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Old 15th March 2007, 01:28 PM   #1
bigboy is offline bigboy  United States
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Default Sound of Low Imp 3886

I have several 28VCT toroidal transformers around and wanted to make a gainclone out of them. With the MTM speakers I am making I can either put them in series (8 ohms) or parallel (2 ohms) and am trying to figure out what GC to make. Looking at the Excel Overature design spreadsheet, I extrapolated the data to look at 2 ohm loads. With 19.8 volts, (this is a no load voltage so it will never stay at this) I would end up drawing around 8.5A of current...bad. But after the voltage drops with a load to say around 16 or 17 volts, the current draw is 6.8A and 7.25A, respectively. With needed heatsink values well above 3C/W. Ok, so by the theoretical math it looks ok. But will my THD go WAY up if I am running at such a low voltage and impedance? Is it going to sound like garbage or am I better off taking two or the transformers and making a 39v PS and running them at 8 ohms. The 8 ohm option would give about 5 watts more output, but I am not needing much more than 40 watts or so. I would like to have the low impedance option for the future, but if it is going to sound bad, I won't. Any advice?
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Old 15th March 2007, 03:20 PM   #2
Fenris is offline Fenris  United States
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Use a pair of LM3886s in parallel or (better) use different drivers in your speakers. 2 ohm is really too low.
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Old 15th March 2007, 04:07 PM   #3
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The LM3886 does not really like 2 ohms, even at low voltage. You can run them up to about +/-20V into 2 ohms but that is about all. It just gets too hot and the Spike protection circuitry will kick in way before the part can heat up a heat sink. If you can get the supply voltage down to +/-20V then you will have about a 50W solution. The LM3886 really prefers 4ohms or higher to get the most output power while keeping the thermals in line enough to work without any protection kicking in. If you really want to drive 2 ohm loads then a parallel set up will work best.

-SL
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Old 15th March 2007, 04:20 PM   #4
bigboy is offline bigboy  United States
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Quote:
Originally posted by SpittinLLama
The LM3886 does not really like 2 ohms, even at low voltage. You can run them up to about +/-20V into 2 ohms but that is about all. It just gets too hot and the Spike protection circuitry will kick in way before the part can heat up a heat sink. If you can get the supply voltage down to +/-20V then you will have about a 50W solution. The LM3886 really prefers 4ohms or higher to get the most output power while keeping the thermals in line enough to work without any protection kicking in. If you really want to drive 2 ohm loads then a parallel set up will work best.

-SL
I was wondering if the theoretical numbers were a bunch of crap. It looks like parallel is the way to go. I didn't realize that the Spike protection would kick in earlier either.
Is my THD going to go up considerably running it with a low voltage like that?
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Old 15th March 2007, 04:52 PM   #5
macboy is offline macboy  Canada
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Why not use two 28 VCT transformers, ignoring the center-tap. This would be the same as using a transformer with dual 28 V outputs, which is what a lot of people here use. It is just about the perfect transformer voltage to use with 8 ohm loads.
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Old 15th March 2007, 05:29 PM   #6
Nordic is offline Nordic  South Africa
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I would stay away from the 2ohm loads...
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Old 15th March 2007, 05:52 PM   #7
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Dear,

Just put 8 LM3886's parallel It raise the system dampingfactor as well

Best regards,
Bas
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Old 15th March 2007, 06:30 PM   #8
bigboy is offline bigboy  United States
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Quote:
Originally posted by macboy
Why not use two 28 VCT transformers, ignoring the center-tap. This would be the same as using a transformer with dual 28 V outputs, which is what a lot of people here use. It is just about the perfect transformer voltage to use with 8 ohm loads.
I have enough transformers that I could try both options of the high and low PS voltage (parallel GC). I'll have to see which sounds better...they both have advantages and disadvantages. Obviously the lower impedance, higher Vs option will sound better, but how much is the question.
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Old 5th July 2007, 01:06 AM   #9
Arx is offline Arx  Canada
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Quote:
Originally posted by Sebastiaan
Dear,

Just put 8 LM3886's parallel It raise the system dampingfactor as well

Best regards,
Bas

Will it? You usually put resistors in line with each amp so they don't fight. Wouldn't that send the damping factor out the window?

-Nick
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Old 19th August 2007, 09:18 PM   #10
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Default 3886 design that laughs at 2 ohms

The 3886 can handle 2 ohm loads, you just need to parallel them. At the bottom is a pic of a design I have been messing around with for awhile. It is bridged and paralleled with 4 devices on the + output and 4 devices on the - / inverted output to create the bridge.

In this pic the amp is putting 30v p-p across 2 ohms. If I push it much harder, the amp starts clipping because the PS is sagging so much.

Right after I snapped this pic I put a 8 ohm speaker in paralleled with the 2 ohm dummy load and cranked up the music. Sounded great!

To deal with the sharing issues that come with paralleling devices, I put each device under servo control (half of the parts on the board are for the servos) to take the output offset down to almost 0 and then had each device feed a .1 ohm ballast resistor.

So for you 2 ohmers out there, don't despair, it can be done.

Click the image to open in full size.
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