back to

To get more information contact me at:

Hi-end Balanced Circlotron  Hybrid Amplifier  started in the 2014


This amplifier born to try if it is possible increase the sonic performances of the Amplifier End and to create a complete system with the HQ reference DAC ES9018S.
This design is inspired to the
Circlotron US patent n. 4229706 by James W. Bongiorno and to the Thorens TEM 3200 by Frank Blhbaum.
I have used the LTspice IV free simulation tool to design and optimize this circuit.



The main characteristics are:


This design use the interstage transformer LL1660/PP and the D3a pentode tube connected in triode mode.
The D3a  is used in many my new projects because it have special features that make it unique:

The output mosfet is the 2SK1058 used in many projects and I consider this the best after the result obtain on my test on Circlotron.


circuit used in the LTspice simulation

complete circuit

complete circuit with the transformer connections



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:

I am looking about these models:

Jensen electr.    4 pole  axial    cod. 012023 10000F -20/+20 63 DC 035x050    $61,00
Jensen electr.    solder term.    cod. 022670 22000F -10/+30 63 DC 050x065    $68,51
Silt Foil  Guided Current 4TTN capacitor - 10,000 F 63 Volt 4 pole  Length 58mm, Diameter 40mm 30.00
-73633 10000uF / 50VDC, KG Series, Gold Tune, 35mmD x 80mm, Solder Lug  $12,95
Nichicon-73619 22000uF / 63VDC, KG Series, Super Through, 63mmD x 100mmH, Solder Lug $49,95



Follows some measurement of distortion decay on 8ohm load, 24Vrms 70W with 0.7% thd and only 2 x 0.75Vrms input.

Follows the frequency response at full power on 8ohm load, dual channel with sound card compensation



The my choice is the HiFi-2000 mod. Pesante dissipante 4 units with 10mmm frontal panel got at Audiokit webshop.
On paper each lateral heat sink of this chassie have a capacity of 0.35 C/W so it will keep the output devices in a good safe area also when the environment condition are terrible 35 C.



Any serious solid state amplifier need a protection circuit because a fault on output mosfet can destroy the loudspeaker.

The Circlotron design have an intrinsic protection and also if one power is disconnected the max dc output is about 4-5V.

On Ebay store there are many shop with the UPC1237 Speaker BTL Protection Board at less than 10$, you need two modules for two channels.


Vandal Resistant  Push Button Switches

RS Stock No. 174-6381

Manufacturer Apem Manufacturers

Part No. AV021003C900

The panel cut out  is 22mm, the same of noval socket, so this is the ideal choice for the 10mm front panel