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
US patent n.
4229706 by James W. Bongiorno and to the
3200 by Frank Blöhbaum.
I have used the LTspice IV free simulation tool to design and optimize this circuit.
The main characteristics
- 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)
This design use the interstage transformer
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 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 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 $12,95
Nichicon-73619 22000uF / 63VDC, KG Series, Super Through, 63mmD x 100mmH, Solder
Follows some measurement of distortion decay on 8ohm load,
24Vrms 70W with 0.7% thd and only 2 x 0.75Vrms input.
Follows the frequency response at full power on 8ohm load, dual channel
with sound card compensation
The my choice is the HiFi-2000 mod. Pesante dissipante
4 units with 10mmm
frontal panel got at Audiokit webshop.
On paper each lateral heat sink of this chassie have a capacity of 0.35 °C/W so
it will keep the output devices in a good safe area also when the environment
condition are terrible 35 °C.
Any serious solid
state amplifier need a protection circuit because a fault on output mosfet
can destroy the loudspeaker.
design have an intrinsic protection and
also if one power is disconnected the max dc output is about 4-5V.
store there are many shop with the UPC1237 Speaker BTL
Protection Board at less than 10$, you need two modules for two channels.
POWER SUPPLY SWITCH
||Vandal Resistant Push Button
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