Bridged LM386 with 3.2 Ohm 5W speaker?

[ozzu2000]

Hi!
I'm building a Little Gem MK2 (bridged 386), using 2 LM386-4 chips, and about 15 volt supply. (15v when running on an AC adapter, 12v when on batteries).
I'm planning on using a small heat sink for the 386 chips.
Everywhere I read it states a 4+ ohm speaker, but the one I have is a 5W 3.2ohm unit from a sony portable cd player.

What will happen? Burning amp, burning speaker, bad sound?

Thanks for your help

[J M Fahey]

With such high voltage (12/15V) I would use a non-bridged, single LM386 with an 8 ohm speaker.
Don't go beyond what the chip stands.
*If* you need high power, a single TDA2003 supplies 5/6W into 4 or 3.2 ohms; bridging them will give you 15 to 20W.
Really LM386 are very poor and inefficient amplifiers, but popular because of their gutless wonder "no parts" design, and functiononing relatively well with only 6V (and even 4.5V in a radio or Walkman), but if you have above 9V and can use 4/5 extra components, there are *much* better options.
And no, your bridged Ruby won't drive that 3.2 ohm speaker.

[ozzu2000]

My knowledge of electronics is somewhat limited.
I went for the 386 because I can have simple amp with distortion and it has schamatics readilly available at runoffgroove.

I tried to use the power amp that used to power that 5W 3.2ohm speaker, (LA4597 iirc) but I couldn't figure out a way of adding a volume pot between the ruby and the power amp...

[rowdy_riemer]

I built a Little Gem MK2. It's not bad. At 12V, it is plenty loud enough for a practice amp. Put a booster and tone control in front of it, and it sounds pretty cool. I suggest using a cheap 8 inch guitar speaker, like a Jensen Mod 8 or something comparable. Build it and see if it is loud enough for you. I can piss my wife off with mine. If you need more power than that, go with JM's suggestion. There is only so much you can expect from an LM386.

[ozzu2000]

I don't need much power... and for the power it has, the little gem mk2 is as simple as it gets
I just need to know what will happen if I connect the 3.2ohm speaker to the bridged 386.
On the Little Gem mk1 (single 386 and no preamp jfet) it seems to sound fine.

[rowdy_riemer]

If you have sockets for the lm386's and a nearby radioshack that keeps them in stock, just give it a try. If you fry them, you can fork out $3 and pop in some new ones.

[rowdy_riemer]

Check out my Little Gem MkII build: http://www.ssguitar.com/index.php?topic=1456.msg9232#msg9232

[ozzu2000]

I've found that little gem mk2 a while ago when searching through google

Well, I still have the power amp that used to drive that speaker (LA4597), thing is I have no idea on how to use the ruby as a preamp, or how to add a volume potenciometer between the ruby and the power amp
Can anyone enlighten me?

The "grace" stompbox on runoffgroove has a 100k potenciometer on the output, can this be used as a preamp instead off a stomp box?
Maybe I could connect it directly to the power amp I have?
Grace (lm386 based distortion):
http://www.runoffgroove.com/grace.html

[rowdy_riemer]

In many cases, a stompbox is a preamp. This is certainly the case with the "Grace" from ROG. There is also a Distortus Maximus schematic floating arround which uses an LM386.

[ozzu2000]

I've just found that when bridging amps, the amp will "see" a speaker with half the impedance. So my 3.2ohm speaker will become a 1.6ohm speaker to the bridged lm386, not good!
On the other hand, a parallel amp will see a speaker with the impedance doubled, so a paralleled 386 would see my 3.2ohm speaker as a 6.4ohm speaker.

Any ideas on how to to this?
Or the benefits of doing it?

[J M Fahey]

Just use a single, non-paralleled LM386.
If you want more power, use a LM383/TDA2002/TDA2003 ; which will happily drive 1.6 ohms with 10 *real* watts, very loud.
You can use the speaker you have and in the future add an extra similar one. Loud.

[rowdy_riemer]

Ditto what JM said. LM386 is great for not too loud (though I can still manage to piss my wife off with a ruby amp, especially with a 2 x 12 cab). If a single lm386 doesn't do it for you, go with a more suitable chip.

[ozzu2000]

Ditto what JM said. LM386 is great for not too loud (though I can still manage to piss my wife off with a ruby amp, especially with a 2 x 12 cab). If a single lm386 doesn't do it for you, go with a more suitable chip.

I like that description
Think I'll stick to the single ruby with 12+ volts eheh

I tried to connect one of my hi-fi celestion 100W 8ohm speakers to the ruby (still on breadboard) and it is much more louder than the other 5w 3.2 small speaker I have.
Though on that large speaker I seem to have lots of bass and little treble... 

[rowdy_riemer]

That might have more to do with that specific speaker than with speakers that size in general.  Also, like I said earlier, you should really get a speaker suitable for guitar. Something comparable to a Mod 8. Weber makes some really affordable speakers, too. Using a larger speaker with a ruby will probably make a bigger difference volume wise than using a more powerful amplifier with the small 5W speaker.

[bigbeck]

You'll be amazed at how good and loud the Ruby will sound when played through a high efficiency 12" guitar speaker, say 98-100db SPL. There is an extreme tone and volume difference between guitar and Hi-Fi, car,bass or PA speakers. Sure, they all work with guitar amps, but never sound right. Most important, especially with small amps, is the efficiency rating of the speaker.

Even a high end Hi-Fi speaker will sound bad with guitar as you have already found out.

Beware of using 8" speakers from small commercial guitar amps. Many of them sound terrible and are not even voiced for guitar(I have one of these in my Peavey Rage). Rowdy mentioned the Jensen 8" Mod speaker. That is decent sounding for a small speaker. A much better choice would be the Mod 12-70. It would be much louder and fuller sounding.  There are a lot of very nice guitar speakers in the 50.00 to 100.00 range. I think Eminence makes some of the best speakers in this lower price range.

Don