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Old 23rd August 2010, 07:05 PM   #1
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Default LM3886 clipping odd?

I put together a BrianGT kit using mostly the parts supplied with the kit including the Ci cap for low DC offset. No input cap currently.

The amp works great but I felt the way it clips seems strange to me.

I'm using an Antek 300VA 20+20 transformer. My testing was done using a Tektronix 100MHz analog scope and a decent signal generator. load was two Dale 1% 250W 8 ohm resistors. I used two for 4 ohm tests.

The amplifier clips the bottom way before the top and it clips bad all of a sudden. The best way to describe what it does is the bottom of the sine becomes a UU with the right U being much higher. If I run it barely into clipping it alternates between a perfect sine wave and clipped bad, again bottom only.

My power output was 30W into 8 ohms and 54W into 4 ohms. temps were all fine during the tests. I can try and post some pics of the scope tonight if need be.

Perhaps this is just the nature of the chip and completely normal?

Click the image to open in full size.

Click the image to open in full size.

Will be neatening up the wires a bit more with sleeving and zipties.
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Old 24th August 2010, 01:20 AM   #2
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Just took some pictures and did more testing. It doesn't seem as bad as I had originally thought.

top line is just the scope connected to the negative rail. The bottom of the sinewave clips just slightly before the top.

Click the image to open in full size.

Wires are a bit neater now as well.
Click the image to open in full size.

Last edited by thetube0a3; 24th August 2010 at 01:22 AM.
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Old 24th August 2010, 02:42 AM   #3
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Are you *sure* the temperatures are fine? I see isolated chips, it could be the SPIKE kicking in.
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Old 24th August 2010, 03:14 AM   #4
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What I see happens immediately as soon as the amp is turned on and remains the same unchanged. The chip is cool to the touch.

I am also under the impression that SPIKE effects both top and bottom of the wave but not the in this way according to National Semiconductors writeup on it.

http://www.national.com/an/AN/AN-898.pdf
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Old 24th August 2010, 03:27 AM   #5
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Hi TheTube

I performed some clipping tests on my LM3886 P2P a few years back. Basically it is based on the BrianGT schematic, and I have an Antrim 20+20 300VA transformer so similar to you. Main difference is I only have 1000uF / rail / channel of capacitance in the PSU.

The clipping waveform was very ordinary see this post Wintermute's Gainclone Admittedly only a PC based scope but when the same test was run on my 100W mosfet amp I got perfectly flat tops and bottoms on the sinewave, so it's not the test that is at fault.

I had certainly read that once these chips clip it is pretty ugly! I guess the bottom line is don't over-drive them!! Certainly My setup at the moment with 4 ohm speakers which are around 92db efficiency I don't have a problem with it not going loud enough for my tastes

Tony.
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Old 24th August 2010, 03:32 AM   #6
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wintermute : how do you like your amp with the 20+20 transformer?

Everything here seems to add up. I'm down to around 25V+- under full load which yields 30WPC into 8 ohms and around 54WPC into 4 ohms. Almost identical to what the datasheet claims. My mains measured 118V during the test.

I kinda wish I had gone with the 22+22 transformer for a bit more muscle, yet heat would've gone up substantially as well.
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Old 24th August 2010, 03:44 AM   #7
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I'm quite happy with it for the cost (it was less that $150AU to build) I think it would be hard to beat it! It was originally built as a test amp (for testing speakers) with the possibility to end up being the second amp in my planned BI-AMP setup (playing from 200-300HZ and up), but for the last few months it has been doing main duty as one of the channels on my main amp has stopped working and I haven't had the time to investigate.

I do notice it has a slightly more pronounced treble than my old amp (particularly noticeable hissed esses, not something I like...) interestingly on a FR analyses it is very slightly flatter (like 0.5db) in the higher frequencies, ie the mosfet amp starts showing a slight rollof from about 10Khz down by 0.5db by 20Khz whereas this amp is essentially flat. I don't think this is what is making what to me is a fairly pronounced difference though.

I still haven't really had a fair test though because I'm running it off my HT PC (no preamp) which is simply using the on board sound card, which I'm not certain about the quality!

I was actually very impressed at how well it works for HT use (stereo). Even without my 10" woofers running (my active crossover project has been stalled for a while), it is very good for sound effects, and the imaging is very good

Tony.
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Old 25th August 2010, 10:34 PM   #8
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I also just finished one channel of my LM3886 amp board. My supply is very similar to yours. A 330 VA 18V+18V (20 V unload) toroidal and 10000 uF per rail. I will test the chipping behavior of my board too.
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Old 26th August 2010, 12:59 AM   #9
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Panson, i'm curious why you decided to go with 18+18? Reason I ask is with 20+20 i'm seeing around 26V under full load into 4 ohms and its not getting too hot. As I mentioned previously I kinda wish I went with a 22+22 transformer now.

Then again, maybe i'm just asking for trouble.
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Old 26th August 2010, 02:08 AM   #10
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Quote:
Originally Posted by thetube0a3 View Post
Panson, i'm curious why you decided to go with 18+18? Reason I ask is with 20+20 i'm seeing around 26V under full load into 4 ohms and its not getting too hot. As I mentioned previously I kinda wish I went with a 22+22 transformer now.

Then again, maybe i'm just asking for trouble.
Hi,

just because I have this transformer available for testing. It has two outputs per secondary: 18 VAC and 28VAC at rated load. I use the lower one for now. Since this is a 330 VA toroidal, under normal usage for single channel, the output voltage will be higher than the full load output.
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