LM3886 amp

[moving_electrons]

For a lighted power button:

On one recently I put in low cost low amperage separate transformer just to provide the 12v ac (what my light bulb needed).  I separate fuse between the 120v AC and the transformer.  I ran the 12v AC to a pot and then to the bulb so that I could dim the bulb as desired.

A solution I have not tried yet that is more elegent than a separate transformer is a few turns of magnet wire (5-10?)  through the toroid.  This reportedly will produce the right voltage.  I may try this soon.

[SirPoonga]

What type of switch would one need for that?  4pdt to close the circuit to both transformers?

[SirPoonga]

I'm going to draw out the complete schematic, including power supply, in ExpressSCH for the 200W version in that pdf.  I am looking at the pdf and I think something is missing.  Will post findings when I get them.

Plus I will probably understand this better if I look at it part by part

[R.G.]

OK, short lesson in transformer picking.

Power transformers have only a few things to look at if you're picking them for solid state amps.
(1) total power: you need to pick a transformer for something like 1/2 to 2 times the steady state power the amp puts out. This is not a hard and fast rule, because music is not a steady state signal. Music varies in loudness, and the power used is not constant. Stereo amps can get by with maybe 1/2 - or 1/4 - of the total amp power because hifi music has a peak-to-average power ratio of 12 to 24db, and so the average power, which is what heats the transformer, is much lower than the peak. For heavily compressed musical instrument supplies, you can be sure if you use 2 times the output power because the DC power produced causes an RMS current in the transformer about 1.8 times the DC out average. One times the amplifier output power is usually quite safe, and what I usually pick. For one 3886, you need something like 50VA to 100VA. For two , 100 to 150. For four, 150 to 300.

What's a VA? Transformers put out volts, and they heat up with the amps that flow through them. If the amps were in phase with the volts, this would just be watts - volts times amps. But the amps are often not in phase, and they are what overheats the transformer, so transformers are rated in Volts times Amps. It's a subtle point that an EE class spends an hour on, but for choosing amplifier transformers, think of VA as watts.

(2) primary voltage: you have to get primaries that match the AC line voltage where you are. In the USA, that means 110/115/120/125.  Usually you see 120V rated. You may find dual primary 120/120 transformers. These have to be connected in parallel, and the phase must be correct. The transformer data will have the info for paralleling the primaries. If you get the paralleling wrong, you smoke the transformer.
(3) secondary voltage: This is what a lot of "Power Supplies Basics" is spent on. For the sake of our discussion, use the numbers I put here earlier - 21+21 or 42Vct to 25+25 or 50Vct.

3873347429

Plenty enough watts (VA=volts*amps=Watts), but only one 24V secondary. No good.

3873057137

Two of these is 150VA (150W) and 48V if you connect primaries in parallel and secondaries in series. That will work for one LM3886 (50W) or two (100W)

3873326077

Two of these is 48V, 150VA, and that will supply the power. Control transformers are conservatively rated, which is good. They are sometimes hummy, though, and you may need to install a copper hum-strap. OK for two (100W)

3872871277

43VA each, you'd need two for 50W, or possibly 100W if you are not expecting continuous heavy sound. Same comments on belly bands

3874275242

This is marginal; toroids, so they're not hummy, and they're small and light. However, you need two, because they are single secondary, and in series the secondaries are 56V. That's 79Vdc out after you rectify, and if you get a 10% high line surge (and that's really, really common), you get 87Vdc, and your LM3886 goes up in a puff of smoke. You would have to do some thing to keep the voltage down. The simplest is to put in a third transformer to lower the AC line voltage a bit - see the GEO article in the tube amps section "Vintage Voltage Adapter" for how. This is going to up the complexity for a beginner, though. I'd pass on these if I were you.

3874276358

Also marginal, for the same reason. Voltage too high. Otherwise, great choice for 50 or 100W.

3874129918

They only have one, and you would need two.

3873455040

Otherwise nice, but there's only one, and you'd need two.

3874282709

Pair at 24V, 40VA each - OK, that will work for one.

3874254619

You need two to get the secondary voltage. OK for one amp.

