LM3886 Amplifier advice needed

[solderburn]

I'm thinking about taking on a new build, This;http://www.generalguitargadgets.com/pdf/ggg_lm3886_amp.pdf

Now, I have a few questions,
1. In the parts list it has a 10 Ohm 2w resistor with 10 to 12 turns of 22ga insulated wire, after reviewing the schematic I noticed the 0.7 UH inductor. Is this the wire wrapping? couldn't i buy a 0.7UH inductor and not use the wire? This has me very confused as I don't have any experience with a resistors like this.
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2. I like to buy parts from one place, in this case Tayda, and they don't sell 10 Ohm 2w or higher, can i use 2 20 Ohm 1W resistors in parallel, dividing the resistance to 10 Ohms, and boosting the wattage rating to 2 watts. Is this possible? After doing some searching it seems possible, but confirmation wold be nice.

3. It lists a 10k trimmer pot for a volume control. Can I use a regular Linear/Audio Taper pot? Or is the trimmer critical in some way?

And finally can I use a 9v power supply with a 555 timer charge pump? Like this one http://www.555-timer-circuits.com/voltage-doubler.html

Thanks in advance.

[R.G.]

1. In the parts list it has a 10 Ohm 2w resistor with 10 to 12 turns of 22ga insulated wire, after reviewing the schematic I noticed the 0.7 UH inductor. Is this the wire wrapping? couldn't i buy a 0.7UH inductor and not use the wire? This has me very confused as I don't have any experience with a resistors like this.

Yes, the inductor is the wire wrapped around the resistor body. No, a commercial 0.7uH inductor is not practical, unless you buy an air-core inductor. The conditions under which this "inductor" is used are very special. The most effective way to build it is with wire wrapped around a resistor, which is why the project states it that way.

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2. I like to buy parts from one place, in this case Tayda, and they don't sell 10 Ohm 2w or higher, can i use 2 20 Ohm 1W resistors in parallel, dividing the resistance to 10 Ohms, and boosting the wattage rating to 2 watts. Is this possible? After doing some searching it seems possible, but confirmation wold be nice.

Yes, you can. Wrap the wire for the inductor around both resistor bodies.

The point of that inductance paralleled with a resistor is to isolate the amplifier from capacitive loads at high frequencies. The point of the resistor is to damp the resonance of such an inductor/capactive load if and when it happens.

You can also make a suitable inductor by wrapping 12-20 turns of house-wiring wire, #14 "Romex" with one conductor pulled out to use, on a broom handle or some such, and parallel this separate inductor with a power resistor.  The inductor carries heavy currents, and cannot be iron or ferrite cored, and must have a resistor to damp resonances from any capacitive loads.

3. It lists a 10k trimmer pot for a volume control. Can I use a regular Linear/Audio Taper pot? Or is the trimmer critical in some way?

The trimmer is not critical. A normal 10K pot will work OK.

And finally can I use a 9v power supply with a 555 timer charge pump? Like this one http://www.555-timer-circuits.com/voltage-doubler.html

Yes, you can use one. No, it will not work well. A power amplifier does not create power, it only controls the release of power supplied to it.

A power amp is best viewed as a wart on a power supply that lets the power out in a highly specialized way. If the power supply cannot make the power and supply it to the amp, the amp cannot let it through to the speaker. A 9V power supply with a charge pump has no hope of making all the power such an amp needs. I'd have to look at the LM3886 to see if +/-9V is even enough for it to start, but that's another issue.

Some advice: Design the power supply and heat sinking FIRST. If you don't have enough DC power, and cannot remove the waste heat, no power amplifier can work. The LM3886 will go into thermal shutdown if not properly attached to a big enough heat sink, and will not work at all unless there is enough voltage and current to power the speaker. It needs about 40% more power than the maximum it puts out to the speaker. Don't even bother buying the PCB, chip, and parts until you also buy a suitable power transformer ( 24Vct to 44Vct at 75 to 200VA is about right for one of these) large diode rectifiers, and large filter capacitors, perhaps 4700 to 10,000 uF at 35V or more; also a heat sink able to get rid of 10-40W of waste heat without sending the LM3886 into thermal shutdown.

