new here -- wanna make my first small amp

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zx3chris

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Joined 2002
#1
hello everyone -- im new to this forum and i am going to start to build my first amp soon.. currently i am going to school for EE and unfortunately there is alot of emphasis on book work and knowing all theory, but not much on hands on work and actually building things.. i like to put things together and see how they work, because theoretical models never include things such as noise, ratings etc.. anyway, i want to build sometihng small and cheap.. i was thinkign of using a lm387 power op amp chip for the basis of my amp (single channel) and rather than building a power supply (tho i was going to use a 25-0-25 transformer and bridgin rectifier), i found a nice power supply that gives off +/-48/1a for $6..

so, rather than just build a basic op amp amp, i was going to follow the design here
http://www.sound.westhost.com/p72-f1.gif
An externally hosted image should be here but it was not working when we last tested it.


is this good to follow or does anyone have anything else i shoujd try for my first project? im not going to excellent sq here, just an idea to get the physical feel of things. i am ordering everything from jameco, except for the 1watt resistors and heat sink (bc they dont carry it), so i think ill get them from digikey.. any help or input is appreciated -- thanks..

-chris
 
#2
If you already have the 50vct xformer I suggest you build an amp based on the LM3886... It will give you about 50watts in 8ohm. Its almost the same circuit as you show. You can even get free samples from National if you feel cheap. Im building it and it is very easy to do if you have the PCB. Very few parts also. To my hear it has kinda good quality sound for the small package.
Here's a link with a schematic and PCB but its not in english.

http://www.saunalahti.fi/elepal/vahvistin/vahvistin1.html

Now I have a question for the others DIYers, do we really need the input cap for DC coupling? I cant find them anywhere near me and I wonder what would happen if I didnt use it. Im going to use the amp with my SBLive.
 
#4
I might simply be confused but every time I see the inverted 38xx amp I read it as being an inverting amp (!) all the way into real life. Why is the speaker not connected reverse phase to maintain absolute phase? Doesn't anybody care of am I simply mistaking? Just like in the Zen series I would have expected the speaker to be connected in reverse phase.

/UrSv
 
#5
UrSv said:
I might simply be confused but every time I see the inverted 38xx amp I read it as being an inverting amp (!) all the way into real life. Why is the speaker not connected reverse phase to maintain absolute phase? Doesn't anybody care of am I simply mistaking? Just like in the Zen series I would have expected the speaker to be connected in reverse phase.
Click to expand...

Yes absolute phase is important, but the markings on the kit we are using does not tell us much about that. If you put the plus side of a battery on the plus terminal of a speaker it does not necessarily move out. What is the phase of the device plugged into the amp? And more importantly what is the abs phase of the source. It is often the case that the absolute phase will change from track-to-track.

So i see no point in being anal about how things are labeled. The final determination has to be done in the field on a track-to-track basis.

dave
 
#6
i appreciate all the help -- i am not dead set on exactly what i am going to build. i was going to use the +/-48vdc/1a power supply rather than build my own from a transformer for simplicity purposes (and cost) but i still am not sure. i am going to do some more research on this board and the other as posted -- i will post as soon as i start building -- thanks again.

-chris
 
#7
Hi Chris,

"i was going to use the +/-48vdc/1a power supply"

I think this voltage is too high for all of the IC power opamps that have been mentioned, make sure you check the datasheets of any chip you intend to buy.

See ya,
Tim.
 
#8
the data sheet for the lm387 says supply voltage of 20-94V.. +/-48 is 96 volts, but im hoping it should still be ok? or is this supply voltage something else.. i could buy regulators, but then it might just be cheaper to build a powersupply...
 
#9
Hello Chris,
the LM387 is a dual preamplifier. You obviously mix up some numbers.
If you mean the LM3875, which is a power-opamp, the maximum rating for supply voltage is 84V in total.
With +-48V you could use the TDA7293 for example.
Using your power supply for this amplifier you won´t get the full power cause it´s limited to 1A.
Kicking the amplifier a little and your supply would probably die.
What power supply is this? A switching power supply?

greetings
Jens
 
#12
Interesting, I was just looking at this thread and some neat questions came up.

About the coupling cap. I'd say it's smart to use the cap, that way you won't fry your sound card with DC (sound cards are very vulnerable to this) and it's safer in general.

Also, in that updated picture with the inverted opamp, the negative supply rail cap is backwards, that would cause a nice explosion eventually.
 
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