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INTRODUCTION
This is a 20-25W Class A Single ended no feedback amplifier and only 2 active components on the signal path.
If you are looking for an extreme quality amplifier and
you don't need high power this is the project for you because nothing can have
higher performances.
After 22 year I have
rebuild the same project because this is perfect on all aspects except the
efficiency but this is normal for a true single ended class
A.
This project is an hybrid amplifier composed by a tube voltage
amplifier followed by a single ended mosfet current amplifier with an exclusive
configuration.
I have published several
articles about this my current amplifier and this design was the winner of the
award in the Circuit ideas in May 2000 on Electronics World which
is the most important electronic magazine in the world.
This amplifier was
built by a lot of people around the world and everyone are enthusiastic about
the result, the sound is compared to that of the best single ended vacuum tube
amplifiers but this have a higher performances in terms of driving capacity
(damping factor) and low distortion.
To read these articles search on the web page: www.audiodesignguide.com/doc/index1.html
It seems
a silly and common circuit but it has got great differences from any other
similar design, that’s why this is unique.
We have a typical source
follower (as an emitter follower but with a Mosfet) working in pure class A with
a current generator.
Please note that this circuit works only in pure
class A, so it requires enough bias current for the requested output power.
There are no particular limits on the power that can be delivered but obviously
the efficiency of this amplifier is very low, 25% theoretical and 15% real so
you need a big heatsink also for medium power about 20W.
To drive this
current amplifier is necessay a voltage stage
with an output swing not lower than 10Vrms and Rout < 1000ohm.
.
In my design the first feature
making a big difference in the sonic results is the negative power supply and
the signal ground tied to the drain of source follower.
With this design we achieve:
The discoupling
The power supply integrate a Virtual Battery to get a reduced ripple and to use lower capacitors.
The 500
Ohm precision trimmer should be set to balance the output fuse voltage to 1/2 of
the regulator output.
This
single trim should be set after the switch-on, because it just optimizes the
output swing to the maximum available.
The 2 x 18V 1W zener are optional and these can be used to
prevent crash of the mosfet when it receive bad input signal like a vacuum stage
startup (these could decrease the sonic performances).
The
phase on output terminals has been inverted because here I will use a voltage
amplifier that reverses the phase.
The
quiescent current is set by the resistence on current generator source pin, and
can be changed with the simple formula Iq = 0.65 / R where 0.65 is the
transistor Vbe.
With a
bias current of 3A you can get about 20 Wrms on 4 and 8 ohm load.
The power
can be increased with the unique drawback of the dissipated power (heat)!
I have already specified the only problem with this
project is efficiency so first of all it is necessary to establish the maximum
power that we are able to dissipate.
We will have about 20W with a power
to dissipate near to 130W per channel so the efficiency is
15.6% but a single ended with the same output power is very similar, for example
the my GM70 SE give this result:
description | voltage (V) | current (mA) | power (W) |
anode current GM70 | 1100 | 80 | 88 |
anode current D3a | 180 | 20 | 4 |
filament GM70 | 20 | 3000 | 60 |
filament D3a | 6.3 | 315 | 2 |
total | 154 |
Ibias (A) | Vdc (V) | Power to dissipate (W) | Heatsink (°C/W) | Tamb max (°C) | Tchassie (°C) | Vout (Vp) | Vout (Vrms) | Rload (ohm) | Pout max with voltage used (W) | Iout (Irms) | Pout max with current used (W) |
3.0 | 36.0 | 108.0 | 0.31 | 35 | 68 | 17.0 | 12.1 | 8.0 | 18.2 | 2.1 | |
3.0 | 36.0 | 108.0 | 0.31 | 35 | 68 | 17.0 | 12.1 | 6.0 | 24.2 | 2.1 | |
3.0 | 36.0 | 108.0 | 0.31 | 35 | 68 | 17.0 | 12.1 | 4.0 | 2.1 | 18.1 |
The
output device, IRFP150, can be substituted by their TO3 equivalents or by other
similar MOSFETs like IRF250, IRFP250, IRF240, IRFP240, with a minimal impact but
the IRFP150 has been selected because these have a lower input capacitance and a
low Zout.