Other people on this and other forums have found these items out the hard way.

[PRR]

R.G. typed faster, and hit many of these points. Nevertheless I will toss this out:

_________________________________
This is a POWER amplifier.

You can't fool around and get results.

We get-away with a lot of "silly" things in small-signal work. In POWER work we have to do it solid.

The LM3886 is not even specced to turn-on at 18V total. (No "doubler" will reach 18V from a 9V supply, and the 555 thingie won't come close to the power needed.)

That GGG sheet is pretty incomplete. You need a heat-sink. You need a *real* power supply with appropriate voltages, polarities, and current.

The trimmer is a frill.

This won't take guitar directly. In most cases you will have some kind of preamp in front, which will often have a volume control. Then the power amp's trimmer would be left full-up or just omitted.

The choke must carry HIGH current. You could maybe source one. But the dozen turns of wire does the job, well, cheap. And this little chore is VERY traditional in power-amp construction.

If you are going to "build a power amp", you start by picking a power goal. This pretty-much determines your HeatSink and Power Transformer Size (and cost). Then pick your Load Impedance. Now you can compute your supply voltages. Then look for a suitable amp plan (or chip).

Conversely if you are stuck with a specific chip you figure what range of power, impedance, voltage it is suitable for, and work backward to HS and PT size/costs. The LM3886 may shut-down with less than +/-10V. At full load! This will probably be +/-14V at idle. Power output may be like 2 Watts in 8 Ohms.

Working with power transformers tends to lead to working with Wall Voltage power. Which is dangerous. (I won't post the graphic I just found.)

I advise you to extend your search. Taylor's "Tiny Giant" gives an honest 20 Watts, has a PCB layout you can buy or copy, and uses a "safe" LOW-cost pre-made power supply.

If you are truly in-love with LM3886, look on eBay. People living near the factories can kit-up all the right parts and ship them to you cheap. As with any eBay deal, beware. Especially on "kits" because buyers usually give Feedback on delivery, before they build the kit. Also be aware that you need several things, and the '3886+PCB is usually offered separate from Power Transformer and HeatSink (since you can build 15W-60W on the same PCB with different PT and HS).

[Brymus]

You might heed Paul's advice about building something that doesn't require messing with building a power supply.
Or like he said get a kit matched for a specific voltage and amperage transformer.
If your set on building the 3886 amp try reading these pages,very useful.
http://www.amp-fix.com/watts_it_all_about_alfie.htm
Explains the basics needed to start calculating which transformer you need for your amp.

This site is very useful for building a 3886 (Gainclone) amp,read all the info there BEFORE you start,its aimed mainly at HiFi use but
is one of the best GC amp pages I have found.
http://www.decdun.me.uk/gainclone_3886.html

More useful info from its home page.
http://www.decdun.me.uk/gaincloneindex.html

Great place to get parts on the cheap for Gainclones.(3886amps)
http://www.apexjr.com/

The 160VA one would be ideal for a GC amp
http://www.apexjr.com/miscellaneous.html#Toroids

[solderburn]

Thanks for the replys!

I didn't know much (If anything) about Bipolar power supplys. So, i figured that a wallwart would work. Hence my question about a charge pump. I'm still learning as you can tell.

Could anyone recommend a power supply design? I would prefer a traditional chassis/foot mount transformer.

If you are going to "build a power amp", you start by picking a power goal.

In the 50w+ range. Just something that could be heard over a drummer if I ever join a band. Which doesn't seem likely since I live in a rural area. So i really don't have a specific goal, gust something with a bit of headroom at louder volumes.

If you have a different circuit that might be better feel free to post. Preferably simpler but good sounding to keep parts cost down.

I'm about to start a guitar build, afterwards I'm still considering this project, but I operate on an tight budget so after realizing the power supply parts I may have to hold off a while. But on the bright side hay season will be here soon and hopefully Ill get hired to load wagons!

