I believe in valves, but I have some friends they believe in transistor. I heard some good ss. amplifiers (Krell, Mark Levinson), and I try to built a good sounding transistor amp. I want to forsake the common design.
The schematic:
I
don't use long tailed input stage. This amplifier don't need phase splitter
as the push-pull valve output stages. Symmetrical complementer common
emitter input stage was build here. Zener regulated power supply gives
the bias current. I want to build this amp as simple as I can, thats
the reason why don't use current source.
The second stage is emitter
follower, it gives low impedance driving to high level voltage amplifier.
This driving results high speed amplifier, eliminates Miller effect.
Third section is a double
common emitter stage, with high bias current. The high current easy to
drive the output drivers. R20 and R21 help to exactly determine the amplifiing
rates. This resistors eliminates the input impedance changes of the output
stage. Output devices made by Sanken, if you have any problem, you can
use 2SC3281&2SA1302 from Toshiba.
The fully complementary
design help to minimize the distortion without large overall feedback.
Strong local feedback used in first and third stages.
The input stage don't eliminate
the thermal changes. Simple DC servo circuit added to the circuit. The
IC1 work to kill the differences between the GND and the output. The C9
and R27 set the low frequency limit. The servo work very hard. I measure
some amplifier
and there was no more than 2mV offset over
-10 to +50 degree centigrade. Servo circuit help to forsake the coupling
capacitors.
Output power: 40W/ch.
If you need more, change the BD139/140 pair to Motorola MJE340/350., then
you can use +-65V power supply. It can gives 200W/ch on 8ohm.
You must use more than two output transistors!
I try some square wawe
measuring with HP oscilloscope, the output signal was same as the input.
The slew rate was more than 60V/us. Frequency response flat from 0.2Hz-1MHz
(without input filter!).
You can improve the sonic
performance to use separate transformer for driver stage, and other for
output stage. I use 200W transformer for each channel output, and one same
200W transformer for both driver stages. Regulated power supply good for
the driver, but not good for output stage.
I try some output bias
settings..... Pure class A make very good central heating in my room
(need some fun), sound was good. Class AB with 250-300mA is the good choice
if you are not cold. There are no significant difference between the settings.
Use heatsinks for T5, T7,T8
andT9, it can be the same as the output devices heatsink. Put the T6 to
the heatsink of the output transistors.
Now I plan to built this
amp with MOSFET output stage.
Have a nice listening.