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

1. Intro

The circuit is in fact the topology according to the Douglas Self's Blameless Amplifier discussed in his book Audio Power Amplifier Design Handbook and also on his website

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. 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. The amp is able to drive capacitive load up to 4ohm//2.2uF remaining stable.

2. Functional sample parameters

3. Measurements (according to IEC 60268-3)

3.1. Input characteristics

3.1.1. Rated source impedance - 100 ohm

3.1.2. Input impedance - 66kohm/20Hz, 65kohm/1kHz, 30kohm/10kHz, 17kohm/20kHz


3.2. Output characteristics

3.2.1. Rated load impedance - 4ohm (or more)

3.2.2. Output source impedance (including SSR protection board) - 0.15 ohm at 1kHz, 0.35 ohm at 10kHz

3.2.3. Output power (distortion limited at 1%) - 2 x 280W/4ohm

3.2.4. Gain under standard measuring conditions - 34.3 dB


3.3. Overload restoring time
3.3.1. Characteristic to be specified
The time interval (which occurs after the amplifier working under standard measuring conditions is overloaded by a certain amount for a specified period) between the moment when the input voltage is restored to its original value and the moment when the output
voltage has again reached its original value within specified limits

3.3.2. Method of measurement
a) The amplifier is brought under standard measuring conditions.
b) The source e.m.f. is increased by 20 dB during a time interval of less than one-quarter
period of the input signal and kept at this value for approximately 1 s.
c) The source e.m.f. is then reduced to its initial value during a time interval of less than onequarter
period of the input signal.
d) The time that passes before both the positive and the negative output peak voltages have
reached their final value, within 1 dB unless otherwise specified, is measured by means of
a suitable calibrated oscilloscope.

3.3.3. Measurements of overload restoring time

Full record. Rated power is 250W/4ohm => standard measuring power is 25W/4ohm (10Vrms). 20Hz frequency.

Transition area zoomed. Please note power supply ripple superposition on the clipped dignal.


The same measurements at 1kHz

Full record.


Transition area zoomed.


Please keep in mind that overload in this test is +10dB.


3.4. Responses

3.4.1. Gain-frequency response

3.4.2. Phase-frequency response


3.5. Amplitude non-linearity

3.5.1. Rated total harmonic distortion - 0.1%

3.5.2. Total harmonic distortion under standard measuring conditions - 0.01%

3.5.3. Total harmonic distortion as a function of amplitude and frequency



3.5.4. Distortion-limited (0.1%) power frequency range - 250W/4ohm/20Hz-20kHz



page created 03/2021, last edited : March 10, 2023