chipamp.com 3886 amp power supply questions

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Hi Everyone,

I was the lucky winner of the most interesting amp enclosure contest. I most graciously received the 3886 amp in the mail a couple weeks back and am ready to finish up this project - I am working it into a self-contained live rig.

I've built several SS amps before, but always used a thrifted DC power supply of one sort or another. I have no idea about making a power supply.

Honestly, I've been looking at chipamp.com and I am a little confused about which amp I even have. I believe it is the 3886 mono kit - though I think the PCB is different than what he shows there as it looks like on mine the power supply is included on the board (other than transformer), and on the site it shows them being seperate boards. There is no pic of just the 3886 mono kit though, and the PDF manual for building is confusing to me, again because the boards shown there all look different from mine.

SO all that out of the way - my questions:

1. How do I know what to look for in a transformer? As mentioned before, I have never built an AC->DC power supply and don't really know much about transformers in general. I read the choosing a transformer section of the PDF manual and it was largely greek to me...

2. If I found a suitable DC power supply, could I just use that and connect it into the power inputs on the board?

3. If this is possible and I were to look for a pre-made DC supply, could it be single rail, and what type of voltage and amperage output ratings would I be looking for? I have a knack for finding things at thrift stores. Also, if so which connections would go where AC1, AC2, gnd...?


FYI, my hope is for it to be "as loud as possible" so that is what I have in mind when looking for the above components.


Thanks

[JPHeisz]

Hi,
The supply voltage would depend on the speaker load. Less voltage for lower speaker impedance to keep the max current down (and heat dissipation).
You'll need a split supply (+/-) - easily as much as +/-28VDC for 4 ohms and up to +/-38VDC for 8 ohms.
Here's a handy design calculator for these chipamps: http://www.national.com/vcm/national3/en_US/products/audio/files/Overture_Design_Guide15.xls
Works with Excel.

[joecool85]

You won't find that board (or a manual for it specifically) on chipamp.com.  It is essentially a mono LM3886, but it includes the PSU onboard.  Here is the build thread for it: http://www.ssguitar.com/index.php?topic=1255.msg7766#msg7766

It shows how to hook it up properly.  I had problems as well, it's an odd beast but works amazingly well.  I hope Brian eventually starts selling them and makes a manual for that kit.

As for transformer ratings.  For full power you will want a 22v+22v transformer rated 160VA or higher.  I used a 330VA because I was planning on using another LM3886 eventually.

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Ok thanks both of you for your responses. Haha, I had no idea that it was not a commercially available board. No wonder I was so confused about it.

I have access to a decent electronics surplus store, so I may be able to get the transformer I need. I don't think I will be able to dig up a bipolar DC supply at that high of a voltage, or at least I haven't seen one so far.

So I am sorry because I am really ignorant when it comes to transformers.

If you could give me as much detail about the type I should be looking for - I am a little confused, for example about what 22v+22v means? Is that for the output of the transformer? Would that be two outputs that are out of phase with eachother?

Or if you could point me to a commercially available transformer that meets the necessary specs, so that I can look them over and compare them to one I can buy locally?

Thanks so much again.

[joecool85]

No worries.  I can explain a bit better.

You don't need (nor could you use easily) a bipolar DC supply.  The board handles that already.  All you need is a transformer with dual outputs, 22v on each.

Here is a great example: http://www.parts-express.com/pe/showdetl.cfm?Partnumber=122-615

The LM3886 puts out:

68w at 4ohms with +/-28v
38w at 8ohms with +/-28v
50w at 8ohms with +/-35v

When you rectify AC to DC you multiply it by 1.4.  So for 4 ohms to get max power you only need a 20+20 transformer.  For 8 ohms to get the max you would need a 25+25 transformer (I can't find these smaller than 250va, much larger than you need but won't hurt anything other than your wallet if you go that way).  These specs are all for being on the bleeding edge of peak performance.  22+22 will get you most of the way there and is cheaper and available in a 160va size so I normally recommend that.

Please ask questions if you have them and I'd be more than glad to help.

[JPHeisz]

This would be good for your amp if using 4 ohm speaker: http://www.antekinc.com/details.php?p=40
22-0-22 and 100VA

If using an 8 ohm speaker: http://www.antekinc.com/details.php?p=41
25-0-25, 100VA

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Sorry for my absence/lag in response... I am getting back to this project.

