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Class D Switching Power Amplifiers and Power D/A conversion

IRS2092 Amplifier
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Old 19th September 2019, 06:03 AM   #41
Rallyfinnen is offline Rallyfinnen  Sweden
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Hello!

I bought a pair of the LJM L15 Pro boards, and connected them for the first time yesterday. Power supply is linear +-57V with 22mF/rail.
However, first impression of the sound was not that good, mids and highs a bit 'grainy', and also bass did not seem as 'dynamic and contolled' as I hoped for.
Speakers used for test are only basic workshop speakers at 8ohm. Maybe giving a small peak in top end response, but I doubt that explains the impression of the sound.

FFT confirmed that it has pretty high distortion (into 8ohm resistors) with a lot of high order spikes at normal listening levels 1-10W, and IMD pretty much the same, a lot worse than any solid state amp I measured. With 1kHz in, 2nd & 3rd harmonics were at abt -50 to -60dB, and higher harmonics not much lower.

I see a 'carrier' at about 1Vpp, and frequency around 500kHz (a little high?)

I scoped the supply voltage at the main PS caps and it seems nice and steady, as it should with such an easy load and low power output. I also tried 'reversing' one channel (inverted input and output polarity), and saw a small improvement in supply voltage stability, but could not hear any difference.

I'm planning to add a trimmer to be able to adjust the switching frequency, and have some thoughts on increasing the capacitance on the low voltage supplies (over the zeners). My local supplier does not have any caps (that fit) that I would consider an improvement for the main rails on the board.

Is this what is expected of the irs2092, or is it even worth it to try to tweak them? As I understand it, the LJM board are considered well made?
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Old 19th September 2019, 06:52 AM   #42
abraxalito is online now abraxalito  United Kingdom
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Location: Hangzhou - Marco Polo's 'most beautiful city'. 700yrs is a long time though...
If that particular board is using the internal zeners for regulation and doesn't augment them with large 'lytics on the rails then, based on my experience, that sound description is pretty much par for the course. 500kHz is a bit high, the original appnote from IR suggests 400kHz at idle. Around 1V carrier is fairly typical too, 1V p-p seems better than average perhaps due to the higher than normal idle freq.
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Old 19th September 2019, 08:23 AM   #43
Rallyfinnen is offline Rallyfinnen  Sweden
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It does have external zener diodes with 22uF caps, as in iraudamp7d scematic.
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Old 19th September 2019, 04:57 PM   #44
Rallyfinnen is offline Rallyfinnen  Sweden
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Default Distortion measurements LJM L15d-Pro

I took some distortion measurements and screenshots, so this is what I'm seeing, and possibly hearing?
See the filenames for details. The IMD measurements are taken with the same amplitude setting as the 20Vpp. I did not even bother to get a better sound card, since distortion is clearly visible even with the Behringer UCA222.

I tried adjusting the switching frequency with fft running, but saw absolutely no difference. Now it's around 400/425kHz per channel.
Attached Images
File Type: jpg 1kHz 20Vpp 4ohm.jpg (222.2 KB, 138 views)
File Type: jpg 1kHz 20Vpp 8ohm.jpg (210.6 KB, 135 views)
File Type: jpg 1kHz 40Vpp 4ohm.jpg (204.9 KB, 132 views)
File Type: jpg 1kHz 40Vpp 8ohm.jpg (204.5 KB, 127 views)
File Type: jpg 13+14kHz 4ohm.jpg (217.6 KB, 124 views)
File Type: jpg 13+14kHz 8ohm.jpg (216.6 KB, 12 views)
File Type: jpg background noise.jpg (187.3 KB, 12 views)

Last edited by Rallyfinnen; 19th September 2019 at 05:05 PM.
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Old 21st September 2019, 06:12 AM   #45
Turbowatch2 is offline Turbowatch2  Germany
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The screenshots don`t look that bad. Usual 50Hz noise. Maybe you could post some pictures of installation, power supply etc.
I have recently reversed my opinion about "burn in voodoo". Maybe leave them alone at some low load like 5 watt or so, over night. I heard some very strange improvements after such a cure with D-amps recently. Do not ask me why.