There are some differences on mosfet specifications from one manufacturer to
another, a low input capacity is crucial for having a good high frequency
response:
IRFP150 IRF Ciss=2800pF
IRFP150 Fairchild Ciss=2000pF
IRFP150 Vishay Cis=2800pF
IRFP150NPBF Ciss=1900pF
In this new design I have used the IRFP150NPBF by Infineon (RS cod. 541-0856) with only 1900pF.
VOLTAGE AMPLIFIER
The presented topology, has no
voltage gain (actually it looses something 1.3%) so it should be driven by voltage gain stage, with an output swing not lower than 10Vrms and Rout < 1000ohm.
TO DO
PCB available in this Ebay shop
I advise to build this project using a pcb instead of an
air wiring because it is certainly more stable.
PCB available in this Ebay shop
This is a single channel pcb so duplicate the components list.
PCB files available for DIY.
Mount the resistances in parallel configuration in opposite phase or direction.
R1=Rbias 0.2ohm 5W
Mouser 71-LVR5-0.2
R3
1Kohm 2W
Mouser
660-MO2CT631R102J
R9
2Kohm 2W
Mouser
660-MO2CT631R202J
R11/R10
500ohm multi turns trimmer Mouser
81-PV36W501C01B00
R4,R5
470ohm 1/4W 1%
R6,R7
220Kohm 1/4W 1%
Mouser
594-MBB02070C2203FCT
R12
2700ohm 2W
Mouser
660-MO2CT631R272J
R16
220ohm 1/4W 1%
R14
- NOT
USED
R15
-
NOT USED
R13 -
NOT USED
R8
-
NOT USED
Q1,Q2
IRFP150NPBF
RS 541-0856
Q3
-
NOT USED see jump
Q4
MJE340
Mouser 863-MJE340G
D14,D5
zener 18V 1W
Mouser 78-1N4746A
F3
fuse 5A FAST with fuse holder Mouser
534-4628
C1
-
NOT USED
C4 470uF 63V
Mouser
647-UPW1J471MHD3
C2 0.47-1.0uF 400V
MKP
C3 100uF 50V
Mouser
647-UPW1J101MPD
C5 1000uF 50V
Mouser
647-LKG1J102MESZCK
Cout min. 4700uF
50V Nichicon KG
Rout
1Kohm 2W
Mouser
660-MO2CT631R102J
Isolators Bergquist SP400-0.007-00-104 RS RS541-0856
The connections are 63862-1 (CUT STRIP) by TE Connectivity / AMP (cod. Mouser 571-63862-1-CT, cod. RS 718-7987)
PCB available in this Ebay shop
This is a two channels pcb so the components list is complete.
PCB files
available for DIY.
Mount the
resistances in parallel configuration in opposite phase or direction.
R1,R2,R18,R19 220Kohm 1/2W 1% Mouser
71-RN60D-F-220K
R3,R4,R20,R21 2200ohm 1/4W 1%
Mouser
71-RN60D2201F
R5,R6,R8,R9,
R10,R26,R22,R23 39Kohm 2W
Mouser
71-CCF02-J-39K
R7,R24
100ohm 1/4W 1%
Mouser
71-RN60D1000F/R
R37,R39
47ohm 1/4W 1%
R11,R34
4700ohm 1/4W 1%
R12,R30 2 x 100Kohm 1/4W 1% to have 200K
R13,R28 4700ohm 1/4W 1%
R14,R29 150Kohm 3W
R17,R33
3.3ohm 2W
R25,R35
0ohm
R15,R16,R31,R32
220ohm 1/4W 1%
C1.C3 220uF 6.3V OS-CON
Mouser
667-6SEPC220M+TSS
??
