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Amplifier End version 2022  started in the 2021


INTRODUCTION

This is the last version of the incredible Amplifier End, the original design has been reengineered on pcb to allow a easy assebly. 
Using the
ECW20N20 mosfet we have a good damping factor, low distortion and high output power.
In the driver section there is the very good Russian 6Z51P / 6Ж51П (cheap pentode used in triode connection tested on Bartola articles).
I have used these russian pentode also in the my last Power Folllower and in the Inpol/Mofo and these vacuum tubes sounded better than the NOS Mullard, Jan Philips and Siemens. 
Like all my amplifiers also this can be driven directly by a CD, DAC or Phono pre-ampl.

This is a reference amplifier, in the future it will be difficult for me to find something that goes better.
If we compare this amplifier with my last Power Follower 2021 we have a higher transient speed obtained by driving the mosfets with a lower impedance and higher current.
The mosfets used have also have a lower input capacity than the IRF and there is another dynamic while still maintaining the purity of the sound of the Power Follower.

The main benefit of a Circlotron design is an high power amplifier with no switching output stage like a pure class A but higher efficiency, with only 1A bias current still 1mA with 2.3A on 8 load.

Specifications:

Pout = 60w on 8ohm with 1.5% thd
Pout = 105W on 4ohm with 1.6% thd
Pout = 21W on 8ohm no-switching with 1A bias
Pd = output stage 2 x 1A x 38V = 76W each channel with 1A bias

Zout = 0.4ohm to have a damping factor of 20.
Ft(-3dB) = 20Hz-56KHz at 0dB and  10Hz-65KHz at -3dB

NOTE

Please do not touch with multimeter probe the mosfet gate after the power on because this operation will destroy the mosfet.
Before to solder the mosfets set the trimmer to have 0.2VDC on all trimmer outputs,  then turn off the amp, unload the capacitors of the power stage, solder the mosfets, screw them on the heatsink with a silicon pad isolators and after the power on increase both trimmers, any time one half turn for each, until you get the correct bias current.
Turn only one trimmer to get zero DC voltage on output terminals and after check again the current to have about 1A.
The faster method is to use 2 multimeters, one to check the bias current on mosfet in parallel to a 0.1 - 0.33ohm resistor and one on output terminal before dc protection to keep the dc voltage near to zero.







SUPPORTERS

These companies have supported this project.

 

SCHEMATIC

It is very simple design, only few components on the signal path.

This give an incredible good sound with high slew rate, low dirtortion also on 4ohm loadspeakers, a high power over the 100W and good frequency band.

The interstage transformer is the core of this very simple design.

It is used to reduce the output impedance of the vacuum tube, to increase the output current and to create a differential signal in order to drive the mosfet with opposite phase as required by the Circlotron design.

D3a and 6Z51 tubes in the triode connection have about 70 - 80x of voltage gain and an internal resistance about 2 - 2.8Kohm.

CONNECTIONS GUIDE

This new version use pcb for all the sections.

Here the Ebay shop with pcb

PCB DRIVER STAGE

R1,R2,R18,R19   220K    1/4W   Vishay Dale RN55 or Caddock MK132
R3,R4,R20,R21   1K      1/4W   Vishay Dale RN55 or Caddock MK132
R7,R24          68ohm   1/4W   Vishay Dale RN55 or Caddock MK132
R14,R29         47K     3W     Vishay PR03
R13,R28         22K     2W     Vishay PR02
R11,R34         4700ohm 1/4W
R12,R30         220K    1/4W  
R45             68K     1/4W
R46             220ohm  1/4W
R48             1K      1/4W
R27,R39         47ohm   1/4W   Vishay Dale RN60 or Caddock MK132
R47             100ohm  multi-turn trimmer
R5,R6,R8,R9     500ohm  multi-turn trimmer
 
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   33uF    16V  Nichicon UFG 

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
U$1,U$2         6Z51 + tube socket

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

PCB INTERSTAGE TRASFORMER

    

For this project assemble two of this module.

LL1671 / 20mA or LL1671 AM / 20mA

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

PCB OUTPUT STAGE

For this project assemble two of this module.

R1,R2,R5,R6     180ohm 2W      Vishay PR02
R3,R4,R7,R8     1800ohm 1/4W   Vishay Dale RN55 or Caddock MK132
U$1,U$2         ECW20N20
2 x FUSE        5A fast with socket

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

PCB DIODE BRIDGE

 

For this project assemble four of this diode bridge.

D1,D2,D3,D4        MBR40250 ON 250V 40A Schottky Power Rectifier

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

 

VACUUM TUBE

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

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.

 

INTERSTAGE TRANSFORMER

The interstage transformer is used to reduce the output impedance of the vacuum tube, to increase the output current and to create a differential signal in oder to drive the mosfet with opposite phase as required by the Circlotron design.

It is necessary a good 4 : 1+1 with a minimun inductance of 50H to have a good low frequency response if the internal resistance of the vacuum tube is about 2000-2700ohm and the Rdc of primary is more than 300ohm.

