Questions about the LM3886-One more question

[ninoman123]

Real quick questions. Keep in mind that I did read R.G.  and Sir Poonga's thread about this amp. Found here

1)I'll be using +/- 35 volts for the power supply of the chip because I read on that thread that 35 volts DC is the best to run this amp at. Whats the easiest way to get +/-35 volts? I read R.G.'s explaination of power transformers and it made no sense to me, and I read the power supply basics at GEO, but none of it is clicking.

2)Also. I saw that there was a LM4780 or something like that. This chip is just two LM3886s put together. However the data sheet application doesnt use the inductor coil/resistor combo like the LM3886 data sheet does. Why is this? And should it be added to make the amp more stable?

3)For the #22 wire for the LM3886...does it matter if its stranded or solid core?

4) The LM3886 application says there is an optional standby switch addon. What is the point of this? If there even is one...Its not a tube amp and has nothing that needs warming...is it just for fun/cosmetics?

Ehh I think thats all now. Thanks any help is appreciated.

[R.G.]

1)I'll be using +/- 35 volts for the power supply of the chip because I read on that thread that 35 volts DC is the best to run this amp at. Whats the easiest way to get +/-35 volts? I read R.G.'s explaination of power transformers and it made no sense to me, and I read the power supply basics at GEO, but none of it is clicking.

You need 70Vdc (35+35) out of the power supply. The rectifier diodes eat up 1.4V, so you need 71.4Vdc peak from the transformer. The factor that relates the peak value of a sine wave to its RMS voltage (which is what transformers are specified in) is 1.414, so the secondary voltage you want is 71.4V/1.414 = 50.49Vac. That needs a center tap so that you get +/- voltages, not 0V and 70V. The current needs to be sufficient to run the amp and supply the losses. If you're putting out 68W (and that's optimistic) you'll need something like 1.4 times that if you run a full power sine wave  all the time. Real music has peaks and valleys in power, so you can get by with less rating on the power transformer. A tranformer rated at 48Vct to 50Vct at 1A to 2A will work fine. Use filter capacitors of at least 4700uF at 50Vdc. More capacitance is better, more voltage is wasted.

2)Also. I saw that there was a LM4780 or something like that. This chip is just two LM3886s put together. However the data sheet application doesnt use the inductor coil/resistor combo like the LM3886 data sheet does. Why is this? And should it be added to make the amp more stable?

Applications notes are notorious for leaving out small details. The inductor/resistor combo is a good idea for every amplifier. So is a 0.1uF/10 ohm Zobel network inside the inductor/resistor. Some amps work fine without them - until that one difficult speaker comes along.  Put them in.

3)For the #22 wire for the LM3886...does it matter if its stranded or solid core?

No. The solid core is easier to bend into place and it will stay. The stranded is simpler to wire a chassis with. They have equal resistances per foot. The electrons don't care. YOU care about wiring simplicity.

4) The LM3886 application says there is an optional standby switch addon. What is the point of this? If there even is one...Its not a tube amp and has nothing that needs warming...is it just for fun/cosmetics?

The point of a standby switch is not really warming up. It's for muting. Standbys are to let you mute a tube amp without having to wait for warming when you come back from that break. You can simply turn off a solid state amp all right, but there are some circumstances where it's nice to have a "mute" pin that mutes the amp without having to restart the power supply. It's optional for solid state.

[ninoman123]

Awesome. Thanks R.G. That explaination for getting +/- 35 volts explained it. I might end up powering another seperate amp off the same power supply so I can have a seperate amp for stereo use if I want.  I will be using the same voltage but I should probably get one rated at more amps correct?

[R.G.]

I might end up powering another seperate amp off the same power supply so I can have a seperate amp for stereo use if I want. I will be using the same voltage but I should probably get one rated at more amps correct?

Correct. For two of these, a 48Vct, 3A rated transformer is about perfect for musical instrument use. You can back that down to about 2A for two amplifiers if it's in stereo duty because hifi use is not as consistently high level - there's more quiet periods.

