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LM386 Bridge configuration

Started by n9voc, January 20, 2008, 12:52:32 AM

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n9voc

Good Day All!

In my harvesting of speakers from old television sets, I ran across this 32 ohm speaker, 4 1/2 inch in diameter.  Kept it around for a while, and was experimenting with using the LM386 in a bridged configuration.  I found out that the chips get prohibitively hot when running 12 volts and a speaker load of less 8 ohms.  I tried 16 ohms, and though the chips got "good and warm" under full operation, they were survivable.  With the 32 ohm speaker, they work GREAT.

I put this in a kind of small box, thus the title "The Mini".  I was still experimenting with values for the headphone amp portion, and have since determined that changing 10 kilohms to give the headphone amp a gain of 10 works better.  Holding it at 15 kilohm gives it a gain of about 7, which is tolerable, but not best.

R1 and R2 determine input impedance, set to 5 Megohm in the Mini2.  The buffered output provides a means of connecting this amplifier to another unit or a house system.  It's output is approximately 80% of the voltage level of the input to the amplifier.  I use this for guitar, as well as for general listening purposes.

I like the independent headphone assembly, that can be utilized as a secondary "line out" if necessary.

The power supply is a regulated "wall wart" design, similar to that found on the "Mini".

Anway - enjoy!

n9voc

A picture of the finished unit.  I used the grill from the TV speaker and a piece of masonite for the front panel.  The handle came from a hardware store (a drawer pull)  on/off switch is part of the IEC power connector on the back side.

noobiePT

hi, just a doubt, if i wanted to add another lm386 in parallel, what input should i use? the pin2 or 3?

teemuk

In bridged mode each amplifier sees half of the load impedance and the LM386 is rated for 8-ohm loads – now wonder they got hot. 16-ohms is about the minimum load impedance when you run the chips at their full rail voltage.

NoobiePT, you do realize that this is a bridge configuration and you can't just add one parallel LM386 – you need to add two: One identical parallel circuit for each side of the bridge. The gains of the parallel and bridged circuits must be closely matched or the amplifiers will load each other (this causes the chips to get very hot and even fail). Normally you need to put (low value) current sharing precision resistors to the outputs of each parallel amplifier, then bridge after the current sharing resistors. Since the chips have that gain set "loop" utilized the components in it must be closely matched as well.

If you ask me, all this is too much trouble with such a low power chip as this. (Better just upgrade to a chip amp circuit with higher power rating in the first place.)

n9voc, I like the looks of your amps... they're sort of "retro". Do you have "bigger" amps?

n9voc

Teemuk,

The only 'bigger' amp I have right now is the "Bug Eyed Monster" shown elsewhere in this forum.  I am working on a 30 watt amplifier, based on a toshiba chip I harvested out of an automotive radio unit.

I build each amplifier enclosure from solid lumber and coat with three coats of polyurethane.  Pine is usually what I use, but I have recently come into possession of a couple of cypress boards - the next amplifier enclosure!

:)

noobiePT

erhmm didnt know ehe
so what other chip would you advice? just need a bit more power, not alot more, the problem is cuz i got some free speakers but they basically suck! lol, im trying to build a battery amp.. so some chip that works on low voltages
thanks  :)

n9voc

 ;D
Noobie,

I would look at a single chip per speaker of the TDA2003 chip.  It operates quite well on 9-12 volts, and with a 4 ohm speaker at 9 volts will deliver about 2.5 watts into a 4 ohm load.

It is a bit high on current for the typical 9 volt transistor battery, but with 6 AA to D cells you'd be all right.  You also could look into using a sealed lead acid (my favorite for portable amps).

Half the power output for 8 ohm speakers!

Do a google search on TDA2003 for the data sheet, you can also look at the B.E.M schematic - it runs on 12Volts into 4 ohm speakers,giving me about 7 watts per speaker, maximum.  It is a real forgiving chip to work with, and the base circuitry is fairly simple.

good luck! :tu:

n9voc

 ;D
Teemu,
Agreed that this is a lot for a little power return, but this amplifier grew out of an experiment of "can I do this?" and is now used as a sound amplifier for a scanner in my bedroom- but has been used in travel (due to small size) for a guitar amplifier in a hotel room!

I shared the schematic not to advise people to use this, but as a general learning experience that it CAN be done, given the right load - and it worked well for the 32 ohm speaker I had kicking around in the junkbox!
;D

teemuk

The bridge is a pretty nice configuration and what I consider the greatest merit of these bridged LM386 designs is that they provide moderate amount of power without the need of heatsinking stuff. (Although this thread pointed out the simple fact that more power also means more heat and you can't push that circuit too far). I also apologize for not being clear enough, which obviously caused some confusion: My (sort of) off-putting comments were mostly about the bridge-parallel variation that I described. Some issues I brought up in my post concern the bridged circuit as well (e.g. gain matching) but these issues become way more serious when the parallel amplifiers come to picture. I guess what I'm trying to say is that after bridging I wouldn't bother to try any other tricks with that chip.

Since I'm always a sucker for pointing out things that can be optimised I must mention that the two Zobel networks in the output can be substituted with a single one that is connected in parallel with the speaker, from one bridge amp output into the other. Also, inputs of LM386 have 50K resistors to ground so you don't need an external resistor to create this reference (as is the case with most other chips). In fact, you might even gain improvement from omitting that resistor since it's in parallel with the input resistors that are also in parallel. The total impedance at that node is about 20K (not 100K) and this may begin to interfere the proper operation of the volume potentiometer. If the pot doesn't seem very "precise" then this might be the problem.

noobiePT

the site i normally order stuff doesnt have that chip... :( (i normally use musikding) it hasnt got much variety.. do you know any european site that has it??

n9voc

 8)
Teemu,
Thanks for the comments on making the design more efficient, if I build another one, I'll incorporate you suggestions - anything that simplifies building without sacarificing quality is stupendous :tu:

Noobie,
Sorry, being right in the center of the U.S., I don't deal with the EU.  The version of LM386 I get from Mouser is made by New Japan Radio, if that helps any!

:)

noobiePT

didnt help, but thanks anyway
have to do a search, see if i find some store