The main characteristics
dc coupled of all stages
- no capacitors on the signal path
extreme high slew-rate
- low distortion with very low feedback 3 or
- 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
To drive this
amplifier you can use a LL1684 Lundahl to create balanced input or my
new USB DAC with balanced outputs (kit available at the begin of
first version tested
last version on pcb tested by my friend Michele
The driver stage need a good low ripple power supply like a CLC,
a regulated or a virtual battery operation.
In the output stage you can use a normal capacitor only, a CLC or a CRC
like Nelson Pass design.
There are some choices for the power supply capacitors:
The my choice is the HiFi-2000 mod. Pesante dissipante 5 units with 10mmm
frontal panel got at Audiokit webshop.
On paper each lateral heat sink of this chassie have a capacity of 0.25 °C/W so
it will keep the output devices in a good safe area also when the environment
condition are terrible 35 °C.
It is possible use 4 units container but you should use normal power supply
instead of CLC for the output stage because there is no enough space for all the
>>>> PROTECTION CIRCUIT
Any serious solid
state amplifier need a protection circuit because a fault on output transistors
can destroy the loudspeaker.
This design need a relay on output terminal to disconnect the loudspeakers
during the start-up phase at the switch-on for 1min.
Follows some measurement of distortion decay on 8ohm load, thd of
0.58% at 50w.
Follows the frequency response at full power on 8ohm load, only +-0.1dB in the 10-20KHz range.
Here some photos of this project build by Michele Branchini.