LM3886 amp

[Vincent Volta]

Hello, hello-
I ve seen the schem at GGG for this IC- based amp and read the datasheet provided. Seems easy and cool to build, but I have some questions:
1. Whats with the inductor at the speaker output? Can it be left out? Most often you dont see inductors at the output of amp circuits ( safe for the speaker ones    ) I want to know if I can just ignore that and leave the 10 ohm resistor alone, or will the amp explode or have a catastrophic failure if I do? Seriously, it just messes what is otherwise an easy build. Don even know what wire to turn because we measure centimeters not inches here.
2. Whats the "exact" power transformer spec for full output power into 8 ohm? I mean the voltage AND amperage, something like "28 + 28 v/ 2A" or whatever it is. Dont send me over to Radio Shack part number xx cause I live in other country! Thanx for your patience.

[Aharon]

For the inductor use 2mm coil wire.
For the PT I will build mine with an 18+18 3A transformer and see what happens but i think I remember somebody saying in another thread that 4A was more like it.
Hope it helps or at least somebody else will pipe up and correct me.
TAke care
Aharon

[Nasse]

About 0,7 uH, wound  1mm enamelled copper wire 13 times inside diameter 10 mm.  Slide the coil over that 10 ohm resistor at output

[R.G.]

Whats with the inductor at the speaker output?

It decouples the amplifier from capacitive loads at high frequencies. Capacitive loading can make feedback amplifiers oscillate, and this cures that.

Can it be left out?

Yes. But in doing so you take a chance of burning out the chip and/or the speaker under some conditions.

Most often you dont see inductors at the output of amp circuits ( safe for the speaker ones  )

Actually, almost every audio power amp of any significant power uses the damped inductor on the output.

I want to know if I can just ignore that and leave the 10 ohm resistor alone, or will the amp explode or have a catastrophic failure if I do?

Leaving the 10 ohm resistor in there will seriously reduce your power. If you leave the inductor out, leave the resistor out too. I recommend putting in both.

Seriously, it just messes what is otherwise an easy build.

Mother Nature is like that sometimes - she doesn't care how hard it is for you to obey Her laws. Just think how much easier it would be to make PCBs if we could make pencil lead conduct electricity like copper foil. I've asked Mother for that several times and She just ignores me.

Don even know what wire to turn because we measure centimeters not inches here.

We have that trouble a lot. We use calculators to defend ourselves from that problem. It turns out that one inch is 2.54cm or 25.4mm. Converting back and forth does require pushing the buttons though.

[R.G.]

oops. hit the button too soon.

For a power supply:
The chip dies a fiery death at 84.00000V between the V+ and V- pins, so we don't want to go there.

Power line surges of 12% are not uncommon. So we want to run most of the time at 88% of 84V or 73.92V or less. Call it 72V.

To get 72V dc, you have to use a fill wave bridge, and that causes a 1.4V loss (two diodes) so the peak AC feeding the bridge is 73.4V, and the RMS value of that peak voltage is 73.4/1.414 = 51.9Vac. You'll want + and - voltages, so you need no more than 52Vct, or 26+26vac secondaries. Anything from 42vct  to 52Vct is actually just fine because the available power drops very little and it's not particularly audible.

As to current rating: It makes sense for a music amplifier to use a transformer that can supply the full power continuously. This is not necessarily true for hifi loads. If we get the full 68W out and we are using a 70Vdc supply, then the DC power is 70/68 = 1.03A. The current in the transformer windings comes in high current pulses, so the RMS current is higher than the DC average. A lot of modelling and computation was done in the 40's and 50's to figure out how to estimate that, and the simple result is that you should use a factor of 1.6 to 1.8 depending on how constant the power draw is. Use 1.8.

That gives an AC current rating of 1.8*1.03 = 1.85A.

So you need 42vct to 52vct at 1.85A or more.

Don't go over 52V or you'll likely blow the chip from overvoltage.

