2stageEF high performance class AB power amp / 200W8R / 400W4R - diyAudio
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Old 1st May 2013, 01:04 PM   #1
astx is offline astx  Austria
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Default 2stageEF high performance class AB power amp / 200W8R / 400W4R

Heavily inspired from books of Douglas Self, Bob Cordell and other great authors I decided to design a discrete power amplifier to learn how LTSpice can be used for my DIY projects.
Attached is the current simulation file and some information about the real world result: a very good working 2stageEF power amplifier with high performance.
The simulated THD performance is about 0.0004% at 20kHz@200W@8R.
Real life THD+N performance of working prototype amplifier is about 0.0015% at 20kHz@200W@8R (80kHz bandwidth).
Using the HP3585A spectrum analyser (100dB resolution) while running 20kHz@200W@8R test only H2 and H3 are visible above the noise floor.
To download the needed BJT/JFET/MOSFET/DIODE ltspice library file "Cordell-Models.txt" please visit CordellAudio.com - Home
More test results, THD plots and other information will be updated from time to time.

Regards, Toni

Updates:
Attached Images
File Type: png 2ef_simulation.png (207.2 KB, 6176 views)
File Type: jpg SA2013_assembled_1.jpg (250.8 KB, 5779 views)
File Type: jpg SA2013_assembled_2.jpg (326.5 KB, 5347 views)
File Type: png SA2013_discrete_class_ab_input_vas_stage_schematic.png (191.8 KB, 5718 views)
File Type: png SA2013_discrete_class_ab_output_stage_schematic.png (178.2 KB, 5519 views)
File Type: jpg SA2013_high_power_stability_test.jpg (277.9 KB, 3052 views)
File Type: jpg SA2013_THD_20kHz_200W_8R.jpg (151.6 KB, 1350 views)
Attached Files
File Type: asc 2stageEF_class_AB_power_amplifier.asc (37.3 KB, 539 views)
File Type: zip ltspice_libs.zip (1.3 KB, 335 views)

Last edited by astx; 14th September 2014 at 03:12 PM. Reason: Update index / Update to reflect current state of development
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Old 1st May 2013, 02:46 PM   #2
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Using 2N5401 for Q1 and Q2 is a mistake. At low Vce these transistors perform badly.

The BC5x0, BC3x7, and 2SA1015 are known to have excellent okay quasi-sat behavior. There are virtually no transistors with Vcemax>50V that have good quasisat behavior.

Because SPICE does not model quasi-saturation, this problem will not show up in your simulations. However, since the above recommended transistors are pretty close to ideal, if you use Cordell's or NXP (Phillips) models for them they will get you within 6db of real life at low Vce.
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Old 1st May 2013, 03:16 PM   #3
astx is offline astx  Austria
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... think you are referring to ltspice simulation file as the part numbering of the attached schematic pictures is different:
Maybe you are right that 2N5401 is not the best BJT for the current mirror.
Using BC560C instead of reduces THD simulation indeed about 3ppm.
BTW the used transistors for Q1/Q2 are hand selected from a big number of measured devices and have hfe > 220. Perhaps this is the reason, that the real world amplifier THD figure is better as simulated one...
Will try your BJT tipps the next days in real world.

Thx,

Toni
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Old 1st May 2013, 03:25 PM   #4
astx is offline astx  Austria
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Default minor fix ...

Output stage's R48 has to be 33R instead of 150R.
Attached corrected output stage schematic.
Attached Images
File Type: png SA2013_discrete_class_ab_output_stage_schematic_V2.png (178.8 KB, 2293 views)
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Old 1st May 2013, 04:01 PM   #5
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In your setup matching the CM transistors isn't going to do anything if the CM degeneration isn't also matched. I posted about it here:

diyAB Amp - The "Honey Badger"
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Old 1st May 2013, 04:22 PM   #6
astx is offline astx  Austria
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Default Q1/Q2 CM matching data ...

Your explanation is very interesting.

Q1 and Q2 matching data:

Left channel:
BJT#30: hfe=224 vbe=-0.5670
BJT#173: hfe=224 vbe=-0.5660

Right channel:
BJT#35: hfe=230 vbe=-0.5670
BJT#128 : hfe=230 vbe=-0.5660

100R resistors are matched < 0.5% tolerance.

So what would be your choice for Q1/Q2 CM and degeneration (2.2mA per leg)? (Amp is using nominally +/- 66V and max. +/- 72V power supply)

BR,

Toni
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Old 1st May 2013, 04:44 PM   #7
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I go by Slone's advice to degenerate by 50mV. 50mV / 2.2mA =~ 22R. Degeneration is specified as voltage drop because Gm is proportional to Iq and so is the degeneration voltage drop. This way degeneration can be described conveniently independent of Iq.

I would use BC560C in the CM because they are very fast, have high gain, and have good enough Vcesat. For a CM, I think they are as close to ideal as you can get.
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Old 2nd May 2013, 07:13 AM   #8
astx is offline astx  Austria
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Default PCB and input stage assembling details ...

Before soldering the heat sinks place pvc washers between pcb and heatsink solder legs to avoid short circuit with rail voltages. Do this even if your pcb has a very good stop paint!
Attached Images
File Type: jpg SA2013_input_vas_1.jpg (210.1 KB, 1967 views)
File Type: jpg SA2013_input_vas_2.jpg (180.9 KB, 1071 views)
Attached Files
File Type: zip SA2013_pcb_gerber.zip (166.8 KB, 398 views)
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Old 2nd May 2013, 10:48 AM   #9
The Dastardly Dad of Three
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slobber slobber Congratulations on this beautiful piece of engineering.
It looks looks really professional to me - you must do this for work too and not just as a DIY hobby!

Thanks for sharing this btw - really very inspiring stuff
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Old 2nd May 2013, 11:00 AM   #10
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by keantoken View Post
I go by Slone's advice to degenerate by 50mV. 50mV / 2.2mA =~ 22R. Degeneration is specified as voltage drop because Gm is proportional to Iq and so is the degeneration voltage drop. This way degeneration can be described conveniently independent of Iq...............
I have been using 200mV to 300mV as the Vdrop of the degeneration resistor, in LTP, & VAS & Current Mirror.
What is wrong with doing it that way? excess added noise? or many other avoidable problems?
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regards Andrew T.
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