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Old 15th May 2009, 07:38 AM   #1
skandal is offline skandal  Romania
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Default Help creating a audio processor

First of all, since it is my first post here, hello everybody. Second, I have this school asignment i must do...I have to build a audio processor which adds guitar efects using a AVR ATmega16 microcontroller. All in all, it was pretty easy until i hit a serious problem: output. TO be more precise, i'd like my circuit to output to a guitar amp(a low end e-zipy ga10), but also make a separate jack for normal speakers. Problem is this: i thought speaker impedance means the resistance they put in the circuit. But that varies alot(40 ohm - 1000 ohm for my headphones, somewhere around 18kohm my pc 2.0 speakers). Not to mention i couldn't even measure the amp's resistance since it was shown as infinite even on the 200 Mohm scale by my meter.

So, i need to take the 5V max output of my DAC(a simple R/2R ladder with an OPamp), and turn it to 1V max output for speakers(that's what a normal audio card outputs). Also, on a separate jack(they won't both be connected at the same time) i need to output signal for the amp(which i assume receives an even lower voltage, since electric guitars don't produce alot of voltage). Anybody have any suggestions as to what it can be done, and maybe useful links?
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Old 15th May 2009, 09:48 AM   #2
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You will need some basic electronics knowledge for this project. Per example to understand the difference between resistance and impedance which will explain why you are measuring input resistance of infinity (input is cap coupled and the actual resitor is on the other side of the cap). In any case trying to measure this with a multimeter is wrong.

You will obviously need some kind of preamlifier which boosts the pickup output from mV to 5V. As this project is not about quality (the AVR ADC is quite poor) a single opamp will probably suffice. You need a low voltage part which will be happy to work with a single rail +5v. You will also need some kind of buffer at output. It can be just an emitter or source follower.
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Old 24th May 2009, 04:50 AM   #3
skandal is offline skandal  Romania
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Well, truth is, i do know a little about electronics...Just that this impedance thing is still confusing me, even after reading alot of stuff on the internet. But this aside, i actually managed to program the avr to output the signal it receives on the ADC pin. I connected the input to the speaker output of my PC's sound card, and the output to my speakers...To my suprise, it kind of worked...sort of, because it failed myserabily. The volume of the sound is ok, but the quality is so poor, i can barely recognize the songs played. A friend of mine told me it was because of the circuit noise. Assuming this is the case, and it probably is, would it be a good idea to put a anti-aliasing filter before the ADC input of my avr controller, and maybe after the opamp output? I'm asking this because i don't know if it will help or not(most headphones don't have such things and they work perfectly?).
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