Wire Wrapped Resistor for LM3886 amp?

[smashinator]

I'm looking at the power amp project here: http://www.generalguitargadgets.com/pdf/ggg_lm3886_amp.pdf

What's this "10 ohm 2 watt resistor with 10-12 turns of #22 insulated wire"?

Is that something one can buy, or do I wrap a wire around the resistor and solder it at either end of the resistor?  

How do I do that, and why is it there?

Has anyone built this?  Is it a good building block for a guitar amp?

[Peter Snowberg]

That's a home-made inductor using a resistor as the coil form.

If you can find a 0.7uH 2amp inductor you could use that instead, but it will probably be easier to make your own.

My guess is that it was put there to reduce super high frequency transients, but that's just a guess.

[smashinator]

So would I ground the mesh at one end of the wire, and solder the wire to each end of the resistor?

***edit*** and thanks for the quick reply, by the way.

[smashinator]

Also, is an 18 volt Bipolar power supply something I can just buy?  I've searched here, and checked google, and haven't found the answer I'm looking for.   Maybe I'm searching for the wrong thing....

[RDV]

Doesn't that mean 18v AC?

RDV

[smashinator]

I dunno...  :oops:   The project file says 18 dc volt bipolar power

Would this be what I want?

http://www.circuitspecialists.com/prod.itml/icOid/1703

[RDV]

Like this?
http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&category%5Fname=CTLG%5F009%5F001%5F001%5F002&product%5Fid=273%2D1690

RDV

[Peter Snowberg]

It wants a 36 volt power supply with a tap in the middle which is connected to ground. The normal way of saying that is that it wants a +/-18VDC supply, or a dual-polarity 18V supply.

Here's one example using fixed voltage regulators, but the current may not be high enough. You can always just leave out the regulators though.
http://members.psyber.com/dibsed//CIRCATS/opamp-ps.pdf

[Nasse]

What's this "10 ohm 2 watt resistor with 10-12 turns of #22 insulated wire"?

Is that something one can buy, or do I wrap a wire around the resistor and solder it at either end of the resistor?

I wound 0,7 mm copper wire about 15 times around a suitable sized pencil used as former maybe less than ten millimeters diameter so it fits loosely (not much touching) around the 10 ohm 2 watts resistor. And yes it is meant to be soldered at both ends of the resistor.

Quess 2x18 ... 2x24 volts 2,2 amperes transformer is good for 3886 amp. I used toroidal center tapped one

[Aharon]

You need a power supply something like this

http://sound.westhost.com/project04.htm

If you are making two boards for stereo or something,if you want it for just one board only half of that will do.
I would use transformers rated at least 2A.
Aharon
PS:I have the boards made already waiting to populate,I'm making two power supplies to feed the two boards independently,it's sort f a powered monitor for my little studio that I'm making.I'm really excited about this project and I'd like to thank JD and Dean for it,may seem easy but it's prone to oscillations if not done right.

[RDV]

Like this for the transformer?
http://www.mouser.com/index.cfm?handler=displayproduct&lstdispproductid=416550&e_categoryid=24&e_pcodeid=54616I can't believe this sort of thing hasn't been covered here much.

RDV

[RDV]

How's this?


RDV

[Michael Allen]

I believe that after fullwave recification, the voltage from the transformer is multplied by 1.414. So to get 16v, you would need about an 11.3v or 12v transformer since that is easy to find. For a 35v supply you would use a 25v transformer as in the original project.

[R.G.]

What's this "10 ohm 2 watt resistor with 10-12 turns of #22 insulated wire"?

As Peter correctly notes, it is an inductor formed on the resistor.

It's there so that capacitive speaker loads and cables don't make the amp unstable and oscillate. The resistor damps the inductor's natural ringing frequency which could conceivably cause problems.

[smashinator]

Wow, where can I get a 36 volt, 2 amp transformer?  (well, one that will produce 2 amps at 18 volts...) A brief look through the hammonds on mouser's website didn't give me any joy.  Perhaps I'll have to actually go through the catalog when I've got a minute.

Do I actually need 2 amperes?  My plan is just to make a guitar amp, probably powering a 2x12 cab (this is VERY preliminary).

[spongebob]

Toroidal transformers are a good choice for such a project, I found one really cheap on ebay just recently. Try finding one with dual secondaries or a center-tapped secondary, and enough amperes!

A nice starting point for building chip amps is this page:
http://www.decdun.fsnet.co.uk/gaincloneindex.html

Here's a closeup of the TDA2050 amp I built a while ago, I soldered everything point-to-point, no PCB!

[smashinator]

spongebob - That site looks pretty cool.  Cool photo of your amp too!  

Anyone else have any input on where to get one of these transformers?

Also, anybody have any thoughts on whether or not this poweramp would be a good building block for a guitar amp?

[RDV]

Wow, where can I get a 36 volt, 2 amp transformer?  (well, one that will produce 2 amps at 18 volts...) A brief look through the hammonds on mouser's website didn't give me any joy.  Perhaps I'll have to actually go through the catalog when I've got a minute.

Do I actually need 2 amperes?  My plan is just to make a guitar amp, probably powering a 2x12 cab (this is VERY preliminary).

http://www.mouser.com/index.cfm?handler=displayproduct&lstdispproductid=416095&e_categoryid=24&e_pcodeid=54616

RDV

[R.G.]

OK, here's the poop. For an amp with an LM3886, you want about 42Vct to 45Vct. I copped four 44vct toroids on ebay for $7 each a ways back for exactly this. This lets the 3886 put out about 45-50W on the +/-32Vdc you get from that.

If you go over +/-42Vdc on the 3886, it vanishes in a puff of (metaphorical) smoke and never does anything again. They're very sensitive to power supplies being too big.

[smashinator]

RDV - I guess I missed that one!   :wink:

R.G. - That looks like a significantly higher voltage than what's on the schematic.  Would I have to change anything on the schematic to use that voltage, or should I regulate it down to 18 volts?    I guess I need to do some reading about transformers....