Power Amplifiers with Valves

In my opinion this amplifier is now at the limits of what is possible when the output power is about 30 Watts and the concept is input stage – phase-splitter – output stage.

The output resistance of 0.6W is still high by today’s standard and it could be reduced with more negative feedback. A further 10 dB of NFB would lead to an output resistance of 0.2W , but I do not like this approach. NFB should be used but not abused. Output resistance could be taken down to zero ohms or even become negative when positive current derived feedback is applied.

Unfortunately the sonic results are rather unpredictable. What is an improvement when used in conjunction with one speaker system and cable can be quite the opposite when used with another speaker/cable combination.

The effect can be impressive and it can be disappointing. So unless you know exactly what you are doing and are able to measure and evaluate your results, I would not recommend the use of positive current feedback, but it can certainly make an

under the right conditions The power supply could benefit from the inclusion of a filter choke of 5-10 Hy and an additional chain of 220 + 220mF capacitors – see diagram. These components are necessary in a stereo amplifier but no harm is done using them in mono blocks as well.

Claus Byrith