18V bipolar power supply for LM3886 amp

[El Heisenberg]

I wanna build this amp from GGG:

http://www.generalguitargadgets.com/pdf/ggg_lm3886_amp.pdf


But it needs an 18v power source. There's this one:

http://www.generalguitargadgets.com/pdf/ggg_bipolar_ps.pdf


But I don't know if 1A from each of the regulators will be enough to get the full power from the amp? Will this power supply do?

[El Heisenberg]

I'm having a tuff time finding the 10 ohm 2 watt resistor with 10-12 turns of #22 insulated wire. Anyone know how to find these on mouser??

[Nasse]

That supply is for low power stuff like pedals and opamp circuits. For such power amp use full wave rectifier

[PRR]

> tuff time finding the 10 ohm 2 watt resistor with 10-12 turns of #22 insulated wire.

For this same +/-18V amp? Power is small. You can use 1 Watt.

Mouser Part #:  294-10-RC  Manufacturer Part #:  294-10-RC  Carbon Film Resistors 10ohms 0.05  

Use #22 or #24 insulated wire. Strip a bit of one end. Solder to the resistor lead very close to the resistor body. Wrap it around the resistor like a coil (what it is). Try to get at least 8 turns (why you may need to go #24 on this smaller resistor). 10 turns is fine. Cut the wire with about 1/2" end, strip, solder to other resistor lead, trim.

If it is easier, wind 10 close turns of #22 on a pencil. Leave short 1/2" ends. Whack excess pencil. Strip wire ends and solder across resistor.

Resistor must be non-inductive. Carbon (comp or film) is fine. Metal-film may be OK but is not the best plan. Wire-wound may not be fine. In normal use the dissipation is negligible, but it must survive occasional bursts of hypersonic frenzy. A 2 Watt part is usually OK on larger amps. 1W should be fine for a 10W-15W amp.

[PRR]

POWER amps usually get UN-regulated power.

The difference between a regulator and a power amp is details. A good power amp (the '3886 is very good) can both amplify audio and reject power ripple. You do want to avoid over-volting the power amp, but the '3886 will stand over +/-40V (more than regulators!) so your +/-18V is not even close to trouble.

Transformer, rectifier, caps.

Use a 24V CT transformer, a FWB, and two caps. For 8 ohm load use 1A transformer and 2,200-4,700uFd 25V caps. For 4 ohm load use 2A transformer and 4,700-10,000uFd 25V caps.

[El Heisenberg]

This particular design says the amp is 68w.


Also, if I have a 24v CT transformer at 3A will that be fine?
 

[El Heisenberg]

Paul I was actually going to also use the 12AU7 preamp you whipped up for my last amp with the TDA2003 again on this amp. That amp was 10 watts. For this I was going to use another 12v 1A or 3A transformer in parallel. With 68w I don't mind having two transformers. I'll mount them on top along with the tube.

Still tho, you said a 2 watt part is OK for a larger amp. But 1 watt is ok for 10-15watts. but this is sposed to put out 68w

[El Heisenberg]

I'm curious why the resistor has to be wound with insulated wire

[El Heisenberg]

I drew this up real quick. This is what I'd be doing:







Will this work? I've never made anything grounded to a metal chassi. I've always worked with wood or plastic.

[El Heisenberg]

Oh PPR said to use a CT transformer. What do I do with the center tap?? And a full wave bridge rectifier?

[Tubebass]

Use a 48 volt 2 amp center tapped transformer and a 3 or 5 amp bridge rectifier. Ground the center tap. Then + and - of the bridge go to filter caps then to v+ and v- pins of your 3886.

[El Heisenberg]

Blah I just got ahold of a 25.2v CT 2A transformer!

[PRR]

> says the amp is 68w.

That's the show-off rating.

The actual power will depend on supply voltage and load. Ues Ohm's Law, account for peak-to-peak versus RMS, and expect loss.

The power supply you pointed to gives +18V and -18V (at not enough current for a Power Amp). Ideally you have 36V peak to peak; in fact probably 31V p-p. This is 11V RMS. 11V in 8 ohms is 15 Watts; in 4 ohms is 30 Watts.

The 3886 will work fine at this power level. While it is a fraction of what LM3886 "can do", there isn't a better/cheaper chip in this power range. 



> Blah I just got ahold of a 25.2v CT 2A transformer!

That's fine for the 15W-30W zone.

> Use a 48 volt 2 amp center tapped

Using Dave's suggestion you could have 60 Watts in 8 ohms. The 4-ohm rating could be over 100 Watts but you will need VERY big heatsinking for long-term high power. (The show-off rating is 68W for good reason: the chip can't handle the heat or current for higher powers.) Also 48V 2A would be very saggy pulling a 4 ohm load; this is probably a recipe for 8 ohm work.

> What do I do with the center tap?? And a full wave bridge rectifier?

Basic power supply for any tranny amp bigger than a Ruby. Study/plagiarize.

[El Heisenberg]

where the heck am I gunna get a 48v 2 A transformer??

[El Heisenberg]

i've been looking everywhere. I can't find a transformer like that

[R.G.]

The best target transformer for this chip is 42Vct to 44Vct. 48Vct is a little hot, and may kill it on AC power line surges. The reason this is fussy is that there is a hard stop on total DC voltage for the LM3886. I always try to not exceed the "Absolute Maximums" voltages on any part I use, and a 48ct tranny could exceed it occasionally. But you might get away with it. You could also stick more diodes in series with the rectifiers into the filter caps to wipe off a few volts too. That's cheaper than a new LM3886.

48Vct just happens to be a very common value. So does 24Vct. You can use TWO 24Vac transformers in series to get 48Vct. Same issues with voltage. Better would be two 20V or 21/22V transformers.

Where to get them? http://apexjr.com/miscellaneous.html#Toroids The Chia Yu toroid is just about perfect for a single channel, but not too good for two of them. the Avel-Lindberg one is on the hot side, but you could use them.

There are many other places to get 48Vct at an amp or two. ApexJr is just one I'm familiar with.

[petemoore]

Blah I just got ahold of a 25.2v CT 2A transformer!
 This should do it, probably the same thing I have here.
  If you have to, you can whip up a different PS for it anyway.
  Very loud like this too, of course 'loud' has more to do with speaker effeciency.
  It's a bit like shovelling dirt, the perfect shovel load is too heavy, takes too long to load up, is hard to carry, spills. Much better off with a medium shovel load, since you can repeat it neatly..the dirt will get moved just as fast or faster than trying to perfect the shovel load, then having to clean up because it keeps spilling.
  IE...gleaning every last drop of possible juice from the amp...I suppose it's possible, but why when a somewhat more conservative application can be set up much more easily, then simply duplicated.
  
  

[davidallancole]

My feeling when reading this is that your not up to snuff on your power supply knowledge.  It can be dangerous working with this kind of stuff.  I suggest doing some reading.  The following links have some info about designing power supplys for amplifiers:

http://sound.westhost.com/power-supplies.htm
http://sound.westhost.com/psu-wiring.htm

As far as safety, you'll have to do some looking around for that.

[El Heisenberg]

Chia Yu Co Ltd       
Input: 2 X 120Vac 50-60Hz 3" Dia X 1 1/2" Tall    
Output 42Vct 21V- 0 - 21V 80VA



Really, I should get this one?? Will 80VA work?

Cos i'm about to buy it....I wanna wait for reply but,....



wait nevermind. I can't buy from this guy. You have to mail him a check or something.

[head_spaz]

Antec
Dual 22V @ 200VA
AN-2222   
$29.00

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