Why a push-pull amplifier?

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1 Unison Research P70 Fully balanced dual mono valves Amplifier in Push- Pull topology Why a push-pull amplifier? A push-pull topology amplifier is characterised by a power stage in which active devices pairs work together amplifying separately the positive and the negative signal half waves. The output transformer, with its particular structure, reconstructs the amplified signal and makes it available at the output terminals. The main benefit of this topology is the possibility to design amplifiers which have a much higher power than their single-ended counterparts, using the same power valves in the output stage. This advantage reflects in a better efficiency, which also means a significant reduction of the dissipated heat at the same power rate. Nonetheless the push-pull stages, because of to the simultaneous action of two different devices, needs a very careful design and set up to avoid the large distortion that always occurs when the amplification of the two half waves is not equal.

2 A bit of history... In the past Unison Research had designed and produced different push-pull models, mainly because it was complicated and expensive to obtain large output power with single-ended stages. Among the others let us mention the Triode 20, the Power 35 and the early Nimbly. The P70 Designing the P70 has required about one year of work. The very first prototype has been developed in 2006, and it immediately showed all the critical elements related with this kind of design. It took time to find all the electronic solutions that have lead to the current cirquitry. The P70 is now an amplifier capable of delivering more than 70 W per channel into a 6Ohm load, with a THD lower than 0,2% over the whole audio band. Its performances are comparable to those of a good single ended amplifier. Structure of the amplifier The amplifier is laid out in dual mono configuration. Both the amplifier stages and the power supply circuits are completely independent. The signal path in the amplifier is fully balanced from the input through the output. This means that the signal at the output terminals is balanced with respect to the amplifier ground. The P70 has four line inputs. All of them can be balanced or unbalanced, an external switch allows the selection. The volume potentiometer is a four sections motorised Alps blue series potentiometer. The selection of the channel is performed by four high quality relè.

3 All the operations of the amplifier, included the safety functions, are controlled by a microprocessor to ensure maximum reliability and intuitive user interface. The amplifier itself is constituted by an input stage featuring two double triodes ECC83 and one double triode ECC82, and a power stage with two KT88 per channel. The output transformer has been custom designed by Unison Research and ensure a bandwidth of more than 30kHz at full power. The design During the development of the first prototype, we had the possibility to observe and analyse how the push-pull power stage requires a driving current significantly higher and differently shaped than that of a single ended power stage. Thus we concentrated our efforts in redesigning the circuit so to keep the output impedance as low as possible. Another point that has required special care is the polarization of the whole output stage, which had to be stable enough to sustain the large signal excursions required to properly drive the stage under the most critical operating conditions. The preamplifier is constituted by a double differential pair in an original configuration. The two differential stages are DC coupled and allow to reach the needed gain. They are followed by a couple of cathode followers, again DC coupled, which guarantee the low output impedance required. The power supply for the whole preamplifier stage has a very high voltage, around 600V.

4 The input stage has a local feedback, characterised by different feedback factors for the bias component and the signal. The two different values are set keeping into account the different need for precision on the value of the bias and of the signal. The importance of a stable controlled bias in the amplifier is due first of all to the choice of the DC coupling between the stages and also to the need to allow the same signal excursion along the paths of the balanced signal. The circuit we have designed amplifies signals up to 200V pp. Conversely, the feedback factor affecting the signal has been kept to the lowest possible value. It worth mentioning that the particular circuit configuration chosen and the presence of a double differential stage guarantee the possibility to use the amplifier also with unbalanced signals. The proper connection can easily be done through the input terminals on the back. Finally, the signal is feedback in a balanced way to the cathodes of the input differential stage, as in a traditional triode amplifier. The power stage is constituted by a couple of KT88 per channel. During the set up, we have considered that a bias current of about 50mA allows an adequate trade-off between maximum power, distortion and power dissipation. We have also observed how, in most of the traditional push-pull stages solutions, the level of the bias is hardly constant when the input signal or the power supply voltage changes, and this makes the tube work, although for short times, in interdiction or over-bias condition.

5 Moreover, these schemes often require a manual tuning of the bias current and the balancing of the currents in the two paths that has to be performed with the amplifier warm and fully working. To solve this problem, in the P70 a brand new circuit has been designed, which ensures a constant bias level with respect either to variations of the input signal or the power supply, or in overload conditions, or to variations of the tubes characteristics due to ageing. The correct polarization is reached immediately after switching on, even though the amplifier has not yet reached the optimal operating temperature. This circuit continuously verifies the bias current level and the currents in the push-pull paths, and automatically provides the eventual needed corrections. The user doesn t have to manually tune the bias, even after the KT88 pair has been replaced. We are considering to have this solution patented. As far as concerning the power supply, according to the dual mono lay-out already mentioned, there are two completely independent circuits. It has a first filtering stage entrusted to a couple of high voltage p configuration LC filter for the 650V of the power stage supply. The second filtering stage is entrusted to a couple of high voltage p configuration RC filters for the 600V of the amplifier stage supply. The tube heaters power supply has a regulated DC voltage. Another negative voltage is regulated to provide the bias of the KT88.

6 Technical data: Power Output: 70W + 70W RMS Output Impedance: 6 Ohms (4-8 Ohms) Input Sensitivity: 500 mv Input impedance: 47 KOhms Frequency Response: 10 Hz to 40 Khz (0.5 db 1W) Signal/Noisse Ratio: 83 dba Total Harmonic Distortion: 0.3% (10W) Negative Feedback: 12 db Damping factor: 8 CMRR: 59 db Power Requirements: / V (50-60 Hz) at rest 360W Dimension (WxDxH): 460x450x200 mm Weight: 35 Kg