An  even better choice is 3873149875, 21+21V, 225VA. this one would power your 200W amp just fine. It has only one primary, and the buy it now price is $23.00. I would nail this one down if I were you. In fact, if you don't, I may. I habitually buy appropriate looking transformers because getting these at a reasonable price is 90% of getting amps built.

The next hard part is heat sinks.

Look at 3872926138. Again, if you don't buy that one, I may, because I keep a stock of good heat sinks. That sink will keep your 200W amp chilly.

Others to look at are:
3873991494 - I would band- or hacksaw this one in half across the fins. Good buy.
3874142624 - another good one if you have a hacksaw and a strong arm.

Heatsinks over 6" long (along the fins) lose thermal efficiency, so you'd like to have them 3" to 6" high and as wide as needed.

[SirPoonga]

Sweet.  Thanks R.G.  That helps out alot.

For item 3872926138.  He  has two in the picture.  I haven't looked at his other auctions or past auctions yet but I have a feeling he might have more than one...  off to email him...
edit:  damn, the company I am temping at blocks listings.ebay.com and contact.ebay.com  I can get everywhere else.  Sneaky sneaky sneaky.  Will have to wait until I get home.

Thinking about what you said about the heatsinks and reading the pdf.  I am thinking of rack mounting this.  Since I am a computer junkie I can get my hands on racks for dirt cheap  3" high would fit a 2U and 6" high a 4U.  I'd put the heatsinks on the back of the chassis, right?  Would I want a fan?  How would I mount a fan outside of the chassis so it blows across the heatsink?   I see the chips are more efficient if using a fan if I am reading that doc correctly.  
How would I power the fan?  Just like the suggestion about the power light I suppose

Also, if I am going to rack mount this, with the possibility of rack mounting other things (like seperate preamp maybe, effects, tuner) would it be wise to make a power station that has +/-28v outputs, +/18v outputs, +/-9V outputs then power the amp, effects, preamp, etc.. from that?

And hte reason I am thinking of rackmounting even though I am a beginner is that I have a small apartment.  That would make it easier to store the equipment in a closet and pull it out when I wanted to play.

[moving_electrons]

http://www.chipamp.com/lm3886.shtml

Whoa.  What is this?  There isn't much for docs.  Is this what I am looking for?  I would rather buy a kit if I could.
Is this everything needed except the transformer?

This is a parts kit for the power supply boards and the amp boards.  Nice double sided boards with plated through holes, solder masks and thick copper.  You need to supply transformer, switches etc., and connectors, wire etc.  The 3886 kit is a stereo kit and may fit what you are doing.  Usually the kits on the site are cheaper than ordering the parts from Digikey, Mouser etc.  So a very good deal.  The LM3886 has a high capacitance power supply suitable for what you are doing.  

For a 200W bridged+parallel version, the LM47800 kit will likely make more sense.  The LM4780 kit has smallish power capacitors for certain audio reasons so you would change them to something larger such as 10,000uF  50v off board.  These are pretty cheap apexjr.com but about $5.50 each otherwise

Ebay is a great source for heatsinks.  It used to be you had to search for "heatsink" and "heatsinks" to get all the hits.

[R.G.]

Ebay is a great source for heatsinks. It used to be you had to search for "heatsink" and "heatsinks" to get all the hits.

... and "heat sink"...

[SirPoonga]

http://www.chipamp.com/lm3886.shtml
You need to supply transformer, switches etc., and connectors, wire etc.  

So basically the kit is all the onboard parts which include the chip, caps, resistors, IE everything that would show up in the lm3886's datasheet schematic.

I know the LM4870 is basically two LM3886.  What would I do then.  Get two of those instead of 4 LM3886?  I'd need to wire each LM4870 as two parallel LM3886 then bridge the two LM4870's together?  So I basically end up with the 200W Bridge/parallel schematic as if I put a dashed box around the two LM3886 that represent one LM4870?

In otherwords, If I get a LM4870 kit form chipamps I have almost everything I need.  The kit comes with two boards and parts to populate both.  I would just need to bridge the two?  That might be a more safe and better deal then.  I know the parts are most likely in the right place and no bad soldering job

The advantage of doing this would be cost reduction I assume as you buy one chip that may be slightly more expensive but less costly than two chips.

