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Hi-End Class A-AB Balanced Circlotron Hybrid Amplifier
started on January 25  st ,2011

INTRODUCTION

After the incredible and unexpected result of the Hybrid 2011 amplifier I am searching to increase the performance on measurements adding these features:

I don't know if this new design have the same sound of the first version so I will try to create a direct compare.

In the following images are show the signal path of these two versions.

 

 

work in progress ................................


INTER STAGE TRANSFORMER

The quality of the interstage transformers is very important in this design and you cannot use any other type without compromise the final result.
The normal Lundahl  LL1671/20mA is very good but you can ask for the Amorphous strip core version used in my Hi-end headphone amplifier and in my DAC End.

  The Spice simulation show an incredible slew-rate (20V in 200ns) because the mosfet are driven with a tube with a low internal resistance (near to 2Kohm) and an interstage transformer with a turn ratio 2:1 reduce it (4 times) this value (2Kohm / 4 = 500ohm) (here the .cir).

MOSFET

In my test 2SK1058  (Hitachi/Renesas) and ECX10N20R are much better than BUZ900P (Magnatec) as distortion value but the ECX10N20R (Exicon) are better than 2SK1058 because have the same distortion value but a better damping factor.

The minimum distortion with BUZ900P is at the bias point 30V DC and 900mA at any output level.

Using the 2SK1058 and the ECX10N20R the minimum distortion is at the bias point 30V DC and 1600mA and the THD level is 60-70% less than BUZ with the same input stage.

If we increase the power supply to 40 or 45VDC the ECX10N20R show a limit and for all the best bias point is 1000mA.

The 2SK1058 and the ACD100NSD have the same distortion and the same wide voltage range to get high power output. 

You cannot use IRF types and 2SK1529 in this project because these need a thermal compensation circuit.

In the simulation the output impedance is about 630mohm on all the frequency range with the mosfet ECX10N20R and about 1 ohm using the 2SK1058 and ECX10N20R.

It is a good value for a no feedback design with only one output device.
 

 

VOLTAGE AMPLIFIER

To get a good output distortion decay in any condition I have decided to use a single tube in this stage.
I have searched a tube with a very good linearity for large swing and low anodic resistance to drive the output stage without problems.
This  tube should have an amplification factor greater than 40 to connect this amplifier directly to the CD player or phono pre-ampl.

To get 20w on 8ohm we need:
     P=V * I  and  I = V / R  =>   P = V * V / R  =>   V = sqrt (P * R ) = 12.5Vrms
so if we want a sensibility of 0.5Vrms we need
    Amplifier factor tube = 12.5 / 0.5 = 25

Using the LL1671/20mA with the turn ratio 4:1:1 => 2:1 we need a amplifier factor tube of 50.


Many of these tubes have a plate resistance about 2Kohm but only the D3A, 6C45 and 5842 have an amplification factor greater than 40.
D3A specifications in triode connection:

filament voltage: 6.3V
filament current: 315mA

Max plate voltage:  200v
Max plate dissipation: 4.2w
Max cathode current: 33mA
Plate resistance: 1.9Kohm
Amplification factor: : 77

 6C45 specifications:

filament voltage: 6.3V
filament current: 440mA

Max plate voltage: 150v
Max plate dissipation: 7.8w
Max cathode current: 52mA
Plate resistance: 1.2Kohm
Amplification factor: 52 ±16
Max resistance in grid circuit: 0.15Mohm

 Inter electrode capacitance:
input: 11±2 pF
output: 1.9±0.3 pF
transfer: 4.5pF
5842 specifications in triode connection:

filament voltage: 6.3V
filament current: 300mA

Max plate voltage:  200v
Max plate dissipation: 4.5w
Max cathode current: 38mA
Plate resistance: 1.7Kohm
Amplification factor: : 43
 

CURRENT AMPLIFIER

The most simple design to create a class A current amplifier is the mosfet.
This design is inspired to the Circlotron US Patent n. 4229706 by James W. Bongiorno and to the Thorens TEM 3200.

POWER SUPPLY

I have design the power supply to follow a single ended amplifier style but you can use a common solid state design.
The CLC power supply skip any noise problem and it is superior to any pure capacitor design also if large value are used.
In the following simulations with PSU Designer II you can see the big difference in residual noise, with CLC type we get a pure sine wave so it mean low harmonics.

The Hammond 159ZJ (L=10mH Rdc=0.16ohm Imax=5A) used in this amplifier are not difficult to integrate in any amplifier because these have a compact size and these are not expensive.

There are many good capacitors to use in audio amplifier power supply but only few of these give hi-end performances:

For this project I have decided to use the Nichicon Super Through on output section and BlackGate (old stock) on vacuum tube stage because both these types will give extreme performances.

To increase the speed of the power supply has been inserted a 39uF Solen MKP in parallel to the electrolitycs.

It is not necessary a DC filament with slow turn-on but it increase the vacuum tube filament and it can be used also for the mosfet gate bias voltage.

 

MEASUREMENTS

 

PHOTOS

 

CONNECTION GUIDE

 

 

WINSPICE SIMULATION

To optimize the circuit I have simulate it with Winspice.

PROTECTION

see Amplifier End