C2,C4
33uF 400V Mouser
647-UVY2G330MHD
C5,C6 100uF 400V
Mouser
647-LGU2G101MELZ
CY1-CY8 10nF 440VAC
Mouser
80-R474I210050A1K
D1,D2,D3,D4,D6,D7,D8,D9
UF5406 Mouser
625-UF5408-E3
D5,D10 zener 10V 1W
Mouser
78-1N4740A-TAP
U$3,U$6 IRF840
Mouser
844-IRF840APBF
U$11,U$7 33uF 400V Solen
MKP or Vishay MKP 50uF 500V
KK1,KK4
Extruded Style Heatsink for TO-220 Mouser:
532-513102B25
Isolator
Bergquist SP400-0.007-00-54
RS
169-2177
Isolatos
TO-220 nylon platstic Insulator hole size M3
The
connections are
63862-1 (CUT STRIP) by
TE Connectivity
/ AMP (cod. Mouser 571-63862-1-CT, cod. RS 718-7987)
The heat sinks have been grounded to avoid receiving
radio frequency so these must be isolated from the transistor.
INTERSTAGE CAPACITOR
Obviously it is fundamental to use a high quality interstage capacitor and for
this project I will test 4 different types of a good UK company.
This test
was necessary to decide which capacitor to use because it is not true that it is
enough to buy the most expensive one to be sure of having the maximum sonic
performance.
It happened to me to discard many high-level interstage
capacitors, also very well evaluated.
In all my sonic performance tests I
always search to be sure of an objective result using more persons and these
with different experiences.
The ClarityCap has been manufacturing high
quality audio grade capacitors for over 30 years.
Clarity Cap CMR MKP 630VDC
This is the most expensive capacitor of the set with
a really detailed sound and a good sound stage.
After the test I chosen to
use this but I recommend it only to those who have a really good source
because otherwise it could sound too tiring.
This is definitely a
totally transparent component that adds nothing to the signal.
The micro
details that can be discovered in already known tracks are impressive.
Clarity Cap
CSA MKP 630VDC
This is the second choice,
it is an incredible
value for money,
in some case it could be preferable to the first
because it give a good
detailed sound, of course not like the first, but
it is more soft so this effect helps in the case of digital sources a bit
harsh like CD player.
Clarity Cap
MR MKP
A good result but some details are lost compared to the
first two.
Here the evaluation expressed is in contrast with the verdict
expressed by Humble
Homemade Hifi.
Clarity Cap
ESA MKP
This capacitor has a very convincing value
for money, there is a big difference from the MR and CSA.
The low frequency cut-off is determined by the value of this capacitor and input resistance of the current amplifier (110Kohm).
Ft(-3dB) = 1 / (2 * pi * C *
R) = 1 / (2 * 3.14 * 0.68uF * 110Kohm) = 1 / ( 2 * 3.14 * 0.68E-6 * 110000) =
2.13Hz
I suggest to use values in the range 0.47uF to 2uF.
TUBES
E180F
- CV3898 old Mullard
E180F - 6688 Siemens Gold Pins New Old Stock
6Z9P - 6ZH9P -
E180F
POWER SUPPLY
|
After the good result obtain in the last
Inpol / Mofo
project I have decided to test the switching power supply also in my
best project. I have got the same sound of the normal version but higher efficiency (about 20W saved for each channel), less cost (200 euro saved) and less noise (at 100-120Hz). For each channel I have a MEAN WELL LRS-150-36. It is a 150W Single Output Switching Power Supply with a variable DC output in the range 32.4 ~ 39.6V. There are integrated protections for short circuit, overload, over volatge, over temperature. This model has been choised because it is cooling by free air convection and give until 4.3A. |
The power
supply transformer for the driver stage is an 40W R-core model R26-09. You can buy on Alixpress online shop. The secondaries have 2 x 220V 50mA and 2 x 6.3V 0.8A. |
OUTPUT CAPACITOR
I'm sure someone won't like the output capacitor but this is essential because on the output of the mosfet there is half the power supply voltage.
