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the Power Follower - after 20 years a new test 
original design by Andrea Ciuffoli - 1999 - 2019

INTRODUCTION

If you are looking for an extreme quality amplifier and you don't need high power this is the project for you.
After 20 year I have rebuild the same project because this is perfect on all aspects except one: the efficiency.
This project is an hybrid amplifier composed by a tube voltage amplifier follower by a single ended mosfet current amplifier with an exclusive configuration.
I have published several articles about the my current amplifier design and this was the winner of the award with Circuit ideas on Electronics World which is the most important electronic magazine in the world.  
This amp was built by a lot of people around the world and everyone are enthusiastic about the result, the sound is compared to that of the best single ended finals with something more in terms of driving capacity.
 

To read these articles search on the web page:   www.audiodesignguide.com/doc/index1.html


CURRENT AMPLIFIER DESIGN

It seems a silly and common circuit but it has got great differences from any other similar design, that’s why this is unique.
We have a typical source follower (as an emitter follower but with a Mosfet) working in pure class A with a current generator.
Please note that this circuit works only in pure class A, so it requires enough bias current for the requested output power.
There are no particular limits on the power that can be delivered but obviously the efficiency of this amplifier is very low, 25% theoretical and 15% real so you need a big heatsink also for medium power about 20W. 
To drive this current amplifier is necessay a voltage stage
with an output swing not lower than 10Vrms and Rout < 1000ohm.

.

In my design the first feature making a big difference in the sonic results is the negative power supply and the signal ground tied to the drain of source follower.
With this design we achieve:

The power supply integrate a Virtual Battery to get a reduced ripple and to use lower capacitors.

The 500 Ohm precision trimmer should be set to balance the output fuse voltage to 1/2 of the regulator output.

This single trim should be set after the switch-on, because it just optimizes the output swing to the maximum available.

The 2 x 18V 1W zener are optional and these can be used to prevent crash of the mosfet when it receive bad input signal like a vacuum stage startup (these could decrease the sonic performances).

The phase on output terminals has been inverted because here I will use a voltage amplifier that reverses the phase.

The quiescent current is set  by the resistence on current generator source pin, and can be changed with the simple formula Iq = 0.65 / R where 0.65 is the transistor Vbe.

With a bias current of 3A you can get about 20 Wrms on 4 and 8 ohm load.

The power can be increased with the unique drawback of the dissipated power (heat)!

The power supply transformer can be used with different voltages and currents, from 20V to 37V without any changes.

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 outptu power is very similar, for example the myGM70 SE give this result:

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).

A
t this point we must consider 2 constraints: the maximum output voltage determined by the power supply voltage and the maximum output current determined by the bias current.

The output power will be limited by both these values so after choosing a power transformer and a bias current we can check the output power for each type of load.

Using full wave rectifier
VAC (V) Ibias (A) Rdc prim. (ohm) Rdc sec (ohm) Capacity (uF) Capacity    (F) Frequency (Hz) Vripple (V) Vdc (V) Vmedium (V) Power to dissipate (W) Heatsink (C/W) Tamb max (C) Tchassie (C) R8 (ohm) R13 (ohm) Vgs (th) Vdc lost on reg. > Vripple (V) Vdc after reg. (V) Vout (Vp) Vout (Vrms) Rload (ohm) Iout (Irms) Pout max with voltage used (W) Pout max with current used (W)
36.0 3.00 2.0 0.5 4700.0 4.700E-03 50 3.2 46.6 43.4 130.3 0.18 35 58 10000 500 3 5.1 38.4 18.2 12.9 8.0 2.1 20.8 36.2
36.0 3.00 2.0 0.5 4700.0 4.700E-03 50 3.2 46.6 43.4 130.3 0.18 35 58 10000 500 3 5.1 38.4 18.2 12.9 6.0 2.1 27.7 27.2
36.0 3.00 2.0 0.5 4700.0 4.700E-03 50 3.2 46.6 43.4 130.3 0.18 35 58 10000 500 3 5.1 38.4 18.2 12.9 4.0 2.1 41.6 18.1