[therecordingart]

If you want to save yourself a lot of time and headache:

http://www.jlmaudio.com/shop/index.php?_a=viewProd&productId=18

and

http://www.jlmaudio.com/forum/viewtopic.php?f=2&t=21


A very well built LM3886 kit for $50. Joe @ JLM is a great guy. I bought a bunch of his "BA" pres a few years back.

[R.G.]

Once again, ignore the power amplifier circuit until you have figured out the power supply.

Let me add to that - ignore the amplifier circuit until you have figured out the power supply, heat sink, (probably) wooden enclosure, metal chassis, and speaker.

These will be vastly more expensive and complicated to get running right than the amplifier circuit. I speak from experience, as my earliest electronics work was to do what you're trying to do, build myself a guitar amplifier. I eventually did that, but as I say, everything other than the amplifier circuit was what took the time and money.

Here's a usable setup for the power supply:
Power transformer: http://www.antekinc.com/as-1220-100va-20v-transformer/  $23 plus shipping, and an incredible deal at that. If you're in the USA, you wire this up by paralleling the primaries and placing the secondaries in series, getting the polarity of the windings correct or it will go up in smoke. The secondaries then go to the:

Bridge rectifier: 200V or more, 4A or more bridge rectifier module, many sources, about $2-3

Filter capacitors: 4700uF/35V or more, two of them, about $2 each, preferably snap-in PCB style or chassis mount screw terminals with clamps. The screw terminal types may be MUCH more expensive.

Heat sink: http://www.ebay.com/itm/150x60x25mm-High-Quality-Aluminum-Heat-Sink-Cooling-for-LED-Power-Memory-Chip-IC-/360744193465?pt=LH_DefaultDomain_0&hash=item53fe078db9 is a great place to start, and a good deal.

The whole issue of speakers, boxes and enclosures is big. Good speakers are $50 and up each. Sometimes a lot up. Making a speaker enclosure is one thing, making them look good is another. Same comments if you make a head for a pre-existing speaker cab.

[tca]

I think that R.G. and PRR said almost everything that is important. But here goes my 2 cents.

I like building amplifiers, much more than building stompboxes, it is much harder and expensive than building 9V devices, but you can have much more fun!

Get a laptop power source, it can do 18V up to 6A, that will give you lot of safe power with very few risks, i.e., you don't get electrocuted and die in the process. And there are a few that even have an electric fuse, if you accidentally short circuit the output (it will happen), it will not start a fire!

My next advice is, get a few 10W power resistors (from 10Ohm to 100Ohm), a bunch of mosfets (IRF610, or any other), a big heatsink and play around with them and build a class A amp. You will not get much power out of it (few watts and a lot of heat) but you learn a lot on the way. And they sound good!

Cheers.

[R.G.]

Yep, building amps is fun. I still do it, all these decades later.

One thing I forgot to mention to the original poster. Often, you can buy a no-name entire guitar amplifier that's broken in some way, and do a "soul transplant" on it, gutting it and using the enclosure, chassis, speaker, and maybe the power supply and part of the wiring. Craigslist is a good place to look for things like this. A few years back I bought a "Rogue" 100W combo for $40 from craigslist. "Rogue" is Guitar Center's house brand of entry amplifiers.

It was broken, and the owner was eager. The power amp was dead, but then I didn't want either the power amp or preamp. I put in a clone of the preamp circuit from a UK Vox "Supreme" and a couple of ... LM3886 power amp chips on the pre-existing heat sink.

It still looks like a no-name amplifier from Guitar center, but the sound is rather better.

There are options. My own preference is to buy the difficult stuff (enclosure, speaker, chassis, power, etc.) preassembled in some factory somewhere, and then add the much smaller "intelligence" parts, circuits that make it sound the way I like.

[PRR]

> well built LM3886 kit for $50

Yes, an excellent product. But--

No heat-sink.

No power-transformer.

None of the mechanicals (like enclosure to keep your fingers out of the Wall-Voltage and the not-safe DC voltage; also keep your coins and keys from falling in and shorting.)

If you want a "car", an engine is a start but not a whole-deal.

