back to www.audiodesignguide.com |
To get more information contact me at: webmaster@audiodesignguide.com |
Solid State Amplifier
2022 INTRODUCTION
After the good experience made on hybrid amplifiers with circlotron
configuration (see the various version of the my
Amplifier
End)
I thought about making a completely solid state amplifier
of this type.
Please do not touch with multimeter probe the mosfet gate after the power on because this
operation will destroy the mosfet. |
|
SCHEMATIC
These are the main characteristics:
- dc coupled of
all stages
- no
capacitors on the signal path
-
high slew-rate
- good damping factor near to a
solid state
amplifiers with high feedback
- no switching output stage like a pure class A
- high efficiency because
only 70W of consumption per channel
- only single ended stages
- only a single output device
-
only a transconductance amplifier with I/V
- global feedback easy to modify without high
frequency compensation
- full balanced circuit but it accept also
un-balanced input signal
PCB
This is the last pcb version
J1,J2,J3
J113D74Z
T1,T2
BC546ABU
T3,T4,T5,T6 BC556B
T7,T8
MJE340
U$1,U$2
ECW20N20-S or
ECW20N20 or
2SK1058
R1,R2
100Kohm 1% 1/4W Vishay Dale RN55
R3,R4,R7 1Kohm
1% 1/4W
Vishay Dale RN55
R8
500ohm 1% 1/4W
Vishay Dale RN55
R5,R6
47ohm 1% 1/4W
Vishay Dale RN55
R13,R14
470ohm 1% 1/4W
Vishay Dale RN55
R9,R10,R11,R12 15Kohm
1% 1/4W
Vishay Dale RN55
R17,R18
3900ohm 1% 1/4W Vishay
Dale RN55
R15,R16
10Kohm 1% 1/2W
Vishay Dale RN65
R19,R20 2700ohm
5% 2W
Vishay PR02
R21,R22 2000ohm 1% 1/4W
Vishay Dale RN55
R23,R24
120 or 240ohm 1% 1/4W
RN55
CZ
0.1uF Vishay
F339X1
RZ
10ohm 2W
Vishay PR02
The driver stage need a regulated power supply module to
have a stable +36VDC and -36VDC.
The normal linear regulator does not support
40V input so I have design a shunt regulator following the application note in
the datasheet of TL431 and I have optimized
this circuit with
LTspice.
In the pcb there are the pins
to connect all the secondaries of the r-core transformer and also to get the DC
protection power supply.
D1,D2,D3,D4
UF5407
C1,C2
4700uF 50V +105°C Nichicon 20 x 40mm
C3,C6
10pF
C4,C5
1nF
R1,R2,R3,R4
47ohm 1W (use only R1 and R4 for dual mono configuration)
U$1,U$2
TL431
R5,R9
1Kohm 1% 1/4W
R8,R11
15Kohm 1% 1/4W
R6,R10
1100ohm 1% 1/4W
R7,R12
100ohm 1% 1/4W
Q1,Q4
BC556
Q2,Q3
MJE350
C7,C8
22uF 50V
UFG Nichicon
KK1,KK4 HEATSINK SK104 11C/W
513102B02500G (cod. MOuser
532-513102B25)
PCB
2 x Amplifier
4 x Diode bridge
JFET SELECTION
The main problem of this project is the fet selection necessary
to get the max performances.
This design using a little feedback cannot
compensate the large differences between two fet J113 therefore it is necessary
to buy more pieces and make a selection for a matching.
Below I report 2
measurements made with a different selection of these components.
At this time I am testing the Peak DCA75 Pro semiconductor analyser to check the J113 differences but I will give a procedure for making a selection without purchasing this special tools.
Here the plot of measurements of 3 different J113 fet.
Here the plot of measurements of matched J113 fets.
An alternative cheap method to match the fet is this procedure:
before solder output mosfet
add a jump on R21 and R22 (see on photo yellow wire)
set R8+R25 = R7 = 1Kohm
power on driver stage
set R31 to have 0volt on R19
using Arta or other software generate from teh pc a 1KHz sine wave with a level about 0.42Vrms
measure the driver output on R19 and R20
change J113 until both the
output give 10Vrms
STARTUP PROCEDURE
SIMULATION
Frequecy response with Exicon mosfet about Ft(-3dB) = 2.5MHz
Zout=178mohm
Follows the simulation of the distortion at 23Vrms on 8ohm 66W with 2 x 36VDC, about 0.8%.