 

 

Using bridge rectifier
VAC (V) Ibias (A) Rdc prim. (ohm) Rdc sec (ohm) Capacity (uF) Capacity    (F) Frequency (Hz) Vripple (V) Vdc (V) Vmedium (V) Power to dissipate (W) Heatsink (C/W) Tamb max (C) Tchassie (C) R8 (ohm) R13 (ohm) Vgs (th) Vdc lost on reg. > Vripple (V) Vdc after reg. (V) Vout (Vp) Vout (Vrms) Rload (ohm) Iout (Irms) Pout max with voltage used (W) Pout max with current used (W)
36.0 3.00 2.0 0.5 4700.0 4.700E-03 50 3.2 45.8 42.6 127.8 0.18 35 58 10000 500 3 5.0 37.6 17.8 12.6 8.0 2.1 19.9 36.2
36.0 3.00 2.0 0.5 4700.0 4.700E-03 50 3.2 45.8 42.6 127.8 0.18 35 58 10000 500 3 5.0 37.6 17.8 12.6 6.0 2.1 26.5 27.2
36.0 3.00 2.0 0.5 4700.0 4.700E-03 50 3.2 45.8 42.6 127.8 0.18 35 58 10000 500 3 5.0 37.6 17.8 12.6 4.0 2.1 39.8 18.1

Download this Excel table to customize your Power Follower (start download).

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:

The my voltage stage with the 6072A have an output impedante of 670ohm so the high frequency cut-off can be calculated with:

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.

 

 

VOLTAGE AMPLIFIER

The presented topology, has NO voltage gain (actually it looses something 1.3%) so it should be driven by voltage gain stage, with an output swing not lower than 10Vrms and Rout < 1000ohm. 

The input impedance of the current amplifier is 110KOhm - 1800-2800pF and its resistive value can be adjusted by a pretty wide range, just using a different input resistance (max 220Kohm). 

The current amplifier do not introduce any alteration on the signal so is very important take care of driver stage/voltage amplifier.

Many persons drive this amp. with E88CC in SRPP (Totem pole)  or D3a/E182CC/5842/6C45 in single ended.

Always consider the high frequency cut-off because a D3a or a 6C45 have an output impedance near to 2Kohm so we will have:

Ft(-3dB) = 1 / (2 * pi * C * R) = 1 / (2 * 3.14 * 1900pF * 2Kohm) = 1 / ( 2 * 3.14 * 1900E-12 * 2000) = 42KHz

I thought of this voltage amplifier looking for something that would sound great without using anodic inductors and interstage transformers.

Compared to the 100W hybrid amplifier, in this case we don't need an extreme dynamic to drive a current amplifier, 14Vrms are enough.

Both the following designs give good performances (voltage gain and output impedance), the tubes used have a good reputation for sound quality and the vacuum tubes are in the current production.

 

6072 version

This 6072A driver powered at 280V and with an input signal of 0.58Vrms give about 18Vrms with 1% thd.

 

12AX7 version

This 12AX7 driver powered at 280V and with an input signal of 0.33Vrms give about 19Vrms with 0.56% thd.

Here the distortion is lower but has been apply a local feedback on first stage becasue there is no cathode capacitor.

12AX7 - 6072A socket.

This is the complete schematic of the voltage amplifier including the power supply section allocated in the same pcb.

 

 

TO DO

I advise to build this project using a pcb instead of an air wiring because it is certainly more stable.

A few years ago a shop on ebay was authorized to sell the printed circuit boards of this project, I do not take money from these sales.

Ebay shop for the PCB   new release 2019

This is a single channel pcb so duplicate the components list.

Mount the resistances in parallel configuration in opposite phase or direction.

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)

Ebay shop for the PCB   new release 2019

This is a two channels pcb so the components list is complete.

Mount the resistances in parallel configuration in opposite phase or direction.

