back to

To get more information contact me at:

Amplifier End  started in the 2011


This an "End" level design so, like the Phono End and DAC End, it give the max sonic performances.
Now you may be perplexed about a hybrid design if you are a purist of vacuum tubes but I tell you all my friends are shocked by the result of this amplifier.
In a direct comparison with the my new
813-GM70 SE, the Hybrid 2010 version 2 and the PP2010, the difference is huge and it is very difficult to return to hear these other amplifiers.
All sounds are clear and detailed with incredible speed issue that leaves you speechless.

In the past 20 years I have made dozens of amplifiers with different topologies, but the results obtained with this was totally unexpected and unexplained.
I suggest this project to all those who want a definitive amplifier because it can be used to hear any kind of music and it drive any kind of loudspeakers without problems (multi-way and full-range).
A great experience is not required for this project because the assembly is very close to an electrical system rather than an electronic amplifier.
A pcb is only necessary for the protection circuit, but you can buy an assembled module and modify it.
Also if the
Circlotron output stage avoid high dc output voltages in case of mosfet fault I suggest the use of a protection circuit like in my all solid state design.
There is the possibility to reduce and increase the output power with very simple modifications in the transformers and in the heatsinks.
The majority of benefits are available even implementing this project in a simplified version with a cheaper power supply (no chokes for the output power section).
Using a CLC (capacitor, choke e capacitor) power supply is possible skip any problem of noise and spikes generated in the rectifier.
In the driver section you can decide to use the Siemens D3a (hi-end pentode to use in triode connection) or the Russian 6C45 (pure triode).
Both these tubes give a very similar sound performances and it is impossible decide which is the best.
The test show as best mosfet to use the
ACD100NSD  and the 2SK1058 (attention to the fake on Ebay).
In these last years many person prefer to see circuit with the new
depletion mode mosfet instead of enhancement mode but the linearity of the depletion is necessary only when the mosfet is used as voltage amplifier (drain output) like the Nelson Pass F5, in my amplifier the mosfet are a follower (source output).
To use the depletion mode mosfet is necessary add a thermal compensation in the bias circuit.

UPDATE August 2012:

There is a prototype of this type of design developed in 1995 and published in 2004 in several editions by Yury Nagorny:

UPDATE September 2012:

After many listening tests I decided to eliminate the bypass capacitors on power supply so the 10uF and 47uF Solen MKP are no longer needed.
The bias current must be set to 1.2A (0.19V on 159ZJ choke) to prevent the shutdown of mosfet and to get a pseudo class A operation.


Please do not touch with multimeter the mosfet gate after power on because this operation will destroy the mosfets.
Before to connect the mosfets set the trimmer to have about 1.8V DC on 470ohm resistors and after the power on increase both trimmers until the correct bias is set and turn only one trimmer to get zero DC voltage on output terminals (after check again the current).
This is an output bridge amplifier so both the output terminals are active and these must be isolated from ground.
If you don't use the chokes on power supply add a 0.1ohm 2w resistor in series to each power supply to measure the bias current (0.12V on 0.1ohm mean 1.2A bias).


These companies have supported this project with some discounts on the components.





I have design the power supply to follow a single ended amplifier style but you can use a common solid state design without chokes.
The CLC power supply skip any noise problem and it is superior to any pure capacitor design also if large value are used.
In the following simulations with PSU Designer II you can see the big difference in residual noise, with CLC type we get a pure sine wave so it mean low harmonics.

The Hammond chokes used in this amplifier are not difficult to integrate in any amplifier because these have a compact size and these are not expensive.

Following the experince of my last amplifiers I have used Schottky Fast Soft recovery diodes in any section of the power supply but you can use normal 36A diode bridge like the IRF 36MB60.

To create a fast diode bridge with these TO220 and large SMD chips I have created 2 simple pcb.
You can ask to Quanghao to get a quotation about these pcbs.

For this design you can use normal transformers produced in stock  but my choice is 2 custom made on my specifications:

Italtras Srl Transformers in the world
30-0       6A
30-0       6A
9-0         2A
115-0     0.3A

external size 125x65mm, core type M0, flux 80% than normal (13000 gauss instead of 17000 gauss).

There are some choice for the power supply capacitors:

In the output stage I have used 50V Nichicon Super Through because lower voltage sound better and in the the driver stage 100uF 450V  Kendeil K05 + 150uF 200V old stock Blackgate.

After the good result obtain in the Cheap System 3 all the power supply have a large MKP Solen bypass capacitor to be sure that medium and high frequency have a fast road.

The 8.2ohm 10W resistor is necessary to reduce the filament and bias voltage, it function is also skip any noise of peak generated on diode bridge.
If you don't use this resistor and reduce the secondary voltage of filaments in this case the transformer will
amplify the noise peak on higher voltage secondaries.
I have used a 8.2ohm 25W to fix this resistor easily with two screws.


The my choice is the HiFi-2000 mod. Pesante dissipante 5 units with 10mmm frontal panel.
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.

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

I have integrated in the same circuit a soft-start to eliminate shock on electric environment during switch on, a push button to get a beautiful look and the protection on dc output.

You can use the following protection circuit with BC547 without the main board with 4013 and LM311 if you don't need the push button feature and the soft start.




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


I have found 2 little problems in the pcb so there are some corrections to do (in the schematic are in red).


Follows some measurement of distortion decay at 3 different output voltage on 8ohm load, the last show a thd of 1.5% at 50w.

Follows the frequency response at full power on 8ohm load, only +-0.2dB in the 20-20KHz range.


I always used normal power supply cable but for this amplifier I will try this DIY cable with these following parts.

PRVV21A Gold

GOLD CE - IEC 10A/250V high temperature (155C)vertical contact professional cable Holder for cable diam. mm 9 about,

Schuko 1 Black GOLD

GOLD CE- European Schuko for cable diam. max 9mm


3x1,5 OFC Red copper conductors with double Copper+Aluminium  shield ultraflex sheat 



The total cost to create this amplifier is not very low but you are building a reference amplifier without compromise.
It is possible use normal power supply for the output section with only large capacitors instead of CLC and you can use a solid state power supply for the driver stage like in my PF2007, Pre Phono 4,DAC End 2 and Headphone amp. End.

Qty model brand unit price (euro) total price (euro) shop
2 D3a Siemens 20 40
4 2SK1058 Hitachi 7 28 Electronic shop
2 LL1671/20mA Lundahl 100 200 Lundahl distributors
1 10K+10K DACT 145 145  
4 159ZJ Hammond 20 80  
2 159M Hammond 21 42  
1 159ZE Hammond 20 20  
4 10000uF 50V Nichicon Super Through 15 60
4 15000uF 50V Nichicon Super Through 20 80
4 47uF 250V MKP Solen 15 60
4 2.2uF 250V MKP Solen 3 12
2 Custon TR 130 260
20 MBR40250 ON 2 40
8 MBRS3200T3 ON 1 8
2 22000uF 16V Kendeil 8 16 Electronic shop
1 chassie Hi-Fi2000 196 196  
1 PRVV21A IEC Gold 6 6  
1 Shuko 1 Gold 6 6  
2 CY3150 cable 7 14  
1 connectors   50 50  
1 other items   50 50  
      total 1413