Also-- unless you are a true nerd, the Power Amp is not a high-creativity project. It's like an engine.... basically you use a Chevy SBC or a Honda Accord motor, tack-on chrome pipes, and done. Your custom-car creativity usually goes in the "user interface"-- titanium shift-knob, wide-screen audio/video, eye-popping paint job. In stage audio, all that "creative" stuff mostly ends up as "pedals".

Honestly, you can't build a Power Amp from parts as cheap as those clever/motivated Asians can assemble and ship a whole amp to Guitar World. That's been true for many years so there is a sizable pre-abused market on CraigsList etc.

The LM3886 *does* make a little sense if you glom a "dead" 40W-70W power amp with *good* AC/DC power voltages, and heat-sinks and etc.

[solderburn]

Could anyone recommend a schematic for the power supply filtering what works with the Transformer that RG recommended? I'm completely confused after searching and coming up with hundreds of different layouts as i don't have much experience with power supplys.

RG also mentioned a bridge rectifier module, will this work? If not could you suggest one? Even tough I would rather use traditional Diodes.
http://www.taydaelectronics.com/diodes/bridge-rectifier/single-phase-bridge-rectifier-mic-35a-1000v.html



If anyone is wondering about the enclosure, I have a Fender FM 212 DSP amp, that was fried from a broken guitar string landing on a prong not fully plugged in to the wall. needless to say the amp didn't like the wall voltage and blew up. So, I have re wired it to act a speaker cab. I don't think all of the parts are done for considering the it turned on afterwards, hummed and lit up, but no sound came out. I'd like to test the pwr transformer but I don't know how. So the current guts will be ripped out and a new amp put in, Hopefully the power amp that where discussing and a preamp that I have yet to design.

Thanks again, This is really helping me learn a lot.

[R.G.]

You're scaring me Dan. If you don't have much experience with power supplies, you're taking a big chance on doing something wrong and electrocuting yourself or someone else. It's not practical to learn safe wiring of AC power on the internet. You need someone teaching you in person to catch your mistakes before others get hurt.

The connections are similar to this:

excepting that for that transformer, you connect the primaries red to red, black to black, and the 120V power connects across red to black - that is, the two red and black windings are in parallel. You connect a green to a blue, that's the "0V" center tap in the above diagram, and the remaining unconnected green and blue are the "25V" leads in the diagram.

Yes, that rectifier bridge will work fine. It's easier to use these than traditional Diodes, as all you have to do to mount them is to drill one hole and screw them down. Then the two transformer leads go to the "squiggle"  or "AC" marked terminals on the bridge rectifier, + DC comes out of the "+" marked terminal to the positive filter cap(s) and - DC comes out of the "-" marked terminal to the negative lead of the filter cap(s). It's really handier than mounting diodes on a board IMHO.

But back at safety: you're messing with AC wall power, and if you do the wrong stuff, you can get electrocuted, or/and start a fire, or at the least burn up some expensive parts. Don't get into this unless you're sure you can do the power wiring correctly and safely.

[PRR]

> fried from a broken guitar string landing on a prong not fully plugged in to the wall.

A short on the wall-plug really should not hurt the amp. (Worst case could start a fire inside the wall. With guitar strings, more likely the string goes white-hot and tries to set fire to wood paneling or carpet underneath.)

If they done something strange, you might have popped an internal fuse.

But you say it lit up.

There is something more to this story that you are not telling us.

And if you haven't diagnosed this recently-working amp already, you may not be ready to assemble and debug a new build.

[PRR]

> Fender FM 212 DSP amp

IIRC, the problem on those is two large power-drop resistors, which are not large enough, and either fry themselves or cook their solder joints. Once you know the problem you can fix them two per hour.

[solderburn]

I will defiantly asses my ability before deciding on weather to do this project or not. If I do decide to build it I'll post detailed pictures of everything before I power it up. Thanks again everybody!!

A short on the wall-plug really should not hurt the amp. (Worst case could start a fire inside the wall. With guitar strings, more likely the string goes white-hot and tries to set fire to wood paneling or carpet underneath.)

If they done something strange, you might have popped an internal fuse.

But you say it lit up.

There is something more to this story that you are not telling us.

