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Someone please help

Started by fumbi, June 18, 2006, 04:19:30 PM

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fumbi

I am an absolute noob at building amps. I am looking to make a 30-50 watt combo amp. I have no clue where to start. also would i be able to do this for less than 80 euros? Is there any tutorial  or somthing that will show me exactly how to make one ( i am kinda worried about blowing somthing up). I have a digitech rp 80 so i imagine i can use that as a preamp. I also live in austria if that effects anything. Any help is appriciated. thanx

el mo

hi,
unfortunately, 80 EUR will be a little difficult unless you already happen to have a suitable speaker. if you have no experience with electronics in general, you could get a poweramp-kit as www.conrad.at offers them.
they come with a good documentation brochure and reasonably good parts. as for the speakers, conrad used to carry cheap guitar speakers in the past too (really cheap)- i don't know if they still do. for the power supply, you could see if there's a
good "bastelladen" near your place where you might find a suitable transformer.
finally, the only really good advice i can give you is to have look at a book called "gitarrenverstärker-sound" by a man called helmut lemme, quite a lot of information but very useful.

fumbi

Hi thanks for the info. I have found an 18 watt amplifier cuircuit on conrad already built for only 15. now how much would it cost for a 8 inch speaker. I am not looking for top of the line but decent one. I can raise my budget because that seems needed. and are there any specs i should know about power supplies. I am kinda considering using 2 of these amplifiers connected to 2 speakers and using the stereo mode on my pedal. Kinda making a 2x8 combo. Would this work. Thanx

el mo

#3
QuoteI have found an 18 watt amplifier cuircuit on conrad already built for only 15
the black module? a decent speaker in the lower price range would be something like that: http://www.tubeampdoctor.com/index.php?cPath=37_39_65- if there's a good music store near your place, you could ask them whether they can order something that will suit your needs.
as far as the power supply is concerned: http://sound.westhost.com/psu-wiring.htm
good luck.

fumbi

Yes it is the black module. Would there happen to be ne decent speakers in conrad? thanks for the help.

el mo

hm, sorry couldn't find any right now is there a conrad store near you?

fumbi

Yes conrad is rather close to me and itwould be nice if i could find most everything there.

el mo

just have a look, they could have guitar speakers in the store- just to make sure: don't let anyone tell you that car-hifi speakers or anything will do the same job.

fumbi

I have one last question. Will a computer power supply work for an amp because i have a couple pretty good ones not being used. Thanks again for all the help.

teemuk

Not very well, I´m afraid. The positive supply (+12 V) can usually put out enough current but IMO the voltage is too low for anything moderately powerful. Using a dual supply of +12 and -12 V would be better but the current supplied to the negative polarity (-12V) is a lot smaller. See for yourself, it should be printed in the side of the supply. If it is one to few amperes you might get away with dual, there won´t be much power: 12V/4R=3A. 3A*12V=36W. This is theoretical maximum power to 4 ohms, I doubt that any amp will swing to the full Vcc and Vee though.  Bear in mind that some higher power chip amps such as LM3886 required at least +-18V supply, (if I remember correct), and most low power amps use at least +-15V supplies.

el mo

#10
with that particular little power amp  it might just about work. i've got one of them lying round here, but lost the data sheet , unfortunately. IIRC,  it's rated for voltages from +12V to +18V ( single ended. ). but it is indeed so
that the full 18W will not be reached with 12V.

teemuk

#11
Looks like it. But with a single 12V supply the theoretical peak power to 4 ohms is only 9 watts and I guess something around 6 watts is more closer to the truth (assuming the amp can swing to +-5 volts of max +-6V). This is only about 3W RMS!

fumbi

Somthing just came to mind. What exactly would stop me from using batteries. They are a lot cheaper than power supplies (i also have some rc ones) and i dont mind wating for charging. would this work. because i could u 3 7.2v 2300mah battery packs in series giving me 21.6v. THe conrad website rates the amp as 6v-20v. would ne of this work.thanx

Wynand

Using 3 batteries will make is difficult to get symetrical power rails, two on one side and one on the other, so one rail will still be 7.2V.

If you use an op-amp to derive a centre voltage, you're limited to the output capability of the op-amp.

Try and use a transformer, the PC power supplies are noisy and just complicate things.

Try to source an old broken stereo amplifier and take its transformer out. You can get a rough estimate of the transformer rating by looking at the fuses used, the service manual (If you can find one) and the amplifier it was used in.

for a schematic, look on www.sound.westhost.com. It's a site run by Rod Elliot and he lives in AUS.  He has a 100W guitar amplifier and if memory serves correct, he sells pcbs for the guitar amplifier. I might be wrong but I think its 22 AUD dollars.

If you want to keep things simple go for a LM3886 chip from national, or something similar.
I'm planning on doing this because i've got a LM3876 lying around.

The most expensive thing that I've found is the speaker. Unfortunately you can build the best amplifier in the world, but if the last step is rubbish, you won't get a good sound. Save a bit more and get a nice (reasonable) speaker. The rest is easy in comparason.

Good Luck.