Is it possible to make a mini amp using 741 IC Op amps

[peps1]

Is it possible to make a mini amp using, and if so......anyone got a layout? 

[dschwartz]

hmm i´m not sure, but i think you can build a headphone amp..

[davidallancole]

If you check out this page, there is a schematic of an amp you can use.

http://sound.westhost.com/project113.htm

I am running this right now with the op-amp being a LM741 without the 4.7 emitter resistors.  The transistors I am using are 2N3904/6.  I am running it off of 9V with very little power and it works awesome. 

[Brymus]

David that is cool.
What speaker are you using with it?
It looks like about 2 watts into 16 ohms.
So it should be as loud as Little GemMKII  (little louder) but much less distorted at louder volumes.

[davidallancole]

Hi Brymus,

I am using an 8 ohm Peavey Black Widow with it.  It puts ~ 0.75 to 1 watt rms into the speaker with my 9V single supply.  I don't actually get to play it loud enough to use all of the power.  I have 2 little babies and the only time I get to play/fool around is when they are sleeping, so its kept pretty quiet.

[ItZaLLgOOd]

What modifications do you have to do for a single 9v supply?  The schematic in the link shows +/- 15v.

[davidallancole]

If you look at the last picture on the Technology of the Tubescreamer page, you will see in the top left hand corner of the picture a bias network.  It consists of the 2 10k resistors and the 47uF capacitor betwen the two resistors to ground.  This produces a 4.5V bias reference that is then connected to the positive pin of the op-amp input through a high value resistor (the 470k resistor).  Thats how you would set up the input properly.

[JKowalski]

You can parallel op-amps pretty easily to squeeze some more juice out of em.

You can stack them on top of each other physically, and connect the pins, but if I remember correctly it's best to stick some low value >50 ohms resistors on the output of each op amp before the mix bus.

You could parallel like 50 of them and make like a 25 watt amplifier! 

[davidallancole]

If you use a chip like the NE5534 and hook it up to a 600:8 ohm transformer, you can push ~200mW out of the chip too.  Its kind of cool that you can use a standard op-amp like that.

[ItZaLLgOOd]

It consists of the 2 10k resistors and the 47uF capacitor between the two resistors to ground.  This produces a 4.5V bias reference that is then connected to the positive pin of the op-amp input through a high value resistor (the 470k resistor).  Thats how you would set up the input properly.

I was referring more to the transistors.  What do you do with the -15v side of things.

[davidallancole]

Attach the -15V side to ground.  The output will center around 4.5V so you will have to use an output cap.

[davidallancole]

Here is a schematic of what needs to be done.

[brett]

Hi
older op-amps like the 741 will have trouble running at +/-4.5 V.  A rail-to-rail opamp would be better.
This design is excellent (as are all of the ESP projects).  To get maximum value from it, I suggest that you use +/-9 V (or more).  Even then, you'll still only get 4 or 5 V RMS out of it (into 8 ohms = 0.5 A = 2W).  You could reduce the emitter resistors for a little extra oomph, but basically you'll always be limited by the supply voltage.  What about using a power supply from a defunct inkjet printer (mine is 0, 16, 32V at 1 A.  Use the 16V supply as ground and you've got +/- 16V.)

just my 2c....

[davidallancole]

Those are some valuable comments Brett. 

The 741 did work for me at 9V, but the datasheet says the minimum voltage should be 10V.  So thats cutting it pretty close.  I guess the original poster never did mention how much power they were looking for or what supply voltages they have to use either.

[ItZaLLgOOd]

Looks simple enough.  Thanks a bunch.  I usually like to use the 12v AA battery packs instead of a 9v battery on my Ruby builds so I'll just use the same deal on this.  The Ruby is OK for dirty sounds but it's not clean enough with enough volume for me.  This looks like the solution I've looking for.

[davidallancole]

John E,

If you build it like the way I have shown with the 12V single supply, you will have to use the BD139/140 pair.  The 2N3904/6 pair will not cut it if you plan on playing it loud.  Also, you should up the resistance of R45/46 to 6.8k to reduce the current through those resistors.  The 4.7 resistors in Rod Elliotts amp can be reduced to ~0.22-0.33 ohms.  Thats what is usually used in an amplifiers output stage.  That will allow you to get most of what is posible out of this amp.

The drawing I did is just a general setup.  C19 can be reduced as well.  Increase R49 as well to increase the gain.  Right now its gain is 2.

[ItZaLLgOOd]

I'll stick to 9 volts then.  I have a bunch of 3904/6's.  There doesn't need to be any heatsinks on 3904/6's does there?

[davidallancole]

No you don't need heatsinks on the transistors.

[brett]

Hi

The Ruby is OK for dirty sounds but it's not clean enough with enough volume for me.

You'll get about the same amount of clean from this if you run it at 9V. (*Maybe* a bit more, but not much).
The 2N3904/6 combination are only good for 100mA or so.  100mA x 5V RMS = 0.5W, which is the same amount of clean as a Ruby.  The BD139/BD140 WITH heatsinks will give you at least twice that (and won't burn out).  For $2, don't skimp. 

The emitter resistors can be reduced, but you might not want to go too low for technical reasons of stability, etc.  1 ohm won't rob much power.

The only way to get more clean from 9V is to go for something exotic like Class D or H.  But they aren't usually much good for guitar, and they'll cost several $$$ more.

Why not buy a dozen rechargable NiNH batteries ($15 to $20 ) and use +/- 9V (4 times as much power!).  Or scavenge an old notebook or printer power pack (? $5).
cheers

[davidallancole]

Bridged-tied load (BTL) is another option if you need to stay at 9V.  That should give some more clean headroom.  That can be done with LM/NJM386 as per the Little Gem MkII.  To build a proper amp with a higher degree of reliability, definetly go with the BD139/140s and a wider supply voltage, but if you are just fooling around the 2N3904/6s will cut it, but like Brett said, you won't get much power out of them without distortion.