[SirPoonga]

What gauge of wire should be used to go from the amp circuit to the speaker?  To 18v power?

[R.G.]

As big as you can reasonably get and comfortably bend into place. 18 gauge minimum.

[Vincent Volta]

Hello!
Well I guess I have a lot to dig in now, thanx R.G. and all others for the patience. Not exactly a newbie but there is a big step to take from building pedals to building amps...
Something else: I want to build a WHOLE guitar amp so there will be a preamp section with distortion, and this should be switchable, and there should be a tone stack too. I have like a dozen schems of different designs but they sort of contradict. Its not like fuzztones where everything starts with a Fuzz Face. So Im a bit confused. I have the idea that it must be a "clean" preamp section with a switchable distortion at the front and the tone stack should be at the end of all this... or should there be parallel channels for clean and dirty? R.G, I guess Im needing a "technology of the guitar amp" article here!    All this because Im paranoid about power loss if the preamp output doesnt reach a certain level ( 800 mV said in some circuits?). Well, any guideline will be just fine. Thanx a lot for your patience and help!

[SirPoonga]

I'm in the same boat.  I want to make a bass amp.  I was looking at the spec sheet for the lm3886.  
After reading this thread I am wondering that wattage it runs at when using 50V for a 8ohm speaker?  And how to make a 50V supply?
I think I have everything to make my combo amp set expect for what to use for a power amp.  I plan on using a 360+ preamp.  I think an Eminence Delta or Kappa 12" or 15" driver.  I found a 12" that only requires 2cu ft

[moving_electrons]

I'm in the same boat.  I want to make a bass amp.  I was looking at the spec sheet for the lm3886.  
After reading this thread I am wondering that wattage it runs at when using 50V for a 8ohm speaker?  And how to make a 50V supply?

I generally don't see folks push the 3886 past a +/- 35V supply.  Remember max specs assume you are doing the maximum job in gettting the heat away via the heatsink at optimum cool tempatures.

At +- 35V the 3886 can provide 56watts into 8 ohmz. If you drive it to clipping the protection will kick in and really ugly noises can result.

I see 3886 used for guitar amps but I guess I would be surprised if it had enough  oomph for bass.

Take a look at the LM4870 as it contains two 3886s in one package and corresponding greater output.  An example is here:

url]http://www.chipamp.com/lm4780.pdf[/url]

The actual kit on the site has power supply caps that are smallish 1500uF for a particular audio purpose.  But by using ofboard 10,000uF or higher caps you could have the current reserve needed for bass.  It is a pair of amps in the kit though.

Also note the alternate method to a inductor at the output.  There is a "zobel' of .1uf and a 2.7 Ohm 2w resistor to prevent the issues from cables etc. that were discussed earlier in the thread.  A zobel as the advantage that the components are not in the signal path.  The inductor is a very reliable way to go though.

As an alternative amp project look at project 27 at Elliot Sound products.  The boards are pretty inexpensive and the design is a guitar/bass amp.  if you buy the boards you get access to the build instructions for the preamp and amp boards.  Also there is a lot of info in the articles etc. on the site pertinent to constuction etc.  Power supply diagram is shown in project 27.

Some things to be aware of when going from making stomp boxes to making amps is
1) With aps you are now wiring and dealing with 120vAC in the chassis which can kill you, start a fire etc.
2) The rail voltages can kill you as well and don't drop off instantly when you unplug the power.  The capacitors are big and they can deliver a lot of amps after the power is turned off.
3) Capacitors can explode.  This is usully if they are reversed in the circuit.  This is true for low voltage caps as well, but bigger caps have more flying metal heading toward your eyes and face.  So if you have the chassis open on an amp you really need to have eye protection on.

http://sound.westhost.com/projects-5.htm

[Elektrojänis]

I don't have much to say about the poweramp part, but for preamp, how about one of the the www.runoffgroove.com amp simulators.