BTW, if I wanted to use the MUTE I'd need a 4 pole switch to handle all 4 LM3886's at once, right?


And I am sure "heetsink" will show something too.  That's where you usually find the deals, on bad spellings.

[moving_electrons]

So basically the kit is all the onboard parts which include the chip, caps, resistors, IE everything that would show up in the lm3886's datasheet schematic.

Yes I think that is basically right except that 1) the kit has some component differences from the National data sheet based on sound quality tweaks and 2) as I mentioned the 4 1500uF caps you will not use and you will substitute in 4 10,000 uF caps offboard.  Still the price will likely come out on par or less than sourcing the parts yourself.  

Keep the 1500uF caps for something else or sell them in the "Trading post" forum on DIYaudio.com.  Someone will by them at a discount to implement a "gainclone classic".

I know the LM4870 is basically two LM3886.  What would I do then.  Get two of those instead of 4 LM3886?

Yes

I'd need to wire each LM4870 as two parallel LM3886 then bridge the two LM4870's together?  So I basically end up with the 200W Bridge/parallel schematic as if I put a dashed box around the two LM3886 that represent one LM4870?

In otherwords, If I get a LM4870 kit form chipamps I have almost everything I need.  The kit comes with two boards and parts to populate both.  I would just need to bridge the two?  That might be a more safe and better deal then.  I know the parts are most likely in the right place and no bad soldering job

What you say would be true except that the kit I believe is a bridged kit and board.   (I need to read through the posts to find out).  So I suspect you would build the boards and then parallel them.  I would like to confirm this via the posts in that forum of other documentataion before assuming that would work though.

The advantage of doing this would be cost reduction I assume as you buy one chip that may be slightly more expensive but less costly than two chips.

Yes.  Also the number of circuit boards and other components is halved resulting in a lower cost implementation.  The chip cost it self is a relatively small part of the overall cost.

BTW, if I wanted to use the MUTE I'd need a 4 pole switch to handle all 4 LM3886's at once, right?

I believe that is correct.  But I have not implemented a mute switch.  

And I am sure "heetsink" will show something too.  That's where you usually find the deals, on bad spellings.

[SirPoonga]

What you say would be true except that the kit I believe is a bridged kit and board.   (I need to read through the posts to find out).  So I suspect you would build the boards and then parallel them.  I would like to confirm this via the posts in that forum of other documentataion before assuming that would work though.

http://www.chipamp.com/lm4780.shtml
They link to that schematic pdf which is parallel.  I'd assume that is what they are suing.  


Is it me or is the documentation lacking on their site?

[moving_electrons]

Ooops, mistake on my part.  

Yes the schematic on the site for the kit is a parallel implementation.  So as you originally said you would then bridge a pair of them to get the 200w version.

Sorry for the confusion.

I also probably typed LM4870 when in fact the correct number for the chip with two LM3886s in it is LM4780.

[moving_electrons]

I don't think there is a build instruction documentation for the LM4780 kit.  I think one can mostly use the LM3875 kit instructions .pdf and then ask questions as needed on the forums.  

Brian doesn't make much on the kits since they are so low cost and most of the money gets donated back to the forums.  Most time goes into shipping orders and getting new kits in place such as the new LM3875 kit.  It is new as of this month.

[SirPoonga]

Want to move the conversation to diyaudio? Seems like a good way to go.

[SirPoonga]

This one would work, but a little overkill, right?
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=71390&item=3872322235&rd=1

[R.G.]

Nice try. But no, it won't. There's only one output. It *is* three phase, so you don't need much in the way of filter caps. Rectified 3 phase has only 10% ripple with *no* filter caps.

But three phase is so nice for so many things...

[SirPoonga]

I found where to move the thread
http://www.talkbass.com/forum/showthread.php?t=165490

Actually, that may not be the best place.  I think diyaudio would be.  I just registered.  I will start something there.  But I will have to do some searching there, I see this has been discussed there before.


Vincent Volta, I think your number 2 question is answered now

[SirPoonga]

http://www.diyaudio.com/forums/showthread.php?s=&threadid=51661