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Old 10th September 2011, 12:15 AM   #11
CBS240 is offline CBS240  United States
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A speaker is around 8 ohms, the output impedance of the 741 is about 100ohms so all the power is being lost inside the 741.
2.2Wrms @8R (6Vp) is 750mAp. The circuit in post 7 simply adds a current amplifying stage to lower the 100R output impedance so it will drive a speaker. Use 47-100R for the base resistor and maybe 0.47R for the emitter resistors. You could use TO-126 transistors for the outputs or rather multiple paralelled small signal devices like 2N4401/4403 just make sure each transistor has its own emitter and base resistors. Increase the emitter resistors as well, for example, 3R if you use 6 pairs. J-fet can be any device, Idss maybe 2-10mA. Mount the diodes against the BJTs. I cannot think of a cheaper solution since those transistors cost only pennies a piece.
All the trouble I've ever been in started out as fun......
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Old 10th September 2011, 12:30 AM   #12
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hehe I see that the subject is not as easy as I thought :-)

It seems that I have to study more this field before I continue...

Do you recommend me any good books?

All your answers helped me a lot!
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Old 10th September 2011, 12:39 AM   #13
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I tried to emulate what this guy did... he build a guitar amplifier by using 4 op amps each with a gain of x10.

Op Amp Guitar Amplifier - SparkFun Electronics

this is the circuit:


the operational he used is this one:


Why does it work then? Because the output impedance of the op amp he is using is lower?

My mistake is that I tried to do the same with a 741 :-)

Does using 4 op amps with a gain of 10 help to drive the speaker? Could he just achieved the same using one op amp and choosing the resistors if his inv-amp so he gets x10000?

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Old 10th September 2011, 12:44 AM   #14
CBS240 is offline CBS240  United States
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It is simply a non-inverting op-amp circuit that includes the output follower BJTs within the FB loop. The output of the op-amp is conducting DC current through the diodes and through the J-fet. The J-fet used in this way creates a constant current source. So whatever the Idss of the J-fet is, check a datasheet for whatever one you have on hand, that current will be conducted from the op-amp output pin. This sets up a bias voltage across the diodes to bias the BE juntion of the BJT's, two forward PN junction drops. The emitter resistors limit current and add stibility to the bias. The diodes and BJTs should be in contact so they share the same temperature, and this also adds stability to the bias. This is class AB, where both devices conduct a small current but the NPN transistor drives the positive half of the waveform and the PNP drives the negative. Another option, and perhaps simpler, would be to use class A, but this would require dissapating ~10W of heat assuming a +/-6V supply.
All the trouble I've ever been in started out as fun......
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Old 10th September 2011, 01:12 AM   #15
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Default Opamps

Why not Google the Application Notes from the relevant manufacturer?
Also, you can go to an industrial supplier website like Mouser, Digi-key, Newark, Farnell etc.and download datasheets there for the particular type available, then search for the ANs you see there. These ANs can be for generic types which will be fine for understanding of the the quick and simple sort and get you up and going, assuming you already have some understanding of electronics.

If you need the A-Z of electronics and how op-amp circuits (or rather, amplifiers in general) work, you really need to start at the fundamentals with a basic textbook. Set aside more than a few evenings for this, since you won't learn much in just a few forum posts or reading shallow descriptions on the web.

A comprehensive but very accessible text is "Op amps for everyone" by Carter & Mancini (TI). There are plenty more and I think there is even an "Op amps for dummies" title.
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Old 10th September 2011, 01:29 AM   #16
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Hello xchip

The LM741 are a very bad sounding opamp.

At least use a LM5532 who sound so much better, you can use half of this opamp for your guitar amp input. And you can use this LM5532 to drive a small chip amp.



Last edited by gaetan8888; 10th September 2011 at 01:34 AM.
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Old 10th September 2011, 02:09 AM   #17
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Default RUBY

Build the Ruby from runoffgrove.com
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Old 10th September 2011, 09:56 AM   #18
AndrewT is online now AndrewT  Scotland
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Decibel Dungeon
and many others.
regards Andrew T.
Sent from my desktop computer using a keyboard
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Old 10th September 2011, 10:15 AM   #19
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thanks for the pointers guys! I've been using op amps to amplify small signals and feed them into a scope but never drove a speaker, this was my first naive attempt.

So the 741 is bad because of its high output impedance and low output current... if I stack one 741 on top of another wouldn't I get double output current and half output impedance?

gaetan: So what makes an opamp a good candidate for audio? Just output current and low output impedance?
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Old 10th September 2011, 10:56 AM   #20
lineup is offline lineup  Sweden
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You need either one Opamp + transistors or
one dedicated power Chip like for example LM386
LM386, LM380 are Opamps for driving speakers.

See this RUBY link which was recommended by Darkstar52 a couple of posts above:

Here is the circuit:
Click the image to open in full size.
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