A few details I might add;
The string was touching for a while, 5 seconds give or take, and it did get white hot, because I was alerted by my sibling that my guitar was on fire, which was not the case, So I the reacted by grabbing the neck in doing so electrocuting myself and dispersing sparks from the outlet and started the carpet smoking. I immediately trend the power off and and put out the embers.

My guitar was plugged in to my pedalboard (running off a 9v adapter) No pedals where hurt surprisingly.

After I was shocked just before I unplugged everything, I heard subtle crackling sounds and then a loud bang.

I soon collected my thoughts and plugged it back in. The power, green channel led on the faceplate and the green led on the footswitch came on. pressing the footswitch and faceplate channel button has no effect.

The fuse was not blown.

I figured it blew up most of the preamp and digital effects unit. so i assumed that it would cost WAY more to fix than to replace. I also could not find a schematic for the FM 212 DSP.

If you think its worth hooking back up and testing I'll do it. only problem I see is that when I converted it to a speaker cab I took the pcb for the footswitch,pwr in and pwr out and de-soldered the jacks. I'll just have to re-solder them back in and re-solder a spade back onto the main pcb that i accidentally pulled out.

Hopefully makes sense as I am very tired right now.

[bluebunny]

. . . it did get white hot . . . my guitar was on fire . . . electrocuting myself . . . dispersing sparks . . . started the carpet smoking . . . put out the embers . . . I was shocked . . . I heard subtle crackling sounds and then a loud bang . . .

  Now you're scaring all of us!    Please be careful with this stuff.  Paul and R.G. have quite rightly pointed out that this stuff can KILL YOU (or others - you decide which is worse...).  If you have the slightest concerns about your ability or understanding, then please keep well away.  We all want to see you back here, not on the six o'clock news...

I soon collected my thoughts

I think you're lucky to have any left...       Seriously, stay safe, Dan.

[solderburn]

I know this stuff is deadly. the scary part of that story is it could happen to anybody.

As i stated before, If I do this I WILL post pictures to this thread, Displaying the wiring before I power it up.

And I do have experience wiring pedals (which doesn't have any relevance to high voltage) household lights and receptacles. and actually a main breaker on our house. which was scary, because it hooked up directly to the meter base. Hillbilly electrical companies really let you do alot.

I just didn't have any idea about bipolar power supplys. that's where all my questions came from.

Thanks guys, I can't imagine how many lives, time and money you all have saved people over the years.

[PRR]

> hooked up directly to the meter

Was that 240V 3-wire or is it still 120V 2-wire in your sticks?

> any idea about bipolar power

US 240V power is sorta-like bipolar DC supplies. (But don't extend this thought tooo far.)

[solderburn]

> hooked up directly to the meter
Was that 240V 3-wire or is it still 120V 2-wire in your sticks?

120V I think 120A.

Tca got me thinking about computer power supplys, So, I made a pilgrimage to ebay and found these, similar to that he discribed;http://www.ebay.com/itm/24V-5A-120W-Watt-AC-DC-Power-Adapter-for-adapter-connector-2-1-2-5-Charger-PSU-/350865927526?pt=LH_DefaultDomain_0&hash=item51b13d2d66
24V 5A 120W
And this,http://www.ebay.com/itm/20V-6A-AC-Adapter-for-LI-SHIN-0227A20120-Laptop-Power-Supply-Cord-Charger-/261434502508?pt=Laptop_Adapters_Chargers&hash=item3cdeb5ed6c
20V 6A 120W

Will any of these work? I don't know that the current requirement is.
I'm not going to use a transformer. To dangerous for my current skill level. So using a computer power supply will get me into familiar territory and extend my life expectancy.

[R.G.]

Wise choice. Get two of those power supplies, and use one for +V and one for -V.

The LM3886 will barely be turned on by a total of 24V, and will put out only a few watts. Hooking two of them in series for +/-24V or +/-20V gets you into the 40+ watts range.

Note that this will still give you some problems with AC wiring, as you'll have to provide 120VAC to two power supplies, but it's a simpler problem that wiring a mains transformer.