lm3886 power supply question

[Brymus]

OK I know the lm3886 likes a bipolar supply around 50VCT.
But I am having trouble with another chip amp that was supposed to use 12V 2A supply for 12watts could I run the Lm3886 on that and just get reduced output say around 15watts would be great at .01 %THD
I know it works on a straight DC supply as well, just wondering if 12V 2A is enough (isnt that roughly 20VA?) and what the output power would be around?
I got the chips cheap enough to experiment with too BTW so I dont mind not getting its full potential.

[joecool85]

Hi Brymus, unfortunately the minimum supply voltage for an LM3886 is 18v (+/- 9v), so 12v won't quite cut it for you.

[Brymus]

What about using two wall warts to make a bipolar supply like 2X9V 1amp or 2X12V 1amp supplies.?
I was even thinking along the lines of 16AA nihm cells for a +/-12V supply would that have enough current to run the lm3886 to 15-20watts?

Also I have a 42VCT 80VA toroidal for my lm3886 amp just wondering could I use two of these transformers to make a 84VCT 160VA supply for two 3886 chips bridged together?

Sorry I know that's 3 questions But I really appreciate the help before I start messing with these chips.

[joecool85]

What about using two wall warts to make a bipolar supply like 2X9V 1amp or 2X12V 1amp supplies.?
I was even thinking along the lines of 16AA nihm cells for a +/-12V supply would that have enough current to run the lm3886 to 15-20watts?

Also I have a 42VCT 80VA toroidal for my lm3886 amp just wondering could I use two of these transformers to make a 84VCT 160VA supply for two 3886 chips bridged together?

Sorry I know that's 3 questions But I really appreciate the help before I start messing with these chips.

I'm thinking both of the first ideas probably won't work, or at least very well.  Although, until you try it you never know, and sometimes the craziest ideas are the best.  I'd say its worth experimenting on, but just don't expect great results (although it could happen).

As for question #3, I'd use a larger transformer myself.  I recommend at least a 100VA (ideally 160VA) per LM3886.  This keeps the chip's voltage from sagging under high load.

[J M Fahey]

Your 42Vct80VA will provide +/-29V dc nominal, perfect for *one* LM3886 , driving *one* 8 ohm speaker. It will supply around 38W, and "eat" 50% or 60% more than that, around 65/70 VA. In that case, your 80VA xformer should be about right. Of course, if you try to pull its almost 70W power loading it with 4 ohms, your transformer will fall short of the task, as was correctly stated above, since you would need 100 to 120 VA.
Anyway I think that almost 40W into a reasonably good 12" (or a very good 10") is more than enough for serious work.
For your supply you will need 4x 1n5402 or a 3A200V bridge and 2x 4700uFx35/40/50V.
You can get +/-15V for your preamp from there.
If you have two transformers, you can build a stereo version or, even better, two 40/112 combos, which can be used together, open in stereo, borrowed , or left one at home, other for rehearsal, a most versatile rig !!!

[Brymus]

Thanks JM
It was RG who pinned me a list (at DIYstompboxes)of items for my LM3886 build.
It included the 42VCT 80VA toroidal, 2x 6800uf 100V caps a MONTER sink,the 3886 chip
All purchased from Apex Jr for a total of 40$(I bought 3 chips for 3.50 each)
Were it not for this forum (your builds included)I wouldnt even attempt this project.
I am still working on sawing that huge sink in half.
I did get more AA nihm cells now I can make a +/-12V supply that is rechargable.

QUESTION what would be the VA rating of 16 AA nihm cells rated at 2600mah each?
I tried to read the 12Vpack of 8 but it fried the fuse in my DMM I guess I could check one and multiply by 16.
Doesnt the 2600mah mean the total current before the battery is dead?
Even at 200ma a piece thats 1.6 amps per pack x2 would be 3.2amps at 24V is about 50VA.
Did I do that right? Even if I could get 1-2 hrs use at 30-40 watts that would be perfect for my uses.

[joecool85]

Thanks JM
It was RG who pinned me a list (at DIYstompboxes)of items for my LM3886 build.
It included the 42VCT 80VA toroidal, 2x 6800uf 100V caps a MONTER sink,the 3886 chip
All purchased from Apex Jr for a total of 40$(I bought 3 chips for 3.50 each)
Were it not for this forum (your builds included)I wouldnt even attempt this project.
I am still working on sawing that huge sink in half.
I did get more AA nihm cells now I can make a +/-12V supply that is rechargable.

QUESTION what would be the VA rating of 16 AA nihm cells rated at 2600mah each?
I tried to read the 12Vpack of 8 but it fried the fuse in my DMM I guess I could check one and multiply by 16.
Doesnt the 2600mah mean the total current before the battery is dead?
Even at 200ma a piece thats 1.6 amps per pack x2 would be 3.2amps at 24V is about 50VA.
Did I do that right? Even if I could get 1-2 hrs use at 30-40 watts that would be perfect for my uses.

Thats not how amps work.  Also, those are mah, they should put out more than 2600ma over a shorter span.  Lets say for easy math that it is 3amps (3000ma) for their max output.  If the batteries are in series to increase voltage, the amps will stay the same.  So 3amps x 24volts = 72VA.

It would probably run an LM3886 alright for a little while anyway.  You won't be getting 50w RMS @ 8ohms though, thats for sure - that requires +/- 28v if I recall.  But, you should get 15-20w RMS, and I'm guessing an hour or two of play time on your batteries.

[Brymus]

Hey thanks for that Joe
I think I will give it a go and let you know how it works out.
FWIW I will be using this lm3886 build with a spkr/spkrs with a 99db or greater SPL
So getting the max potential isnt key.
I do plan to build another with my toriodal 42VCT 80VA or I have another thats 50 VCT at 3 amps but its not toriodal.

I think I will take your advice and wait until I can get a 200VA tranny before I try bridging two 3886 chips though.

[Brymus]

OK I now have a better understanding of rated Mah and ,
The fact that batteries have an internal resistance.
And the discharge curves that are used to give the rated Mah are often based on an average or a 20 hour rate.
Also the rating drops as the charge is depleted,
And that the AA size cannot supply the full rated Amps for more than a millisecond if that.
So more realisticly I think the D cell in a Nimh rechargable rated at 12,000 Mah Or 12amps(in theory)would be better to get a draw of 2-3 amps for one hour (maybe depending on internal resistance and heat)
Also (as I understand now) since as less amps is needed to produce more watts as current increases then raising the V to +/- 18 or 24 would yield far more volume for a longer period of time per the same Mah rating Vs +/- 12V.
I tried this with my amp and at 16V the volume was comparible with rechargables as to 12V with MUCH more current available from transfromer.
So for car chip amps maybe 15 rechargable D cells for several hours use, the LM3886 would need 30 D cells to work as a decent 40 watt amp.

I dont know if any of you remember the Music Man amp that took 40 D cells to get a 60V supply? (from 1970s)
The batteries lined the bottom and both sides of the inside of the amp (must of weighed a ton.

[J M Fahey]

A practical option is to use two 12V (4,5 or 7 AH) gel cell lead acid batteries, which can supply a *lot* of current, and +/-12V.

[joecool85]

A practical option is to use two 12V (4,5 or 7 AH) gel cell lead acid batteries, which can supply a *lot* of current, and +/-12V.

This is a good idea.  If you happen to have a couple car, motorcycle, boat or other 12v lead cell batteries around it would work also.  Just make sure both batteries are the same size (electrically).