PP2011 - Hi-End Push Pull Amplifier

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PP2011 - 20W Hi-End Push Pull Amplifier
started on July 9^st , 2010

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 sound quality 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.
The very good result got with the PP2010 and availability of a pair of old 10Kohm Tecnotrafo (Italy) push-pull transformers have forward me to create new amplifier.
The low distortion of the PP2010 has been obtain using a perfect phase inverter.
I know many DIY does not like the printed circuit board and the hybrid stage with fet of PP2010 so I have created a new driver stage with the same performances but using only vacuum tubes.
This new driver stage can be used without changes in the PP2010 to get 50W.
Using only a pair of EL34 I have got 18-20W using the ultra linear configuration.
This power could be considered low for a pair of EL34 but I want low feedback and good damping factor.

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 input tube 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.

The power supply fix the filaments reference at +150v instead of 0v for the PCL82 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	0v	0V	60V
PCL82 Triode	200v	150v	150v	0v
PCL82 Pentode	250v	380v	150v	130v
EL34	100v	1V	0V	79V

ECC83 socket, bottom view

EL34 socket, bottom view

PCL82 socket, bottom view

TRANSFORMES

DRIVER

This driver stage give these performance:

incredible low distortion at high output swing
high voltage gain, about 34dB
symmetrical outputs with same output impedance and level
dc coupling of all components
single power supply if we exclude the negative used also for the final tubes bias
low output impedance

FINAL SCHEMATIC

In the triode connection the power is limited and the damping factor is not enough good.
The ultra linear connection since to be the best configuration, see test here on PP2010.