They need to be run on 9v (or somewhere near that) though, but a 7809 or 317 regulator could be probably used to make that from the poweramps positive suply... Check the voltage limitations on the regulators though... I'm not sure if they can handle over 30V.

If a real guitar (or bass) speaker is used then the output filter in ROG designs might not be needed (I would try it with and without to see what I like best).

[R.G.]

The LM3886 is a landmark chip. It's cheap ($5) and effective as long as you understand and live within its limitations. It can be elaborately series/paralleled up to big power levels. It is usually cheaper than just the output and driver transistors for an equivalent discrete amp.

And it opens an interesting avenue of approach. It's one thing to make a simple one-chip amp. It's quite another to make a fire-breathing monster amp capable of welding speakes together. The fire breather usually involves killing several hundred dollars of parts before it works right. A simple approach is what I call the unit amp.

50W is NOT a small amp. 50W will get you ear-burning speaker levels, and is only 12db down from a 200W Marshall Major. The MM is not twice as loud as a 50W. What if you had four 50W amps instead of one 200W?

The 3886 makes a 50W amp trivial. One or two 12" speakers is a good match to a 50W amp. How about mating the two? Build a box with one or two 12's in it, and permanently locate an LM3886 and power supply in the box. Then do it again, and again, and...

You see where this is going. You wind up with a collection of identical speaker boxes, each self powered, that you distribute preamp-output level to. Each box produces an honest 50W of output power, and you have no electrical difficulties with combining the outputs - they're combined acoustically. You have no difficulty with beaming of treble from a flat wall of 4x12's because you can orient the speakers to spread the sound. You can play a club with one of your 50's, and a hockey rink with six. Shoot, you could probably use a dozen and be heard in the next county, although you might need a permit from the EPA because the sound levels would kill birds.

[R.G.]

Oh, yeah, forgot.

It responds down to DC, so you can put out 50W of bass into whatever speaker you like - say, a 15 or an 18 pro audio woofer. And do that several times. You get plenty of bass, and all the cheap power you can pay for.

[bigjonny]

...Build a box with one or two 12's in it, and permanently locate an LM3886 and power supply in the box. Then do it again, and again, and...

Nice side effect of this suggestion: you can easily feed a stereo preamp signal to the different powered speakers!

[moving_electrons]

50W is NOT a small amp. 50W will get you ear-burning speaker levels, and is only 12db down from a 200W Marshall Major. The MM is not twice as loud as a 50W. What if you had four 50W amps instead of one 200W?

Focusing in the bass amplifier question though:

The power demands rise as the frequencies go down.

Factor one: The amount of power needed to produce each octave lower of apparent volume doubles as the frequency goes down.  So while 50 watts can be earsplitting at 1kHz it will not be at 41 Hhz.  Ever hear a 50w sub you thought was earsplitting? I would be cool if it could be though, we could have 50w subs that shake the floor and go below hearing range.

Factor two:
Bass speaker efficiency is necessarily lower than higher range drivers.  Each 3dB lower in efficiency requires doubling of the power.  if you go for higher efficiency drivers you generally loose low end range.  There is no free lunch.

I am not an "old hand" at bass amplifiers and will certainly defer to those who are.  But I have built a number of audio amps including LM3886 recently. (I am listening to one right now for frequencies above 90Hz â€" very nice!).  Lately I have been researching quitar/bass amps.   I have tended to see a recommendation that bass amps need to be 3-4 times the wattage of a guitar amp.  

You are right that paralleling LM3886s or bridging (but this limits the speakers you can use to 8Ohm-16Ohm) will give you more throughput.  That is why I mentioned the LM4870.  It has two LM3886 in a package, simplifying things.

I think it is also worth noting the need for "headroom".  LM3886 does not degrade gracefully when you hit max output.  the SPIKE protection kicks in and produces some awful effects.  So the practical range is somewhat less.  The output is also limited by your ability to get the heat away effectively though good mounting techniques on the heatsinks. It has to be near perfect with a big heatsink to get 50w.