So there has been links to transformers in this thread that are rated 160VA and 100VA. What does that rating mean?

At 100VA, will I not be able to get the full potential output of the chip?

Thanks again for your patience.

[J M Fahey]

in a nutshell, the 100VA one is fine for a single 3886; the 160VA one handles two of them.
even if you'll use only one chipamp, often the price difference is small, and the larger one will always run cooler

[joecool85]

I would go with the 25-0-25 100VA one that JP linked to.  For $18 you can't beat it and you would get full output from your chip.  At full bore it would be maxed out, but trust me, you will rarely if ever be full bore on it assuming you have a decent speaker hooked up - it is LOUD!

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I would go with the 25-0-25 100VA one that JP linked to.  For $18 you can't beat it and you would get full output from your chip.  At full bore it would be maxed out, but trust me, you will rarely if ever be full bore on it assuming you have a decent speaker hooked up - it is LOUD!

I ordered that one last night. I hope it is loud.

I am planning to use the amp for my portable pa system for my live setup. Currently I am using a 30W rat shack PA head from the 70's and it is not cutting it.

[J M Fahey]

I am planning to use the amp for my portable pa system for my live setup.

I feel cheated. :grr
Why do you say so only now, after the fact?
50W is LOUD for any guitar, *moderately* loud for bass and maybe not enough for PA.
You will need *two* 3886 power amps, each driving at least a 12" efficient guitar type speaker plus 2 or 3  Piezos for highs in *each* enclosure.
If your 30W PA does not cut it at all, a 50W one won't be that much louder, now talking 100W plus two cabinets is closer to real-world needs.
There's not such a thing as too much PA power.

[joecool85]

I feel cheated. :grr
Why do you say so only now, after the fact?
50W is LOUD for any guitar, *moderately* loud for bass and maybe not enough for PA.
You will need *two* 3886 power amps, each driving at least a 12" efficient guitar type speaker plus 2 or 3  Piezos for highs in *each* enclosure.
If your 30W PA does not cut it at all, a 50W one won't be that much louder, now talking 100W plus two cabinets is closer to real-world needs.
There's not such a thing as too much PA power.

I agree.  50w is more than enough for most guitar needs given an efficient speaker is hooked up to it.  However, PA is totally different.  I'd actually recommend 150-200w for a PA along with what Juan suggest for a cab (2 x 12" speakers plus some piezos).

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Maybe I should clarify.

I have been using this radio shack 30W PA head (which I am sure is less efficient than a modern design) driving a couple of home made cabinets with dept store announcement speakers and car audio speakers (all thrifted) for about 1 year for live shows. It works, but it gets distorted at the levels I use it. I don't play super loud.

By PA I mean that I mix a couple of synths, a guitar, and a seldom used mic into one amp.

I'm going to see how this amp works. I can always add more in some way or another. Maybe I can use it alongside my rat shack PA.

This isn't to mix drums, amps and a full band of mics through. Maybe think of it more like a "keyboard amp."

[J M Fahey]

Ok, now I understand you.
Rather than a regular PA, you are talking about a multi-instrument amp.
That's an easier task.
The main problem is, do you play with or without a rummer?
If without, you can barely make it, considering your amp serves 4 instruments, meaning you'll have 12W at most for each of them.(4x12=48)
If "with", he alone sets a certain volume standard or level you won't be able to match.
Simple Physics.

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Ok, now I understand you.
Rather than a regular PA, you are talking about a multi-instrument amp.
That's an easier task.
The main problem is, do you play with or without a rummer?
If without, you can barely make it, considering your amp serves 4 instruments, meaning you'll have 12W at most for each of them.(4x12=48)
If "with", he alone sets a certain volume standard or level you won't be able to match.
Simple Physics.

No drummer.

At any one time, I usually have two instruments going at the most. One is a synth with a sequencer and several voices, so it is a little hard to say really how many instruments are playing at any one time... I have been thinking of splitting off my guitar to it's own amp at a miminum in addition to the "PA." I used to do it that way for a while, but then started using FX on a whole submix which includes my guitar.