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 PCB mounted on the heatsink

and this is the amplifier board in the prototype 19" 4U case (the bottom board). The top board is a CFA amp - it was already replaced

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 worst case simulation with the load that well reflects the woofer impedance shows that the SOA Itrajectory of one output device is just at the edge.

This is the impedance response used in the simulation

and this is the SOA simulation for 1 power transistor, with dummy load impedance schematics

Interestingly enough the amp may drive purely resistive load of 2 ohm up to full output swing and still stay inside allowed SOA boundaries. It only tells that pure resistive loads are inadequate for both simulation and testing and do not reflect real-world speaker load.

Another interesting points are the Q16 emitter follower (beta enhancer) that greatly reduces VAS distortion and increases open loop gain and all the current sources that improve PSR (ripple rejection).

Functional sample parameters


Response to 10kHz square wave into speaker CNO-T25

Sine 10kHz at full power into 4ohm load

THD vs. output power into 4ohm load at 1kHz with measurement bandwidth 40kHz

THD vs. output power at 5kHz 4ohm with measurement bandwidth 40kHz


THD vs. output power at 10kHz 4ohm with measurement bandwidth 40kHz


THD vs. frequency at 50W/4ohm with measurement BW = 40kHz

THD 1kHz spectrum at 25W/4ohm/1kHz

CCIF IMD 19+20kHz at 56Vp-p/4ohm


Capacitive load
Red is pure resistive load 4ohm, blue is 4ohm in parallel with a 47nF capacitor

THD vs. output power into resistive and resistive+capacitive load

Measurements into complex dummy load (dummy load simulates real speaker load)

I have built a dummy load to simulate speaker impedance years ago. It simulates a simple 2-way box and uses highly nonlinear ferrite 18mH coil to simulate woofer impedance nonlinearity.

The load looks like this

This is the circuit schematics

and this is a measurement of the dummy load impedance and EPDR (equivalent peak dissipation resistance)

This load was now used instead of the usual and traditional purely resistive load and THD vs. frequency was measured at 9Vrms and 18Vrms. This would be 13.5W resp. 54W into 6ohm, which is an impedance minimum at some 130Hz.

Measurement in THD %

and THD in dB

One can see fast rise of distortion below 80Hz, which reflects high nonlinearity of the ferrite core 18mH coil, this is reflected in nonlinear load current and this again in voltage distortion at amp terminals due to its finite output impedance.

This complex load nonlinearity near resonance is shown in the following plot, which is THDN vs. amplitude with the dummy load at 70Hz. Please note the fast rise of ferrite coil nonlinearity effect above 10Vrms.

THDN vs. amplitude into dummy load at multiple frequencies


Torture load test

Finally the test with the "torture" load


Torture load impedance and EPDR

THDN vs. output voltage

THDN vs. frequency at 4Vrms


More distortion measurements with the dummy load

Below 100Hz we can see the rising effect of the ferrite coil nonlinearity with output voltage.

A250W4R dummy load fr 7-10-14-20Vrms.png  


More measurements with system with bandwidth of 48kHz (Fs=96kHz)

THD vs. output voltage into 4ohm and 6.8ohm
THD depends very little on load impedance

THD vs. frequency at 20V (100W) into 4ohm load and without any load
Green trace is with 4 ohm load and black trace is without load, "open" output

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 : April 20, 2021