Any change to the design to eliminate this component, such
as dual power supply or virtual ground, compromises the main characteristic of
this circuit: the total decoupling from the power supply..
You must consider
the power supply capacitor on the signal path also on any traditional
amplifiers with dual power supply..
Obviously this capacitor must be of the highest possible quality
and these are my choice::
Nichicon KGNichicon KG the my first choice
Jensen Electrolytic these are the my second choice
BHC Slit Foil the my third choice
The
Nichicon KG
capacitors
has been used on all my last hybrid amplifiers and in the my
Amplifier End I
decided to eliminate the bypass capacitors 47uF Solen MKP originally used
because the sound is much better without these.
I have chosen for the
outptu capacitor the value 4700uF because this give a low frequency cut-off very
low also on 4ohm load.
Ft(-3dB) = 1 / (2 * pi * C * R) = 1 / (2 * 3.14 * 4700uF * 4ohm) = 1 / ( 2 * 3.14 * 4700E-6 * 4) = 8.5Hz
Using this value the output impedance give an acceptable 0.2ohm at 100Hz and 0.8ohm at 20Hz (see simulation below), this is much better than any SE tube amplifier but if you want to get a best damping factor use 10000uF.
RESITORS
All the resistors on the signal path have 2
positions on pcb because ...
otherwise to get a slightly better result using the
MK132 Caddock.
In order to dissipate all the heat generated by this amplifier in my case I chose this container by HiFi 2000.
Dissipante 04/300B 4U 10mm SILVER
Product
Code: 1NPD05300B
temperature coefficient 0,31 C°/W per each side
Inner baseplate for Dissipante 300mm
Product Code: 1BASEPD300
I used the HiFi 2000 company for almost all the mechanical processes and here there are some specifications used for this phase.
Here the cost of the chassie
1 x Dissipante
04/300B 4U 10mm SILVER (1NPD04300N) €149.00
1 x B) Drilling front panel 10mm
(LAV10MM) €25.00
2 x A) Drilling panel 2/3/4mm (LAV4MM) €50.00
1 x Inner
baseplate for Dissipante 300mm
€14.15
The panel cut out for thid button is 22mm, this is the same size of
noval socket so this is the ideal choice for the 10mm front panel.
Vandal
Resistant Push Button Switches
MEASUREMENTS
Here follows the final measurements on the complete PF2019 in this condition:
Voltage 36VDC
Rbias 0.2ohm 5W 0.6V = 3.0A
Here the frequency response on 4 and 8ohm load measured with ADALM2000 USB and Scopy software.
PHOTOS
TOTAL COST
My configuration | |||
description | unit price | quantity | total (euro) |
Vacuum tubes with shipping | 16 | 2 | 32 |
Vishay MKP 50uF 500V | 10 | 2 | 20 |
Components + pcb | 110 | 1 | 110 |
Mosfet | 3 | 4 | 12 |
Interstage capacitors | 40 | 2 | 80 |
HI-Fi 2000 chassie 10mm front panel + processing | 250 | 1 | 250 |
Vandal Resistant Push Button + relay | 20 | 1 | 20 |
10000uF 63V | 8 | 2 | 16 |
LRS-150-36 | 25 | 2 | 50 |
Connectors | 26 | 1 | 26 |
550 |
Cheap configuration | |||
description | unit price | quantity | total (euro) |
Vacuum tubes with shipping | 16 | 2 | 32 |
Vishay MKP 50uF 500V | 10 | 2 | 20 |
Components + pcb | 110 | 1 | 110 |
Mosfet | 3 | 4 | 12 |
Interstage capacitors (min acceptable) | 10 | 2 | 20 |
HI-Fi 2000 chassie 4mm front panel + home made processing | 150 | 1 | 150 |
Normal switch | 6 | 1 | 6 |
10000uF 63V | 8 | 2 | 16 |
LRS-150-36 | 25 | 2 | 50 |
Connectors | 26 | 1 | 26 |
374 |