2W push-pull BJT power amp?

[Alpha579]

Hi all,
I found this wee sharp 10cm speaker in an old TV, and i want to make a portable two watt amp for busking . Im fine with designing a preamp, but ive never had any expierience with power amps. I thought i might use a push pull bjt stage, but i have no idea how to do this. Also, i want to run it off a 9v batt if poss, or if i have to 18v. The speaker is 18 ohm, and i want to use BD139 bjts because i can get them easily at my local elec store. Any schematics or ideas? Or info on push pull stages?
Thanks for all input,
Alex

[Tim Escobedo]

It's probably be a lot easier to just use a IC amp chip. A TDA2822M or JRC2073 can both be had for under a buck, are easy on batteries, easy to use, and will power up to 2W in bridged mode.

[Alpha579]

got any schematics? I acn easily get the LM380 and LM386 ic's, would they be alright?

[spongebob]

You will hardly get more than a few hundred milliwatt at 9V into a 18 ohm speaker, you can calculate this the following way:

Power = Vrms ^2 / Rload

where Vrms = 9V / 2*sqrt(2) = 3,2V

so your maximum RMS power output will be 3,2^2 / 18 = 570 mW

A LM386 based design might really be the most convenient approach in this case...

I finished building a small mini amp myself two days ago, it's the Ruby from runoffgroove powered by a 12V rechargeable lead battery in a small (20x20cm) wooden box, the speaker is a cheap generic 8 ohm speaker. It's amazing how loud this little thing can be! :twisted:

[Alpha579]

so if i used 18v, i could get 1 watt out of it?

[Peter Snowberg]

Remember that loudness is measured on a logrithmic scale. 1W is not much louder than 500mW.

100 Watts is only twice as loud as 10 Watts which is only twice as loud as 1 Watt.

Just build a 386 amp and you'll be as happy as you can be with that speaker.

I gave up a long time ago on using small speakers like that because they're VERY inefficient and tinny sounding (no bass to speak of). If you want more volume, break down and get an 8 inch speaker or even better a 10 inch. That will do you much better. The sound quality from little speakers is just ugly. If you put that little speaker into a larger cabinet it will help, but it won't ever be really good.

Sorry to rain on the parade, but that's reality for you.

Take care,
-Peter

[Alpha579]

Lol, thanx peter, ur absolutely right, but ive got the speaker lying around, so ill just try it anyway.

[Peter Snowberg]

When I was in High School, I made a 386 amp in the case that my last phonograph stylus came in. It was just about the size of a ring box and just barely big enough to fit a 9V, a 1/4" input jack, a 1/8" output jack, and the 386 amp with NO room to spare. I used the speaker that came with an old speech synthesizer for output. It was about the same size you're talking about but it was in a little plastic box which made it sort of a "closed back cabinet" LOL. It sounded OK for jamming at school or playing amplified as I walked down the street with the amp in my pocket :twisted:, but one day I hooked it up to the 16 ohm cheapo 10 inch Celestion that came from a little Marshall 2505 practice amp (I blew out the Marshall circuit one day after the input lead touched 120VAC :shock:), and WOW!! It sure sounded good!

I would suggest building a Ruby from Runoffgroove and using a 1/8" jack for the speaker connection. Make up a pigtail with a 1/8" plug on one end and alligator clips on the other, and try every speaker you can! You'll find an amazing amount of variation. When you want to settle on a guitar speaker.... check out the signature series from http://www.webervst.com and you'll get some really nice tones.

Take care and happy experimenting!

-Peter

[Ansil]

i finished mark hammers or it might have been peters suggestion of bi amping a small little combo amp


got a plethra of little micro speakers..  for the highs and mids..  and for the lows i got a 4" subwoofer

put it all in a a little box i got from the chiaropractor.  

wicked evil sound

i got an extra sub speaker from when i modified a buddys computer hometheater system, he upgraded the old one and gave it to me, i modded it with a new preamp and adjustable crossover so when we listen to music in his jeep we aren't bombarded with the sevenstring pumping soley thru the sub only.