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Single Ended/Push Pull Amplifier

THIS IS ONLY A PREVIEW   because I am in designing phase

by Andrea Ciuffoli.

 

Introduction

The target of these projects is increase the performance of single ended design
using
PLITRON toroidal output transformers.

The toroidal output transformer have very high performances but cannot be used in normal
single ended design because have no gap.

Someone to resolve this problem insert a capacitor before the transformer and
use an inductance with gap on the anode but I don't like this solution because
in any case we need a good quality inductance and a big capacitor about 100uF
  
see here the circuit

Together with the Plitron OPT designer, Mr. Vanderveen, I have choiced to use for this
project the PAT4070-CFB that have the following unusuall characteristis for an output transformer.
 
Nominal Power, RMS (note 1)   70  Watt
Primary Impedance (Raa)  4000 ohm
Primary Current (IDC), maximum 258  mA
Sec. Load Impedance, nom. (Rls)  5  ohm
Turns Ratio (Np:Ns)   28.3:1 Ratio
Ultra Linear Tap   33 %
Cathode Feed-back Tap  7.1 %
Frequency Range, -3dB  222 kHz
Power Bandwidth Start, -3dB 14 Hz
Total Pri. Inductance (Lp) 1056  H
Leakage Inductance, Pri to Sec (Lsp) 3.1 mH
Leakage Inductance, Pri to Sec (Lsp) 3.1 mH
Effective Primary Capacitance (Cip)  358  pF
Total Primary Resistance (Rip)  120 ohm
Total Secondary Resistance (Ris)  0.15  ohm
Insertion Loss (Ilos)  0.25 dB
Quality Factor (Lp/Lsp)  338,000 Factor
Case size, diameter x height  152.4 x 88.9  mm
Weight  5.62 kg

This transformer also allow several output stage configurations including the ultralinear
and the
cathode feedback.
These two configurations add a little local feedback loop in the output stage and this can be used
to reduce distortion and output resistance of the amplifier.

The output tube to get very good sonic performances and max power with this transformer is the KT90.
The KT90 is the higher power pentode designed for audio and it has been produced by Ei.
Ei tubes are made in the former Yugoslavia to the Philips license, using original Philips machinery,
drawings and test procedures.
This tubes can be find at good price by
Korato (Beograd, Yugoslavia).

Follows some KT90s Ei items:

I will test also these folloing tubes: 8417, KT88 Teslovak and 6550A.

About passive components types,  I don't leave many choices: Allen Bradley resistors on anode,
Holco or Caddock resistances on cathode, ELNA Cerafine capacitors on cathode and power supply.
For all my new projects, I am using the very good ELNA Cerafine capacitors that I love and which can
be find at good price by
Welborne Labs.
The ELNA Cerafine capacitors contain super fine ceramic particles which, through chemical reaction,
improve the discharging speed between the anode and electrolyte with very low distortion.
Any other electrolytic or polypropylene capacitor give very lower sonic performances introducing some alteration. Only the Blackgate WKZ could give better sound, but I have not tested it and I know of some problems during startup.

The projects below have been simulated with spice release 3f4 running over
a UNIX  in order to optimize their performances. The spice3f4 tube models have
been computed by a complex function, with a custom program working
on the manufacturer's plot for the output characteristics (Ip vs Vp @Vg)
for any of the tubes he is interested in.
  
see here the match (in red tube model)

The distortion spectrum below has been plot by Spice3f4 using a my custom
command "plotfour" add to the original
Berkeley's programs.

The Spice3f4 sources are available for donwload at ftp://ic.berkeley.edu/pub/Spice3/
and can be compilated on many Unix operation system (on PC use Linux).

A nice porting on Windows could be find at S. C. Wong's Home Page.


First idea - Single Ended with Current Mirror

This is the first circuit idea.

We have a Single Ended amplifier using a push-pull transformer and the current mirror
balance the bias current on the transformer.
The transistor on the rigth must dissipate the same power of the tube (about 45watt)
so is necessary to use a heatsink with high aspect ratio.
The main vantage here is the low cost and ofcourse a triode could be used instead
the pentode in triode connection as show.


Second idea - Single Ended/Push Pull


Here the transistors could be low power as BD139 and the second tube
could be driven to switch in push-pull from single ended when we need
more power.
The current mirror balance always the bias current on the transformer.
We can also set the input of tubes to get the same distortion spectrum of
a single ended but more power

This configuration should run very well in class A when both the grids are driven but when
we drive also one grid to get the best sound of single ended the tube with no signal on grid
will function as a load for the other.
To skip this problem I suggest to use a triode to pentode switch as this following schematic:


 



Third idea - Pure Single Ended

Here there is the Pure Single Ended design and a direct heating tube is used.

If we want use this design in Single Ended mode only, ofcourse we can change the first penrtode with
a  triode and the best triodes are the direct heating types.
The direct heating output triodes are the best because are more linear of any inderect hating output tube.
Searching a direct heating triode that can be used with Plitron output transformers 4070-CFB, with a
primary impedance of 2000ohm (4000ohm anode to anode)  I have found two KR Enterprice tubes:
the KR300BXLS and the KR52BX.
 



Input circuit for the Single Ended/Push Pull

In order to achieve the following points, this project include a digital input circuit.


 



Control's notes

As you can see in the input stage schematic for the Push-Pull/Single Ended there are two controls:


I have designed also a more beatifull solution to implement the single ended to push pull  control that allow to do a true comparation of the sound quality in both the modes beacause keep always the same output power level.
This solution should use the CT3
DACT selector switch with some Cadoock resistances.

The disadvantage of this design is the redution to half of the signal in the push-pull mode (center position of switch) so to get the max output power from the amplifier the voltage stage should have a double gain.

See here the simulated distortion spectrum of output stage in three different modes:
 




Power supply for tubes stages



Digital to Analog converter

As is visible on input stage schematic for Push-Pull/Single Ended I unsing the Evaluation Board CDB4390 from Crystal Semiconductor to have a pre-assemblated solution.
Of the CDB4390 the output stage wth op-amp. is not used.


Power supply for DAC

This power supply has been designed to reduce current peaks on diodes of bridge that create problems for sonic performances.
So, the first capacitor connected on bridge have a little value, only 220uF, and the larger capacitor is only after the stabilizer chip.
The 1N4007 diode protect the 7805 chip from reverve current in switch off pahse.
An improvement to this power supply is possible using  an LC cell on output.



First layout ideas

Follows the layout of power amplifier section

and here the section

Follows the layout of DAC section