Simulated noise on power supply module output.
MEASUREMENTS
ECW20N20 version with driver +36VDC and output stage at 2x 33VDC and 6dB global feedback
Follows some measurements using un-balanced input signal.
2SK1058 version
with driver +-35VDC and output stage at 2 x 36VDC, load 8ohm and 6dB global feedback.Follow the distorsion at 17Vrms on 8ohm = 36W, simulated Zout about 0.5ohm.
DISTORTION CONTROL
In a direct comparison with my Amplifier End which uses
a tube as a voltage amplifier, an interstage transformer and the same output
mosfets, the sound of this solid state amplifier remains more fatiguing.
This sensation was not present in the prototype which did not have the selected
fet but had a higher distortion so I tried to unbalance the 2 feedback resistors
and I obtained the desired effect by increasing the second harmonic.
Follows the measurements with various values where it is verified that a small
variation is enough by adding 56K in parallel to 39K on the negative line and we
obtain the same harmonic decay of the amplifer end and another listening
pleasure.
1 path
parallel
2 path
diff. %
thd )%=
3900
none
3900
0
0.18
3900
68000
3688
5,4
0.23
3900
56000
3646
6,5
0.28
3900
47000
3601
7,7
0.34
3900
39000
3545
9,1
0.42
TRANSFORMERS
It is possible to use any trasformers from 20V to 30V with a power about 200-300VA but in any case 2 separated secondaries are required for each channel.
The my choice is a RS Pro Encapsulated Transformers 2 x 25V 225VA (cod. RS RS123-4028)
The transformer for the driver stage is the LITE R26-10 (diyclub.biz) an r-core 30VA with 2 x 9V 3A and 2 x 18V 0.5A
POWER SUPPLY
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 bridge with these TO220 diodes I have created a simple pcb.Noise on power supply module output.
CHASSIE
In order to dissipate all the heat generated by this amplifier in my case I chose this container by HiFi 2000.
Dissipante 03/300B 3U 10mm
SILVER or
BLACK
Product Code: 1NPD03300B
or 1NPD03300N
Used for the 3U 300mm deep chassis temperature coefficient 0,40 C°/W
2 x 33V 1A = 66W 66W * 0.4C°/W = 13.3C° + 25C° = 38C°
Inner baseplate for
Dissipante 300mm
Product Code:
1BASEPD300
PROTECTION CIRCUIT
Any
serious solid state amplifier need a protection circuit because a fault on
output transistors or mosfet can destroy the loudspeakers.
The
Cirlclotron design never will produce an high dc output but I suggest to use a
dc protection circuit.
I have decided to use
2 x AIYIMA 2.0
Digital Power Amplifier Speaker Protection Board Delay Relay Speaker Protection
available on Alixpress online shop.
This module use 2 optoisolator
PC817
for each input and are necessary only some
little changes to increase the start-up time.
It is not necessary modify
this module with 220Kohm and diode.
SOFT START START AND TEMPERATURE PROTECTION
I am using this module got on Alixpress because it incluse a soft-start and a temperature protection at 75°.
1NO1NC Momentary switch from Alixpress
PHOTOS
The first photos are relative to the test environment.
Here some photos of the test environment to compare the switching power supply to traditional.
Here some photos of the final environment
COST
Qty | model | brand | unit price (euro) | total price (euro) | shop |
4 | ECW20N20 | Excicon | 12 | 48 | |
4 | 10000uF 50V | Nichicon | 6 | 24 | www.partsconnexion.com |
2 | Toroidal transformers | 60 | 120 | ||
1 | R-core transformer | 40 | 40 | alixpress.com | |
16 | Diodes for the bridge | On-semi | 2 | 32 | |
1 | Chassie | Hi-Fi2000 | 170 | 170 | www.hifi2000.it |
2 | DC protection module | 10 | 20 | alixpress.com | |
1 | Soft start | 10 | 10 | alixpress.com | |
1 | connectors | 36 | 36 | ||
1 | other items | 50 | 50 | ||
total | 550 |