That being said. if the applications is a practice amp for a bass in a small room, it is possible that one LM3886 could suffice.  But is you are going on the journey to create your own bass amp, why not go the bit extra to have it be more useful in other situations as well.

[R.G.]

The amount of power needed to produce each octave lower of apparent volume doubles as the frequency goes down.

This is true. The need for large amounts of power increases the need for the power that you *do* have to be cost effective. As nearly as I can tell, the $/W for an LM3886 is about the best going.

The nice thing about an incremental approach is that you can get as much power as you like - need 800W? Fine - that's 16 of the beasts, costing $80 for the amps themselves. 800W of DC is going to be about the same price whether one lump or several smaller lumps. You also distribute the power into more speakers. If you simply must put 200W in one speaker, then the series/parallel 3886's will do it. The 4780 is, as you note, just two 3886's in one package, and so it's purely a matter of dollars and availabilty to choose between them. The 4780 is in effect the same choice as two 3886s.

Bass speaker efficiency is necessarily lower than higher range drivers. Each 3dB lower in efficiency requires doubling of the power. if you go for higher efficiency drivers you generally loose low end range. There is no free lunch.

That is also true. However, you are once again up against physics. It is far easier to use several amps and drivers than it is to pump more and more electrical power into one speaker. It is also more fail safe - if one speaker dies, you lose much less than 100% of your audio.

There is no particular reason that getting power in 50W chunks is worse for guitar or pro-audio than getting it all in one single kilowatt chunk. They are equivalent, except for the ease of replication and redundancy.

I have tended to see a recommendation that bass amps need to be 3-4 times the wattage of a guitar amp.

Cool. Build a bunch.

I think it is also worth noting the need for "headroom". LM3886 does not degrade gracefully when you hit max output. the SPIKE protection kicks in and produces some awful effects. So the practical range is somewhat less. The output is also limited by your ability to get the heat away effectively though good mounting techniques on the heatsinks. It has to be near perfect with a big heatsink to get 50w.

Yes. The laws of physics do not change. So (a) don't let it clip; use a clipping limiter ahead of the power amp, and know the design of the speaker you put it with. That second factor lets you ensure some of the more esoteric modes of SPIKe protection never activate and (b) take note of the fact that it is a truth of physics that getting some defined amount of heat spread out to the environment is easier if it's in several places than if it's in one concentrated place.

So one 400W amp exhausts perhaps 200W of heat. To do that, you need high voltage, high current, high safe area power transistors and lots of them, as well as some handsome heat sinks. In fact, the need for multiple pairs of output transistors underscores that you can't get all the heat out of one pair. Eight 50W (or 13 30W) amps have already spread the waste heat out, so the delta-T each one experiences may be smaller than the temp rise on the single AB amp.

That being said. if the applications is a practice amp for a bass in a small room, it is possible that one LM3886 could suffice. But is you are going on the journey to create your own bass amp, why not go the bit extra to have it be more useful in other situations as well.

I completely agree - as I said, you would build a number of smaller amps plus speakers, each of which produces some unit of power. You only feed and power up as many as you need. Bedroom? Great, use one. Bar? OK, take four or six. Bigger hall? Lug all sixteen down there.  Even neater  - you only have to build as many as you'll really need. If you never play hockey rinks, you don't need to build 24 of them.

[SirPoonga]

Interesting conversation.  I am learning alot.

For now I just want a practice amp.  I'm new to bass.  But i really enjoy it.  And I like building stuff.   There is hardly a time that I don't have some sort of project going on.  I figured it would be fun to try and build my own amp.  

However, since I do have a bass it has been tough finding any info on building a bass amp.  Most website deal with guitar.  All DIY kits that I have found are for guitar.  

Could someone, even if it is just a block diagram, draw out what is going on here?  I work better with pictures.  I sorta understand what has been said.

