INTRODUCTION

After many years designing single ended amplifiers I have decided to start a series of test about the Push Pull amplifiers.
These amplifiers are considered at a lower level than single ended but if you want a power enough to drive ESL loudspeakers and low distortion systems like Audiotechnology, Dynaudio, Scanspeak, Morel and Seas there is no other choice.
The main problem of Push-pull amplifier is the not perfect distortion decay with the cancellation of even harmonics but if we keep the distortion level very low this consequence  can be ignored and the sound will not be affected.
Another characteristic of many Push-pull amplifier is the high global feedback used to get a good damping factor because to increase the output power often are used output transformers with low turn ratio.
As my others amplifiers to preserve the signal I have used the minimum number of components on the signal path so only one inter stage capacitor is present.
Here I would like to create a Push-pull taking only the benefits of this design and minimizing the bad effects.
This amplifier, like the my other designs, can be connected directly to a CD Player or to a DAC without using a pre-amp.

update 23 Feb 2016

Some modification by B-LAB of Stefano Bianchini

Vout=20Vrms with less than 0.5% thd so about 50w on 8ohm

Vin=0.962 Vrms to get 50w

Output impedance near to 0.5ohm

Frequency band 13-20KHz at -0.2db

Global feedback about 4.5dB

This amplifier give an incredible good sound on any loudspeakers, tested on the my Monitor 2 e Hi-efficiency system.

TUBES

To create a push pull  amplifier is possible to use triode and pentode but obviously to test many configurations like the ultra linear we need a pentode. There are many pentode good for audio amplifiers but the most common in the current production are the EL34, 6550 and the KT88 so follows a compare table with the main characteristics and price:     EL34 6550 KT88 Power max 25W 42W 42W Anode voltage max 800V 500V 800V Screen voltage max (G2) 400V 440V 600V Cathode current max 150mA 190mA 230mA Internal resistance in triode conf. 1250ohm 1000ohm 1000ohm Price medium each (euro) 15 27 30 The output plate characteristics have been measured with the Sofia computer-controlled curve-tracer by Audiomatica and I have added the lines to calculate the internal/plate resistance. It easy to consider two EL34 better than one KT88 or one 6550 because with two EL34 in parallel we have a lower internal resistance (1250 / 2 = 625ohm), the power is 20% more and the cost is the same.

To get the best result use matched quad tubes available in the shop thetubestore.com or pmcomponents.co.uk. As driver tubes I have used the Jan Philips series to get the best sonic result. In this amplifier it is possible use any kind of 12AX7, ECC83, E83CC, CV4004 and 12AU7, ECC82, E82CC, 5814 without change other parts. The power supply fix the filaments reference at +80v instead of 0v so to consider the max cathode to heater voltage.

In the datasheet is mention this limit:

tube model used	max cathode to heater voltage (max Vkf)	circuit cathode voltage (Vk)	heater voltage (Vf)	Vk - Vf
ECC83 / 12AX7	180v	20v	80V	60V
ECC82 / 5814 / 12AU7	180v	246V	80V	166V
EL34	100v	1V	80V	79V

The circuit cathode voltage (Vk) is calculated with:

EL34 current = 46mA
Resistance on cathode = 47 // 47 = 23.5ohm
Cathode voltage = 23.5ohm * 46mA = 1V

ECC83 current 1mA
Cathode voltage = 20V

ECC82 current 6mA
Resistance on cathode = 82K // 82K = 41Kohm
Cathode voltage = 41Kohm * 6mA = 246V

ECC83 and ECC82 sockets, bottom view.

EL34 Socket, bottom view

2N5459 pin

BC546B pin

TRANSFORMES

2 x Bartolucci 557E

Nominal power 50w
Primary impedance 4500 ohm
Primary Inductance 36 H
Primary resistance 64ohm
Weight 4 Kg
Secondary grid at 40%

1 x Bartolucci

Nominal power 400w
Primaries: 2 x 115v
Secondaries:
2 x 350v 0.4A
2 x 6.3v 9A
1 x 100v 0.1A

The price for the complete set of these transformers is about 400 + tax 20% (3 item).

PHASE INVERTER

	In all the push-pull amplifiers is necessary a phase inverters to split the unbalanced input signal into two balanced signals out of phase by 180 degrees to drive the push-pull output tubes. The performances of this phase inverters is very important and it affect the final result of the amplifier. Most push-pull amplifiers use the the split load or the paraphase but both have a lot of disadvantages. The split load without a regulation give two precise outputs with  the same level but the output impedance are completely different so it should not be used to drive directly difficult load like the final stage. If used as first stage the split-load need an input capacitor to delete the bias dc voltage. Another disadvantage of this phase inverter is the gain equal to 1x so more driver stages are necessary. The paraphase have two outputs with the same output impedance but it need a regulation to set the same level on both outputs.   In this phase inverter the signal path is not the same for both output signals and for one there is a feedback (see V2, R8,C3).   These first two schematics are parts of Bartolomeo Aloia amplifiers. To get the best performances with the minimum components I have decided to use the differential stage seen in the most of solid state amplifiers. This type of differential is present in any electronic manual and if it is well configured it can give a very high performances without disadvantages. There are two methods to implement this: the first of these is use a current generator build with a fet or a transistor and a little negative power supply, The second method is to use a common resistor with a  large value, about 220Kohm, connected to a high negative voltage about 300v. Some designer create a similar stage using as common resistor with low value but in this case this stage work bad and the two differential outputs have different levels. Often in the solid state amplifiers are used dual fet to get the best selection of the two fet but if we want to use normal single fet if possible to add a multi turns trimmer to balance the little differences on output levels. I have seen a similar phase inverter in the AudioResearch VT100 reference amplifier where  a mosfet has been used instead of the fet to create a current generator.

SCHEMATIC

In the triode connection the power is limited and the damping factor is not enough good also if we use the cathode feedback of the transformer secondary.
So I have decided to switch to the ultra linear connection also if this mean go near to the pentode configuration with an inevitable increment of the internal resistance of vacuum tubes, see test here.
Using this connection we get also more voltage gain in the final stage so the cathode feedback of the transformer secondary compensate the increment of  internal resistance of vacuum tubes and the output impedance without a global feedback is only about 4ohm.
This output stage design with both cathode feedback and ultra linear is inspired to the AudioResearch reference amplifiers.
To have enough voltage gain to use global feedback I have inserted in the first stage to a hybrid cascode with vacuum tubes and fet.
The input fet used in this amplifier create a very good result maintaining a simple design and this should not persuade you to love this design.
It is possible see in many AudioResearch vacuum tube reference amplifiers the fet in input stages.
This driver stage give these performance:

• very low distortion at high output swing
• high voltage gain
• symmetrical outputs with same output impedance and level
• dc coupling of all components
• single power supply
• low output impedance
   

PCB

The driver stage of this amplifier is a little bit complex so I have decided to create a pcb using Eagle Cad.
Follows the schematic of all the parts included in the pcb.

• Here download Eagle files