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Old 9th September 2011, 06:13 PM   #1
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Default LM741 :-)

Hello!


I am trying to build an amp for my electric guitar.

Why cant I just use one op amp to amplify the sound? I am using the typical inverting configuration, when I play the guitar I see the signal in my oscillo being amplified nicely and going from +6 to -6.... but when I connect a speaker to the output and I can hardly hear some sound and very distorted...

What am I missing out? Why isn't is working?

Thanks!!
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Old 9th September 2011, 06:28 PM   #2
lineup is offline lineup  Sweden
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The speaker is a heavy load. Impedance 4/8 Ohm.
It takes a bit more than LM741 to drive drive your speakers.
For example LM386
LM386 - Low Voltage Audio Power Amplifier
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Old 9th September 2011, 06:31 PM   #3
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I agree with lineup, use the little 386 for that application, it puts out close to 2W, which is more than enough for pretty loud solo practice, and can be run off a battery.

Last edited by digits; 9th September 2011 at 06:32 PM. Reason: Because the sky is blue...
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Old 9th September 2011, 06:44 PM   #4
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Just to be clear, the LM386 only has a 50 Kohm input impedance, so it is NOT a good single-chip solution for a guitar. If you use your LM741 as an input stage for the LM386 that will of course work fine.
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Old 9th September 2011, 06:57 PM   #5
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BTW The whole purpose of building this amplifier is to understand a bit how amps work.

I just read that the typical power this op amp can deliver is 60 mW... So to drive some cheapo headphones how much power would I need? Isn't that enough?

Feel free to point me to some docs you believe I should read, I searched myself but didn't find much..

Thanks!

edit: BTW I just read the 328 can deliver up to 1W....

Last edited by xchip; 9th September 2011 at 07:00 PM.
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Old 9th September 2011, 08:35 PM   #6
wahab is offline wahab  Algeria
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TEA2025 has so far the best efficency in the dirt cheap low power ic amps...
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Old 9th September 2011, 09:23 PM   #7
CBS240 is offline CBS240  United States
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Adding a couple transistors to be the output stage will allow a bit more power. Something like this.....
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Old 10th September 2011, 12:03 AM   #8
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Quote:
Originally Posted by xchip View Post
So to drive some cheapo headphones how much power would I need?
Whether it's headphones or speakers you need to do the math to determine the required current and voltage drive based on the desired output SPL, driver efficiency, and expected range of load impedances. This is straightforward Ohm's law and dB efficiency stuff.

As a rule of thumb typical speaker listening powers for home audio usually fall within a couple orders of magnitude of 10mW and typical headphone powers more like a few uW. In most 'phone applications that means you're looking at tens of uA to a few mA peak. Just about any op amp and supply are sufficient for that. The LME49720 is a good starting point.
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Old 10th September 2011, 12:09 AM   #9
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Quote:
Originally Posted by xchip View Post
Hello!

Why cant I just use one op amp to amplify the sound? I am using the typical inverting configuration, when I play the guitar I see the signal in my oscillo being amplified nicely and going from +6 to -6.... but when I connect a speaker to the output and I can hardly hear some sound and very distorted...

What am I missing out? Why isn't is working?

Thanks!!
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.
I have succesfully driven headphones from op-amps with resistors on the output but obviously not very loud. I used an op-amp to drive a disco cueing headphones, right deck went to right ear and left deck to left ear. Despite 225WRMS of disco sound I could still cue using the headphones.

For a 8 ohm speaker you need at least a few watts of power, you can use a chip amp or a discrete amp for this. Loads of circuits on DIYAUDIO.

You can easily buy a module off ebay for a few pounds.
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Old 10th September 2011, 12:49 AM   #10
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Nice! Do you have the explanation of what that circuit is doing? I'd like to fully understand it before implementing it

Thanks!
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