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Old 28th July 2014, 02:25 PM   #1
Carlp is offline Carlp  United States
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Default Quick LM3886 Power Supply questions

3-5 years ago I purchased a set of boards for an LM3886 dual mono that also included a single PS board. It appears to be a copy of the Chipamp.com schematic for amp and PS (a Chinese rip-off, sad to say) except the PS board has one 3-pin AC input and a single bridge rectifier (w/cap bypass). I finally put it together using an Antek AS-2224 toroid transformer (200VA) at 24V paralleled (using 24v as the AC input). With this setup, I'm getting +/- 16v, which makes sense given my mains voltage.

My first question is will +/- 16v be sufficient for this, or should I hook up the Transformer in series and get more like +/- 30v for the rails? Datasheet states a minimum of 20v +/- in one place, but elsewhere states 18v is typical and +/- 9v is minimum - it's confusing to a non-engineer. Is 30v too much given speakers that probably range from 4-8R (from what I've read about the chip it looks like 30v should be fine, but I'm looking for confirmation as I'd hate to have to replace the 3886s).

Also, at 200VA for the toroid, will that be sufficient hooked up in series for the higher voltage? Chipamp.com suggests 160VA as minimun, 220VA as better, but I don't know if that is with secondaries hooked up parallel or series. From what I can see of their PCB, it looks like it is rectified differently, in essence the Transformer windings are each rectified for one or the other rail. That suggests to me it's the same as hooking mine up in series.

My second question is that I'm actually getting +16.5v / -16v, with a 1/2 v difference. I can't seem to figure out what could be the cause as all parts values appear to be quite close. I did briefly hook up the bridge rectifier backwards (+ to - b/c I used a different rectifier than the demo photo) but don't know if that could cause this problem. Is this rail difference likely to cause a problem? How close should they be normally? I've built several dual rail supplies and never had them come out so different. Assuming they should be closer, can I drop a little voltage with a resistor?

Thanks for any input!
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Old 28th July 2014, 02:57 PM   #2
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You should wire the secondaries;
24ac to one side of the bridge ac in. 0ac to second 24ac directly to 0v DC/ground and second 0ac to the other bridge ac in, giving 24 0 24ac, otherwise you will have a floating supply of only 16 0 16dc. With the transformer wired correctly you will have +32v 0v -32v dc
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Old 28th July 2014, 03:43 PM   #3
Carlp is offline Carlp  United States
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Hi, thanks.

OK, so I DO want +/- 32vdc rails? Note I may not have described my setup, so here's 2 photos. First is of the entire setup, second is just the PS board. Note on the PS board the AC input to the upper right and the single bridge rectifier in the lower right. I have 2 AC input lugs with a ground reference between them and only one bridge rectifier. Whatever voltage goes into the two lugs therefore gets split into the two rails. Remember this is not the chipamp.com PS board.

I need to either wire secondaries in parallel (the AS-2224 has two 24v windings, the green and blue wires) which gives me 24vAC out (and my +/- 16vdc PS output) OR wire the secondaries in series to get 48vAC out of the transformer. In that case after rectification and smoothing I should get about +/-32vdc as you recommend.

Thanks again,
Carl
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Old 28th July 2014, 03:57 PM   #4
hongrn is offline hongrn  United States
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1. You need to form a center tap for the ground (GND) terminal. Do this by wiring one blue wire with one green wire together, and connect it to GND terminal.

2. Connect the remaining blue wire and green wire to either side of the terminal labeled "AC", so your connection looks like this:

AC - Green wire
GND - Green and Blue wires twisted together
AC - Blue wire

3. Then check the voltage on the DC side by holding the meter black probe on the GND terminal. Touch the V+ terminal with your red probe. You should have around 35V. Now move the red probe to the V- terminal, you should see -35V.

Good luck and be careful.
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Old 28th July 2014, 04:50 PM   #5
Carlp is offline Carlp  United States
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Thanks, hongrn. I'll try that. I wired it that way at first, but it was shorted (my dim-bulb tester lit up like a flood light).

That may have been b/c my bridge rectifier was in backwards, I don't know. But I'll keep using the dim-bulb tester until I'm sure I've got it right. And thanks, I will be extra careful.
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Old 28th July 2014, 05:32 PM   #6
Carlp is offline Carlp  United States
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OK, I took a moment over lunch to try this, wired up in series per hongrn above. This time, the dim-bulb flashed then shut off, I assume due to the inrush current when the big iron started up.

In any case, I'm reading 50v across the two PS input lugs (transformer secondaries wired in series - about what I expected). Also reading +/- 33.6vdc at the outputs, which seems pretty good to me. There is now still a roughly .03vdc difference from + to - (+ reads about 33.6 while - reads -33.57 or so).

Can I assume that difference is OK on the PS output?
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Old 28th July 2014, 05:45 PM   #7
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Can I assume that difference is OK on the PS output?

Yes that is normal.
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Old 28th July 2014, 06:20 PM   #8
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I guess no two outputs of a traffo are same, there will be some margin of error between them, as long as there is no vast difference in voltages then it is okay...


Enjoy the kit, the design is a good one.
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Old 28th July 2014, 07:13 PM   #9
Carlp is offline Carlp  United States
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OK, thanks all. I appreciate the help. That should be all I need. If I get it fully breadboarded and have a listen, I'll give a quick report.
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Old 29th July 2014, 01:51 AM   #10
Carlp is offline Carlp  United States
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Success so far. Got it completed and have had an initial listen from my workbench. No place to get a sense of quality but it sure works! Thanks again all.

Carl
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