Thanks alot.

Edit:  Since I am trying to figure out how to do this for bass I am looking at all possibilities.  There might be a better solution than the LM3886 or lm4780 that I don't know about either.

Looking at the pdf for the lm4780 where would I put a volume pot?  Before R4/5 or before the speaker output?  IE do I limit the input going into the system or the output going out?
http://www.chipamp.com/lm4780.pdf

I wonder how those sims on ROG sound with bass...  probably the flipster
http://www.runoffgroove.com/flipster.html

[moving_electrons]

R.G.  brings out some good points in favor of a modular approach with LM3886s. In particular"

1) ability to use one, a few or a bunch as needed

2) ability to start with one if it meets your needs to start and expand if your needs expand over time.

So there are some very good reasons to go this way as R.G has pointed out.  He is also on target when he points out that the chip amps are very cost effective vs discrete for the amp board itself because they are about the cost of discrete output transistors and need a lot less other components on the amp boards itself.

However the cost math works agains the modular approach as the number of modules increases.  This is because the amp board itself is such a small part of the overall amplifier and chassis cost.

Each module has to have it's own power cord, fuse, switch wiring, jacks, metal chassis, heatsinks etc. etc.  Each of these costs is replicated for each modual but often stay fixed for one bigger amp.

So cost likely will be cheaper for one right sized amp vs several in one box.

800W of DC is going to be about the same price whether one lump or several smaller lumps.

No.  One large capacity power supply will be much cheaper than 4 power supplies of 1/4 capacity.  Each of the 4 power supplies must have a rectifier, Power capacitors, and a toroid that cost nearly as much as a large capacity torroid. It is true that if you get up into a specialty/custom range things may look a bit different.  But that is not what we are discussion.

You also distribute the power into more speakers.

Yes but are 4 speakers of the same frequency range and sensitivity really less expensive than 1 more capable speaker.  (you might go to multiple speakers to drop the impedence thought)  Again it will be more costly, especially when you consider the speaker cabinet, covering, connectors, connection wire and jacks, handles etc.

It is far easier to use several amps and drivers than it is to pump more and more electrical power into one speaker. It is also more fail safe - if one speaker dies, you lose much less than 100% of your audio.

Yes , that is a very solid reason to consider the modular approach.

There is no particular reason that getting power in 50W chunks is worse for guitar or pro-audio than getting it all in one single kilowatt chunk. They are equivalent, except for the ease of replication and redundancy.

I think there are at least disadvantages to take into account:
1) the modular approach will cost more
2) the modular approach will take up more space (multiple speaker cabinets and the like)

[moving_electrons]

However, since I do have a bass it has been tough finding any info on building a bass amp.  Most website deal with guitar.  All DIY kits that I have found are for guitar.  

I posted one from Elliot sound products that has a PC board avaialble pretty cheap but is not a kit of parts.   It uses discrete parts and so is more complicated to implement, particularly for a first time amp build.  A chip amp implementation is by far the most straight forward.  Although straight forward would be a misnomer.  

Looking at the pdf for the lm4780 where would I put a volume pot?  Before R4/5 or before the speaker output?  IE do I limit the input going into the system or the output going out?
http://www.chipamp.com/lm4780.pdf

In general you always limit the signal going into a power amp and the power amp has a fixed gain.  This makes sense as the contolling circuitry therfore has to handle a lot lower power (perhaps 25 times less) than if you controlled the output.

In most audio now, you really are reducing the input signal via a pot.  CD players etc. put out plenty of signal that needs to be attenuated and rairly boosted given the gain of most power amps.

But the issue with guitars is that the signal from the pickup is relatively weak and needs to be boosted by a preamplifier before it is sufficient magnitude to get enough output out of a power amp.  So you need a preamplifier for guitars and bass.

[SirPoonga]

Would rac mounting the equipment help?  would you have to volume control each seperate part?  Could you make a master volume?