[ninoman123]

Cool. Thanks again for your help.

[ninoman123]

Ok...I acquired a 50 v ct 5 amp power supply from ebay. I plan on building this into a high wattage bass amp. My questions are...


1)How would I connect several of these LM3886 amps together so they will all drive a single output? How would I need to mod the original circuit found here

2)How many of these amps could I build with my 5 amp supply?

[Ardric]

Ok...I acquired a 50 v ct 5 amp power supply from ebay. I plan on building this into a high wattage bass amp.

Nice.  What pre-amp will you be using?  Is it gonna be a head or combo?

How would I connect several of these LM3886 amps together so they will all drive a single output?

National has a design note on a 4x configuration called the BPA-200.  There's good info in there.  It's not a 100% complete amp though... like no Zobel networks.  When things go wrong at these power levels it can get pretty nasty.  These things can eat speakers, so test it with a power resistor first.

There's this thread on another forum that you may find interesting.

It can save a lot of time by using a kit.  I googled up a site that uses the same LM4780 kit I'm going to try:  http://max8888.orcon.net.nz/LM4870.htm.

But stereo output would be nice too, so maybe I'll skip the bridging.  I'm now looking for a transformer.

How many of these amps could I build with my 5 amp supply?

R.G. suggests 3A for two, so you should have 6A for a 4x version.  You might be able to make do with 5A for 4 LM3886's, making it a bridged-parallel 200W mono amp.  You probably shouldn't attempt to drive less than an 8 ohm load with this supply.  If 2-4 ohm loads will be used, it would be better to run 3 or 4 chips in parallel.

Don't forget a big heatsink and fan for this monster.  If you don't put a fan in, it'll need a really big heatsink aligned for good airflow.

[ninoman123]

Yes. Ive seen those kits. They are available from www.chipamp.com. But I think Im just going to make my own using GGGs layout. The kits there are a little expensive for my tastes. I can get LM3886s here for 5 dollars and the rest of the components are nothing. I could probably get all the parts needed that they sell for $60 I could get for $20 or so. I will probably start with 2 LM3886's paralleled, and see if they meet my needs with a 4x12. If not I can always add another or whatever...I guess I'll just see how it goes lol.

Preamp wise. I had this 100 watt SWR amp. The power amp and preamp sections were seperated in the amp so Im just gonna use the preamp from that heh.

It'll be a head.

[R.G.]

I have a mildly heretical view of amps and speakers.

If you have one hulking monstrous amp and an array of speakers, then if the amp has any problem, you're not going to be playing music til it's fixed. Also carrying a 4x12 is a problem, at least to an old guy like me.

I am enamored of the idea of a modular speaker system. Each speaker is in a box designed for just that speaker by itself, and includes an amplifier suitable for driving it. If you want a little sound, you use one amp/speaker. If you want kilowatts, it's simpler to make ten 100W amplifiers each driving its own speaker than to make one 1KW amp driving an array. So you make a number of boxes and use as many as you need.

[ninoman123]

I actually thought about that R.G.  I figure each amp with its own 50vct 1A power supply would cost ~ $40. Each amp could drive 1x12. ~$40 or so for each 1x12 plus wood for a cabinet...$50 each cabinet.

So...$90? For a 50 or so watt 1x12 module. That means ~$300-$350 for a 200 watt 4x12 array.

If I made it into a single amp...$250 bucks for a 4x12 off ebay. $50 or $60 bucks for a power transformer big enough to handle 4 LM3886 amps.
Components for all the amps...$25 bucks...? So around $300-$350?

I found a 50vct 5 A transformer for $10 on ebay. Since I got such a monster for so cheap Im just going to build a monster power amp. I could still make individual boxes with 1x12 speakers though...maybe I'll do that...

Im 17...I have no problem with 4x12s.

So...I'll think about it...I'm really in no rush.