1 150
    Input transformers 22 2 44
    Mosfet 12.5 4 50
    Chassie with heatsink and front drilling 300 1 300
    Vandal Resistant  Push Button 20 1 20
    Soft-start + termal protection + relay 40 1 40
    Power supply modules 10 2 20
    Transformers 80 3 240
    Connectors 25 1 25