THIS IS NOT A COMMON PUSH-PULL!

- very low feedback only 16dB
- low distortion near to 0.3 - 0.4%
- good damping factor near to transistor amp. 
- no electrolytic capacitors
started on March 10 ^st , 2012

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.
Here I am using a 10Kohm UL central tap instead of 4 - 5Kohm normally used for the KT88 in commercial amplifier.
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 this new amplifier.
The low distortion of this project like in 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 KT88 I have got 25W in triode connection and 35-40W using the ultra linear configuration.
This power could be considered low for a pair of KT88 but I want low feedback, low distortion and good damping factor.
To get the best sonic result there are no electrolytic capacitors on the signal path and in the anodic power supply.
I have used this amplifier to drive the my Monitor 3 and a pair of Audiostatic ESL ES240, in both cases the result is very very good better than previous PP2010.

UPDATE March 2013

I have inserted a 0.1uF Auricap to bypass to 2.2uF Solen to increase the sonic performances and the result is very good.
This bypass work from 17Hz frequency because ft = 1 / (2 * 3.14 * R * C) = 1 / (2 * 3.14 * 0.1E-6 * (180000/2)) = 17.
In the first schematic published there was no resistors on cathode but this has been inserted to check the bias current.
Before insert KT88 check the bias voltage on grid setting the trimmer for the max negative voltage.

UPDATE December 2022

The heat generated by the 82K resistors destroyed the solen capacitors so has been replaed with Jantzen Superior Z-Cap 0.68uF without bypass.
Leave the pin long about 3-4cm and far from the 82Kohm resistances (see last photos).

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. In this amplifier you can use only KT88 and KT90.     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.

To get the best result use Genalex - Gold Lion KT88 matched quad tubes available in the shop Audiokit.it, thetubestore.com or pmcomponents.co.uk. After extensive research and engineering, New Sensor Corp. has reissued the famed Genalex Gold Lion KT88. This tube has been recreated down to the finest detail with gold plated grid wire, carbonized screen grids, and a tri-alloy clad plate structure for exceptional performance and sound quality. As input tube I suggest to use the old 12AX7 Jan Philips or the Sovtek 12AX7LPS the to get the best sonic result. In this amplifier it is possible use any kind of 12AX7, ECC83, E83CC, CV4004. About the PCL82 use Philips, Siemens or Mullard and if you change the filament is possible use ECL82. The power supply fix the filaments reference at +150v instead of 0v for the PCL82 to consider the max cathode to heater voltage.

TRANSFORMERS

I have used a pair of old 10Kohm Tecnotrafo (Italy) push-pull transformers with UL.
In triode connection the KT88 with a load of 10Kohm anode to anode give an output impedance of only 3ohm so with 10dB of feedback  we have a good damping factor near to a solid state amplifier.

The power supply transformer has been created on my specification with an extreme low flux.
The primary has been calculated for 260V instead of 220V to keep the flux at 14000 gauss.

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

Complete schematic

All the amplifier in Triode connection

KT88 distortion and frequency at 25W with 10dB of feedback Vin=0.69Vrms

All the amplifier in ULtralinear connection

KT88 distortion at 30W with 10dB feedback and Vin=0.69Vrms

All the amplifier in Triode connection

KT88 distortion and frequency at 25W with 16dB of feedback Vin=1Vrms

KT88 distortion and frequency at 35W with 16dB of feedback Vin=1.2Vrms

For the chassie the my choice is the HiFi-2000 mod. Slim 04/350 4U 1NSL04350B with 10mm  frontal panel + 2 x Vented cover
 3SL0103350ASOLATO  + Inner Pierced Base 1BASE0105MAX  got at Audiokit webshop + 2 custom pieces of 3mm aluminium.

Vandal Resistant  Push Button Switches RS Stock No. 174-6381 Manufacturer Apem Manufacturers Part No. AV021003C900 The panel cut out  is 22mm, the same of noval socket, so this is the ideal choice for the 10mm front panel