Ft(-3dB) = R / ( 2 * 3.14 * L ) = 3000ohm / ( 2 * 3.14 * 50 ) = 9.55Hz

LL1671/30mA in ALT Q configuration have 35H so the version with an air gap for only 20mA will have about 35H * 30mA / 20mA = 52.5H.

Using a 4 : 1 instead of a 2 : 1 + 1 the mosfet grid will be driven with low impedance and more current so there is a better high frequency response and faster slew rate.

You can use the mormal version LL1671/20mA or the LL1671AM/20mA with amorphous core much more expensive.

What is the sound difference between mu metal and amorphous core transformers? Here a good note from Kevin Carter at K&K Audio.

Surely the silicon-iron with the amorphous version have a different sound, after a series of tests conducted with other persons I can say that there are micro details in some tracks that are noticeable only with the amorphous version.

With the amorphous version the sound acquires a greater lightness and adds a slight softening losing something on the metallic sounds but with digital sources this is an advantage because it reduces listening fatigue.

In any case, altough the differences are not too significant, after listening the amorphous version it is difficult to go back to the normal version.

 

There is a pcb for a fast cabling.

 

MOSFET

The mosfet used in the first release was the 2SK1058 Hitachi Renesas.
This new versions use the ECW20N20 Exicon with lower output impedance and the same dynamic range of the 2SK1058.

Simulation of the output impedance with the old 2SK1058 and with the new Exicon ECW20N20.

 

 

POWER SUPPLY

In a normal amplifier using a CRC or CLC in the output stage power supply you can reduce the noise but in this case the mosfet gate is isolated from the high current power supply so there is no benefit and it is possible to use a  4 x15000uF or 4 x 22000uF.

The 4 x 22000uF have a different size so you need to change the layout and the chassie.

Following the experince of my last amplifiers I have used Schottky Fast Soft recovery diodes in any section of the power supply but you can use normal 36A diode bridge like the IRF 36MB60.

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

For this design you can use normal transformers produced in stock so available on market at a good price.

I have used 2 x 320VA with these following secondaries but you can use any value in the range 24V to 33V without problems.

30V-0       5.3A
30V-0       5.3A

If you will follow my layout check the size before buy these transformers.

The Hammond 1182P30 should be a good component for this project.

There are only one choice for the power supply capacitors:

 

For the driver stage I have used a r-core R26-90 got on Alixpress web shop.

9V-0         1A        for the filaments and mosfet bias (see pcb driver)
9V-0         1A        for the dc loudspeakers protection (see the relative modules and invert the phase for any channel)
165V-0     0.1A     for the vacuum tube anodic (see pcb driver)

 

 

RESISTORS

All the resistors on the signal path have 2 positions on schematic because ...


otherwise to get a slightly better result using the MK132 Caddock.

 

 

LAYOUT

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 or BLACK
Product Code: 1NPD04300B or 1NPD04300N

temperature coefficient 0,31 C/W per each side


Inner baseplate for Dissipante 300mm
Product Code: 1BASEPD300

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

 

PROTECTION CIRCUIT

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

The Cirlclotron design never will produce an high dc output but I suggest to use a dc protection circuit.

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.

    

 

SOFT START START AND TEMPERATURE PROTECTION

I am using this module got on Alixpress because it incluse a soft-start and a temperature protection at 75.

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.

On Alixpress there are some cheaper buttons.

 


SIMULATION

MEASUREMENTS DRIVER STAGE ONLY

These are the measurement of the distortion and frequency response of the driver stage after the interstage transformer.

 

 

60W on 8ohm

100W on 4ohm

 

 

PHOTO

 

 

COST

The total cost to create this amplifier is not very low but you are building a reference amplifier without compromise.

This is the cost with Amorph LL1671AM/20mA
 

Qty model brand unit price (euro) total price (euro) shop
2 6Z51 - 5 10 www.ebay.com
4 ECW20N20 Excicon 12 48  
2 LL1671AM/20mA Lundahl 250 500 Lundahl distributors
4 15000uF 50V Nichicon 20 80 www.partsconnexion.com
2 Power transformers   100 200  
Driver transformer    40 40 www.alixpress.com
16 MBR40250 ON 2 32  
1 chassie Hi-Fi2000 230 230 www.hifi2000.it
2 protection module - 10 20 alixpress.com
1 Soft start   10 10 alixpress.com
1 connectors   30 30  
1 other items   100 100  
      total 1300  

and with normal LL1671/20mA

Qty model brand unit price (euro) total price (euro) shop
2 6Z51 - 5 10 www.ebay.com
4 ECW20N20 Excicon 12 48  
2 LL1671/20mA Lundahl 100 200 Lundahl distributors
4 15000uF 50V Nichicon 20 80 www.partsconnexion.com
2 Power transformers   100 200  
Driver transformer    40 40 www.alixpress.com
16 MBR40250 ON 2 32  
1 chassie Hi-Fi2000 230 230 www.hifi2000.it
2 protection module - 10 20 alixpress.com
1 Soft start   10 10 alixpress.com
1 connectors   30 30  
1 other items   100 100  
      total 1000