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Hi-end DC coupled Circlotron  Hybrid Amplifier  started in the 2012, it will be end in the 2013


I don't suggest this project because i have found various stability problems that break mosfets

>>> See the new version on:  DC coupled Circlotron 2021

This amplifier born to try if it is possible increase the sonic performances of the Amplifier End.
t 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

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 2013).


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 parts.


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.