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Old 31st August 2009, 08:04 PM   #21
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So I had some prototype boards made, populated them and have been listening to them for over a week. It sounds fantastic. Compared to the bridged LM4780 I was using prior, which has an input DC blocking capacitor only, it sounds better. The bass goes lower, and louder. The high range has lost some harshness, but hasn't lost any of the clarity. I'm using the same power supply, which was set up for 115 Wpc with the LM4780 bridged, and gives only about 30 Wpc with this design.

There is less than 2 mV DC on one channel (my equipment can't read it), but 19 mV DC on the other. I'm wondering if I may have made a bad solder joint, because I feel that's a little high. There is about 4 mV of noise at about 71,500 kHz, and about 2 mV at 625 kHz (with the inputs grounded).
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Old 31st August 2009, 10:22 PM   #22
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hey redshift the board looks fantastic

can I ask where you ordered the board from?

cheers
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Old 31st August 2009, 11:26 PM   #23
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Thanks! I ordered them from http://www.batchpcb.com/. It's a hobby prototype service that waits until a "panel" is filled by smaller orders, then orders the production. Time from order to receipt is about 4 weeks. Cost is $10 + $2.50 a square inch, so it's a good deal for smaller items.
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Old 31st August 2009, 11:31 PM   #24
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Oops, I meant 71.5 kHz.
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Old 1st September 2009, 12:56 AM   #25
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The four boards I received:
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Old 1st September 2009, 06:42 AM   #26
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are those output offsets ( 2mVdc & 19mVdc) you have measured with the DC servo operating?
I would expect the servo-ed offsets to be <1mVdc.
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Old 1st September 2009, 09:12 AM   #27
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Quote:
Originally Posted by Redshift187 View Post
It sounds fantastic. Compared to the bridged LM4780 I was using prior,
Nice board!

Are you aware of that the LM4780 is two LM3886 chips in the same package?
http://home.pacific.net.au/~gnb/audio/lm4780.html
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Old 1st September 2009, 10:55 AM   #28
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Yes, it's with the servo operating. The one probably is <1mVdc, but my meter only goes down to 1mVdc +/- 1mVdc, which is why I said <2mVdc. The other one still seems to be doing it's job. I can affect the DC offset by changing the volume pot with a source that has DC on the output, and watch the DC on the amp move, then recenter on the same value. It's probably less than 19mVdc. My meter reads it as 10mVdc, but my scope reads it as higher.

Thanks peranders. Yes, I was aware of that. It's the only chipamp I have heard to compare it to. The main difference, aside from not being bridged, is that there is no input capacitor on the new board. This time I used the 220pF cap across the + and - inputs to the LM3886, as it's the only bandwidth limiting feature in the design, and I didn't want to risk oscillations. So it's quite a bit different design, but the (almost) same chip for an interesting comparison.
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Old 1st September 2009, 04:31 PM   #29
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Quote:
Originally Posted by Redshift187 View Post
Thanks! I ordered them from http://www.batchpcb.com/. It's a hobby prototype service that waits until a "panel" is filled by smaller orders, then orders the production. Time from order to receipt is about 4 weeks. Cost is $10 + $2.50 a square inch, so it's a good deal for smaller items.
awesome!
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Old 6th September 2009, 03:10 AM   #30
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you should use a scope to observe the output wave,then judge if the 220p cap is needed.
Usually,the 220p cap is a bane for the sound.
I remember that the 21.5 times gain make the unit unstable,try 25 time gain.
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