Hello all, the thread posted at ASR by @sabristol
https://www.audiosciencereview.com/forum/index.php?threads/luxman-l-85v-integrated-amplifier.20657/
inspired me to build a new DIY amplifier functional sample. The
circuit posted in the link above is called Luxman L-85 but in
fact the topology is rather the Douglas Self's Blameless
Amplifier discussed in his book Audio Power Amplifier
Design Handbook on also on his website
http://www.douglas-self.com/ampins/dipa/dipa.htm
http://www.douglas-self.com/ampins/dipa/dpafig33.gif
The original Luxman PB-1037 main amplifier circuit is more
different with 2 differential stages instead of one, no EF VAS
buffer etc. The circuit posted was a temptation to me to get more
output current and power and less dependence on load impedance.
The main change I made was to use 2 pairs of the output devices,
I was thinking about my favorite and robust MJL21194/93 first but
then decided to go for MJL3281/1302 pairs, which have even better
linearity at high currents and are faster, though only very
slightly weaker in SOA.
This is the complete schematics of the amplifier that I built
It was built into my prototype case with two 300VA toroidal
transformers, that are needed for the dual-mono , which
determined the size of the PCB and also components placement and
drilling. The case is 19" 4U, dimensions 450 x 415 x 180 mm.
It has big side heatsinks and can accommodate 2x250W amplifier
concept with long-term full-power capacity.
This is the amplifier built in the prototype 19" 4U
case
The design is dual mono. There are two transformers, two
rectifier-filter boards, two amplifier boards and two DC
protection SSR boards inside the case. The metal case is grounded
but the signal grounds of the left and right channels are not
directly interconnected, they are connected to the case through
the Rvar//C components (connected to PE) to prevent usual serious
ground-loop hum issues.
Two MJL3281/1302 output pairs make 250W/4ohm power possible with
respect to SOA (Safe Operating Area) of the transistors. It is
possible to use speaker complex load that does not fall below 4
ohm in its impedance/frequency plot.
The amp may drive purely resistive load
of 2 ohm up to full output swing and still stay inside allowed
SOA boundaries.
Functional sample parameters
Measurements
Response to 10kHz square wave into 4ohm in parallel with 47nF
THD and THD+N vs. output power into 4ohm load at 1kHz with
measurement bandwidth 16Hz - 22kHz
THD+N vs. output power into 4ohm load at 50Hz, 1kHz and 6kHz
with measurement bandwidth 16Hz - 22kHz
Frequency response into 8ohm measured at 50W
last edited : January 30, 2023