R1,R2,R18,R19,    220Kohm 1/4W 1%                    Mouser 594-MBB02070C2203FCT
R5,R6,R22,R23             
R3,R4,R20,R21,    1000ohm 1/4W 1%                    Mouser 594-MBB02070C1001FC1

R11,R34        
R7,R8,R24,R25     5600ohm 1/4W 1%

R8,R9,R26,R27     220Kohm 1/2W 1%
R15,R16,R31,E32    220ohm 1/4W 1%
R17,R33            3.3ohm 2W                         Mouser 660-MOSX2CT52R3R3J
R12,R30           100Kohm 1/4W 1%                    Mouser 594-MBB02070C1003FCT
R14,R29           150Kohm 3W
R13,R28            18Kohm 2W

C1.C3             220uF 6.3V   OS-CON                Mouser 667-6SEPC220M+TSS
C2,C4              33uF 400V                         Mouser 647-UVY2G330MHD
C5,C6             100uF 400V                         Mouser 647-LGU2G101MELZ
CY1-CY8            10nF 440VAC                       RS 335-066 o 80-R474I210050A1K

U$3,U$6           IRF840                             Mouser 844-IRF840APBF
U$11,U$7          33uF 400V Solen MKP   


D1,D2,D3,D4,D6,D7,D8,D9  UF5406                      Mouser 625-UF5408-E3
D5,D10                   zener 10V 1W

KK1,KK4           Extruded Style Heatsink for TO-220 Mouser: 532-513102B25


Isolator         
Bergquist SP400-0.007-00-54        RS 169-2177

Isolatos         
TO-220 nylon platstic Insulator hole size M3

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

The heat sinks have been grounded to avoid receiving radio frequency so these must be isolated from the transistor.

INTERSTAGE CAPACITOR

Obviously it is fundamental to use a high quality interstage capacitor and for this project I will test 4 different types of a good UK company.
This test was necessary to decide which capacitor to use because it is not true that it is enough to buy the most expensive one to be sure of having the maximum sonic performance.
It happened to me to discard many high-level interstage capacitors, also very well evaluated.
In all my sonic performance tests I always search to be sure of an objective result using more persons and these with different experiences.
The ClarityCap has been manufacturing high quality audio grade capacitors for over 30 years.

The low  frequency cut-off is determined by the value of this capacitor and input resistance of the current amplifier (110Kohm).

Ft(-3dB) = 1 / (2 * pi * C * R) = 1 / (2 * 3.14 * 0.68uF * 110Kohm) = 1 / ( 2 * 3.14 * 0.68E-6 * 110000) = 2.13Hz

I suggest to use values in the range 0.47uF to 2uF.



POWER SUPPLY TRANSFORMERS


The power supply transformer can be used with different voltages and currents, from 20V to 37V.

I suggest a 2 x 36V 400-500VA to get a power near to 25W on any load.

In any case the secondaries must be separated for each channel because the ground point is created after the regulator.

For each channel I have a 36V-0-36V 500VA so I have used the diode IXYS DSA70C200HB Soft Recovery Schottky 200V 2 x 35A.
The power supply transformer for the driver stage is an R-core buy on Alixpress online shop.

The secondaries have 2 x 220V  50mA and 2 x 6.3V 0.8A.

 

RECTIFIER DIODES

For each channel I have a 36V-0-36V 500VA so instead of a diode bridge I have used a pair of diodes IXYS DSA70C200HB Soft Recovery Schottky 200V 2 x 35A.

Here an interesting article about the use of Schottky diodes in hi-end audio amplifiers.


 

OUTPUT AND POWER CAPACITORS

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.

 

 

RESITORS

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


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

 

SOFT START AND TEMPERATURE PROTECTION

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

 


CABINET

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

Dissipante 05/300B 5U 10mm SILVER  
Product Code: 1NPD05300B

temperature coefficient 0,18 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.