as an alternative, if you can get them, replace the MOSfetīs by some from a trustworthy source.
Many of these amps fail during the first hours, because of blowīn FETīs. They may be fake or refurbished from the manufacturers garbage bin. "Second selection", maybe even worse. While doing so. look for shortcut at the heat sink , apply good pressure and use thermal compound.
Many amps profit from a simple buffer, NE5532 will do, not to have one might be a "no go", as many ampīs input stages are extremely simplified.
There are many small, cheap PCBīs at eBay, Ali etc. that can be soldered directly at the input RCA. Change the OP amp for a real NE5532, as these are usually fake.
The pictured one is inverting (no problem) and has gain of 10x (has to be changed to 1x) Less than 2 Euro, you do not make the PCB for that money.
Attached Images
File Type: jpg DSCN3681.jpg (431.4 KB, 43 views)

Last edited by Turbowatch2; 21st September 2019 at 06:16 AM.
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Old 21st September 2019, 08:35 AM   #46
Rallyfinnen is offline Rallyfinnen  Sweden
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Ok, maybe I should leave them on and see what happens.. smoke or voodoo
I'm not so keen on throwing more money at them with fet's etc, since I 'm not sure if I will even use them. I was thinking about buffers for input impedance, and to invert one channel in case I use them for woofers to prevent 'rail pumping'.

I don't worry about the 50Hz in the measurements, it's 'EMC problems' in my measurement setup/garage. However I'm not so impressed with all the spikes from harmonics in the measurements.
Hiss can be heard from tweeters, so gain should probably be lowered if I want to use them full range.

Will see if I remember to get a pic of the test setup.

Last edited by Rallyfinnen; 21st September 2019 at 09:02 AM.
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Old 22nd September 2019, 08:52 AM   #47
Rallyfinnen is offline Rallyfinnen  Sweden
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First I would like to 'cancel' my measurement results above! I redid the measurements with a better sound card, and now they look 'normal' and not so bad. Noise floor from the amp is a bit high though. I did not remember that the Behringer sound card was actually that bad for amp distortion measurements, I often use if for measuring speakers, and it's ok for that.

I also measured frequency response to 8 & 4 ohm. At 8ohm it's slightly peaking at 20kHz, and at 4ohm sligtly falling. Around 3dB differance between the two at 20kHz, no differance at 10kHz.
I put them to work heating resistors with music overnight, so I guess today I will see if there was smoke or magic

Picture of test setup attached as requested
Attached Images
File Type: jpg L15Dpro test setup.jpg (739.6 KB, 59 views)

Last edited by Rallyfinnen; 22nd September 2019 at 08:54 AM.
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Old 23rd September 2019, 01:50 AM   #48
Turbowatch2 is offline Turbowatch2  Germany
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Your test set up looks fine, but for a final installation I would take solid core (like your yellow/green), 3 for the power and 2 for speaker wires, in different colors. A wireless drill will turn them neatly together in no time. This will lower noise and radiation significantly at hardly no cost. I remove the outer insulation of 5x power line installation cable to harvest this "high end" material
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Old 23rd September 2019, 05:56 AM   #49
Rallyfinnen is offline Rallyfinnen  Sweden
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Yes, for sure if I intend to use them in a 'permanent setup' I will use twisted for power and speaker cables. I tend to do that even with solid state amps.
I'm thinking I should replace the 3k gain resistor with a 4,7k, that should be just around the limit of how low I can go on the gain, and still be able to drive it close to clipping, assuming I can get 2V peak in, and a little bit of sag on the 57V rails. If I put some opamps as a buffer, they could have some gain too if needed. However I'm a bit reluctant to do this, since it requires another supply and all that comes with it.

Not sure if I will hear a decrease in noise with lower gain resistor? I guess metal foil resistor is what should be used?
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Old 23rd September 2019, 06:23 AM   #50
abraxalito is online now abraxalito  United Kingdom
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Location: Hangzhou - Marco Polo's 'most beautiful city'. 700yrs is a long time though...
I doubt there will be a reduction in noise from increasing the input resistor. I've gone up to around 6k and noticed no change. You could power your opamps from the zener regulated +/-5V rails (pins 1 & 6) provided you use low enough power devices. Perhaps decrease the zener-feeding resistors a little to cover the extra current.
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