DIY 250W/4ohm amplifier based on "blameless" topology, and measurements

Hello all, the thread posted at ASR by

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

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


Response to 10kHz square wave into 4ohm in parallel with 47nF

THD and THD+N vs. output power into 4ohm and 8ohm load at 1kHz with measurement bandwidth 20Hz - 22kHz

Noise component of the THD+N measuremnt is limited, below 10W approx., by the measuring system noise. This can be seen in the next graph:

THD part of the measurement may be considered correct above 1W approx.


THD and THD+N vs. frequency at 50W/8ohm with measurement bandwidth 20Hz - 40kHz

CCIF IMD 19+20kHz at 25W/8ohm

Frequency response into 4ohm and without load measured at 20V
Blue trace is without load and green trace is with 4ohm load. There is a difference of 0.18dB at 1kHz and the calculated output impedance is thus 0.084 ohm. This output impedance is mostly defined by the 2Rds resistance of output MOSFET SSR DC protection board. All the measurements taken with the output DC protection board.


Multitone measurements

31 tones with crest factor 12.5dB, Vrms = 6.5V

last edited : September 21, 2021