In a second phase I have add another hole on the heatsinks for the temperature sensor (see below in the photos section)

Here the cost of the chassie

1 x Dissipante 05/300B 5U 10mm SILVER (1NPD05300B) €187.00
1 x B) Drilling front panel 10mm (LAV10MM) €25.00
2 x A) Drilling panel 2/3/4mm (LAV4MM) €50.00
1 x Inner baseplate for Dissipante 300mm   €14.15


Totals Sub-Total: € 276.15
BRT Italia (Weight: 12.00kg): €15.00
IVA 22%: €56.83
Total: €355.20

 

Vandal Resistant  Push Button Switches

Manufacturer Apem Manufacturers 
Part No. AV021003C900
RS  No. 174-6381

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

 

 

SIMULATIONS

Here the simulation of the distortion at the max output level with 44VDC and 3.5A on 8ohm load

Here the simulation of the distortion at the max output level with 44VDC and 3.5A on 4ohm load

Here the simulation of the output impedance

 

FINAL MEASUREMENTS

Here follows the final measurements on the complete PF2019 in this condition:

  • Power supply transformer 500VA 36V-0-36V

  • Diode IXYS DSA70C200HB

  • Capacitor after rectifier 4700uF 63V

  • Voltage after the rectifier 44.3VDC ripple 1.4Vrms

  • Voltage after the regulator (Virtual Battery) 37.5VDC

  • Noise after the regulator 13.2mVrms

  • Rbias 0.2ohm 5W  0.6V  = 3A

  • Load 8ohm

  • Voltage amplifier 6072A

  • Room temperature  19 C

  • Mosfet case temperature 63 C

  • THD 0.59% with an output of 12.7Vrms so 20W

  • Vin 0.4Vrms

  • Load 4ohm

  • Vin 0.3Vrms

  • THD 1.34% with an output of 9Vrms so 20W

 

 

FIRST MEASUREMENTS

Here the distortion of the PF2019 drivern by 6072A at the max output level with 45VDC (after regulator) and 3.5A on 8ohm load measured with Victor 1kHz sine generator, Olivine 2  USB ADC and ARTA software.

Voltage gain 13.8 / 0.48 = 28.75x = 29dB

Here the distortion of the PF2019 drivern by 6072A at the max output level with 45VDC (after regulator) and 3.5A on 4ohm load

Voltage gain 9.5 / 0.35 = 27.14x = 28.6dB

Here the frequency response on 4 and 8ohm load measured with ADALM2000 USB and Scopy software.

 

PHOTOS

 

TOTAL COST

It is possible to find some components at lower cost but the interstage and output capacitors are important for the final result.

The power supply transformers could be reduced to 300-400VA so the cost will be about 70 euro each.

The cost of the chassie could be reduced using the HiFi 2000 Dissipante 05/300B 5U with 4mm front panel and doing mechanical work at home so the 355 + 20 euro will be reduced to 200 euro.

My configuration
description unit price quantity total (euro)
Vacuum tubes  16 2 32
Solen MKP 33uF 400V 10 2 20
Components + pcb 110 1 110
Mosfet 3 6 18
Interstage capacitors 40 2 80
HI-Fi 2000 chassie 355 1 355
Vandal Resistant  Push Button 20 1 20
Soft-start + termal protection 25 1 25
500VA transformers 100 2 200
R-core transformer 44 1 44
4700uF 63V 8 2 16
4700uF 50V 6 2 12
Connectors 26 1 26
      958

Cheap configuration
description unit price quantity total (euro)
Vacuum tubes  16 2 32
Solen MKP 33uF 400V 10 2 20
Components + pcb 110 1 110
Mosfet 3 6 18
Interstage capacitors (min acceptable) 10 2 20
HI-Fi 2000 chassie 4mm front panel 200 1 200
Vandal Resistant  Push Button 0 1 0
Soft-start + termal protection 0 1 0
300VA transformers 70 2 140
R-core transformer 44 1 44
4700uF 63V 8 2 16
4700uF 50V 6 2 12
Connectors 26 1 26
      638