Hi-end Balanced Circlotron Hybrid Amplifier

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Hi-end Balanced Circlotron Hybrid Amplifier started in the 2014

INTRODUCTION

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 Blöhbaum.
I have used the LTspice IV free simulation tool to design and optimize this circuit.

The main characteristics are:

no capacitors on the signal path
high slew-rate
low distortion with very low feedback 6-9dB
good damping factor like pure solid state amplifiers
frequency range about 20Hz to 20KHz at 0dB
no switching output stage like a pure class A operation
output power 70W on 8ohm with low distortion 0.7%
only single ended stages
the global feedback is controlled by a tube
only 4 components on the signal path, including the DAC chip
low voltage power supply is necessary for tubes only 180V
zero noise so it is good to drive high efficiency drivers
long tube life, about 10000 hours
filaments soft start for a long tube life
full balanced and differential design
it need only a balanced input of 1Vrms to get about 50W
direct interface to ES9018 DAC chip (or any other differential DAC chip like WM8740, CS4397 and AK4397)

DESIGN

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:

Internal resistance about 2Kohm
Anode current max 30mA
Voltage gain 77 in triode (g2 connected to anode)
Very low distortion
Very good sound
Vg2 max 180V

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

POWER SUPPLY

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:

Nichicon Super Through <<< Parts Connexion my first choice
Kendeil K01 my last choice
Jensen Electrolytic my second choice
Mundorf M-Lytic my last choice
BHC Slit Foil my third choice

I am looking about these models:

Jensen electr. 4 pole axial cod. 012023 10000µF -20/+20 63 DC 035x050 $61,00
Jensen electr. solder term. cod. 022670 22000µF -10/+30 63 DC 050x065 $68,51
Silt Foil Guided Current 4TTN capacitor - 10,000 µF 63 Volt 4 pole Length 58mm, Diameter 40mm £30.00
Nichicon-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

MEASUREMENTS

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

LAYOUT

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.

PROTECTION CIRCUIT

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.

POWER SUPPLY SWITCH

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