This amplifier born to try if it is possible increase the sonic performances of the Amplifier End.
It is the result of about 6 months of simulations and instrumental testing.
This design is inspired to the Circlotron US patent n. 4229706 by James W. Bongiorno and to the Thorens TEM 3200 by Frank Blöhbaum.
The DC servo loop has been design by my friend Marco Ferrari Bortolini with lot of experience on this type of circuits and this servo does not follow the Frank Blöhbaum European patent n. EP1548934B1 (english).
I have used the LTspice IV free simulation tool to design and optimize this circuit.
The main characteristics are:
- dc coupled of all stages
- no capacitors on the signal path
- extreme high slew-rate
- low distortion with very low feedback 3 or 6dB
- good damping factor like pure solid state amplifiers
- large frequency range about 0Hz to 1MHz at 0dB
- no switching output stage like a pure class
- only single ended stages
- the global feedback is managed by a tube
- low voltage power supply is necessary for tubes only +130 and -130
- open loop feedback easy to modify
- full balanced circuit
- it need only an input of 0.5Vrms to get 50W
- the dc servo loop does not change the vacuum tube operating point