DIY
250W/4ohm amplifier based on "blameless" topology, and
measurements
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 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
- input impedance ... 70 kohm
- frequency range ... 2Hz - 88kHz/-3dB
- full-power bandwidth ... 20Hz - 20kHz
- output noise voltage A weighted
..... -84dBV(A)
- output power ... 2x250W/4ohm for THD
< 0.1% at 1kHz
- S/N at full power ..... 114dB(A)
- harmonic distortion ... THD < 0.007%
at 200W/4ohm/1kHz (see graphs)
- rise time of step response ... 4us
- gain ... 34.4dB
- dimensions ... 450 x 415 x 180 mm
- weight ... 30 kg approx.
- construction ... dual-mono with 2
toroidal PSU transformers 300VA each
Measurements
Response to 10kHz square wave
Sine 20kHz 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
10kHz square response
measured with better oscilloscope
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.
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 2, 2021