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

INTRODUCTION

This is a 20-25W Class A Single ended no feedback amplifier.

If you are looking for an extreme quality amplifier and you don't need high power this is the project for you because nothing can have higher performances.
After 20 year I have rebuild the same project because this is perfect on all aspects except the efficiency but this is normal for a true single ended class A.
This project is an hybrid amplifier composed by a tube voltage amplifier followed by a single ended mosfet current amplifier with an exclusive configuration.
I have published several articles about this my current amplifier and this design was the winner of the award in the Circuit ideas in May 2000 on Electronics World which is the most important electronic magazine in the world.  
This amplifier 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 vacuum tube amplifiers but this have a higher performances in terms of driving capacity (damping factor) and low distortion.
 

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.

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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 discoupling of the signal current from power supply because the current generator has a huge impedance (an open circuit for the signal, in theory) 

• The constant current flow from the power supply also during the music peaks that prevent any stress of the power supply. 

• The simmetric distribution of the power dissipated under static and dynamic conditions both on the current generator and the source follower, so I have used the same type of devices.

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 output 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).

At 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:

• IRFP150 IRF    Ciss=2800pF

• IRFP150 Fairchild   Ciss=2000pF

• IRFP150 Vishay   Cis=2800pF

• IRFP150NPBF   Ciss=1900pF

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.

TUBES



Here 3 different 6072A tubes of current production and of course each has a different sound characteristics.

1. JJ
2. EH
3. TAD

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 40W R-core model R26-09. You can buy on Alixpress online shop. The secondaries have 2 x 220V  50mA and 2 x 6.3V 0.8A.

 

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: