My ZD-50 ultralow distortion chipamp. - diyAudio
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Old 1st August 2008, 03:14 PM   #1
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Arrow Ultralow distortion LM3886 chipamp

Hi everyone! It is my first message on this forum and I hope it will be interesting for you.
So, my power amp is based on LM3886. It has a very deep feedback - about 70dB at 20kHz, and much more on lower freq. It considerably reduces distortions compare to the standard schematic. I started this project almost one year ago, and now I have a very good working one. It has about -120dB THD at 11kHz, 4Ohms, 60W. IMD 19+20 is less, and I could not measure it.
Here is the schematic, parts list, PCB layout.
Some measurements.
That's it for today, I hope I'll post more soon.
Nick.

Last edited by Nickolay Shvydky; 15th January 2014 at 06:46 PM.
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Old 1st August 2008, 04:28 PM   #2
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Default deep feedback

Very interesting, I would like to see 2nd and 3rd harmonic values?
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Old 1st August 2008, 05:21 PM   #3
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22kHz -120dB
33kHz -128dB
But -128dB is a noise floor, so visible harmonic is only 22kHz. I am going to improve my measurements abilities, so then I could say more precisely.
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Old 1st August 2008, 08:08 PM   #4
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Here are some pictures:
http://www.s-audio.com/?p=192
PCB is 4 layers. It was designed to reduce magnetic fields from output stage power lines. Power lines has a very low inductance and a lot of shunt capacitors shunt high frequencies currents as short as possible. This approach can considerably reduce the distortion at high frequencies in compare to conventional design.

Last edited by Nickolay Shvydky; 20th August 2009 at 06:29 AM.
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Old 1st August 2008, 08:22 PM   #5
al2002 is offline al2002  India
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Contgatulations!!

This looks like a very impressive design. Can you comment on its stability into capacitive loads?

Are PC Boards available?
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Old 2nd August 2008, 12:58 AM   #6
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Quote:
Originally posted by al2002
Contgatulations!!
Thanks!
Quote:

This looks like a very impressive design. Can you comment on its stability into capacitive loads?
With an output filter (inductunce ~1uH + resistor 10Ohm) it has no problem to drive 1uF capacitor, with or without actual load.
Quote:

Are PC Boards available?
Not yet. I have to test the speaker protection circuit, make some minor changes, and then....

Last edited by Nickolay Shvydky; 20th August 2009 at 06:29 AM.
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Old 2nd August 2008, 03:23 AM   #7
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Very Impressive.. you have taken DIY to new levels..
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Old 2nd August 2008, 03:42 AM   #8
al2002 is offline al2002  India
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Thanks for the BOM. I look forward to the final version and the PC Board layouts.

A couple of questions, if I may.

Is the 28 V LM 1084 regulated supply essential? Just wondering how much difference it makes to the final measured performance if U3 is just fed directly from an unregulated supply.

If a truly low-Z output signal from the preceding preamp stage is available to feed the U2+U3 composite amp, is the U1 buffer really needed?

Thanks in advance.

By the way your private e-mail is disabled.
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Old 2nd August 2008, 06:34 PM   #9
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Quote:
Originally posted by al2002
Thanks for the BOM. I look forward to the final version and the PC Board layouts.

A couple of questions, if I may.

Is the 28 V LM 1084 regulated supply essential? Just wondering how much difference it makes to the final measured performance if U3 is just fed directly from an unregulated supply.
There is practically no difference in measurable perfomance. It was just done in order to make power lines clean as possible. Here is -28V oscillogram (LM3886 output is 20Vp-p on low inductive 4ohms). http://www.s-audio.com/forum/downloa...d=11&mode=view
Quote:

If a truly low-Z output signal from the preceding preamp stage is available to feed the U2+U3 composite amp, is the U1 buffer really needed?
No, it will be optional. But if you have a noisy DAC it is better to install it, because it is a filter also. Actually U1(AD8065) have no measurable distortion (< -130dB).

Quote:

Thanks in advance.

By the way your private e-mail is disabled.
I don't know why, may be because I am a "newbee" here. My email - Click the image to open in full size.
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Old 4th August 2008, 12:30 PM   #10
al2002 is offline al2002  India
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Thank you for the answers.

When will the final version of the PC Board be ready?
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