Unison Research P70 White Paper - VMAX Services

More documents

Recommendations

Info

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.
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 <strong>P70</strong> 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.
Page 1 and 2: Unison Research P70 Fully balanced
Page 3: All the operations of the amplifier

Unison Research P70 White Paper - VMAX Services

Create successful ePaper yourself

Delete template?

Save as template?