Correct.

The gain is (R8/R9) + 1= 11.

Correct.
The datasheet for the LM324 explains that the output will suddenly go high or low when its input voltage goes negative more than 0.3V.
The output from a guitar's pickup could be driving the input of the first opamp well below ground. If you add the FET preamp (that I posted) to the input then the input of the opamp will not go below ground and there will be no phase reversal.

Probably, then the output of the first opamp will clip and produce many harmonics.

I think something is wrong with your 'scope. A power amp is not needed since the input of a PIC is a high resistance load that is easily driven by an opamp. Maybe the input of your 'scope is a low input resistance mic input instead of a high input resistance line-level input.

Attached Images

LM324.PNG (18.9 KB, 2 views)

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Uncle $crooge

 

This is meaning the input Vmax≤+0.3V Vmin≥-0.3V, right?
Assuming my guitar outputs 3mV; which you thought was too little, the first stage shouldn't have a phase reversal.
If I misread my voltmeter and the guitar output was 30mV, the first stage still won't have a large enough input to have phase reversal; yet 30mV×11= 0.33V will cause a phase reversal on the 2nd stage.

What if I changed R5 value to 3MΩ, would that help? Can't I redesign the opamp based preamp for a higher input impedance like that?

If I was to build the FET preamp you suggested, will that be replacing the current first stage(the preamp)?

Is the reason behind using Hi-Z inputs for audio applications prevent clipping?

I had mentioned this before; I was using my laptop's mic input for scope.Maybe I should switch to my desktop and use the line input?

I'm guessing that this could be my problem.. Dell 700M mic design defect My next laptop is going to be an apple...

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transistance is futile...

 

No, it's for feeding the rare hi-impedance connections to - basically the only one these days are passive guitars, these need 500K upwards.

Yes, the mic input is pretty crappy.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

It is the mic input of your sound card that is clipping at about 2V peak-to-peak.
The Zeitnitz Oscilloscope instructios say the input voltage range of a sound card is 1.4V peak-to-peak!!! They say if you want to see a higher voltage then an input voltage divider is needed.
A loud mic level is about 10mV RMS and a loud line level input is about 300mV RMS.

You need to know the exact input resistance and impedance of the sound card to make an accurate voltage divider.

The software oscilloscope and sound card have an AC input, not DC so you won't know when a signal goes negative.

Your voltmeter can measure sine-waves (not music) accurately only at 50hz and 60Hz.

The designer of the FET preamp I posted limits its gain to only 1.4 times because he knows that a guitar can produce a few volts of signal.

The input voltage range of an LM324 is -0.3V to 32V. If an input goes to a voltage more negative than 0.3V then phase reversal will occur. If an input goes to a voltage more positive than 1.5V less than the positive supply then that input will not function.

The output of the first opamp will clip at +0.1V and +7.8V so the other opamps will not have phase reversal.

The value of R5 is 1M. Increasing it to 3M won't make any difference.
If you use the FET preamp then the first opamp is still needed to provide a low impedance drive to the lowpass filter. The gain of the first opamp should be decreased since the FET has a gain of 1.4 times.

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Uncle $crooge

 

I can't find the J201 N-channel JFET locally, can I build the circuit with 2n3904 NPN BJT and get the same results?

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transistance is futile...

 

No, it has to be an FET, that's the whole point.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

The J201 N-channel FET is pretty old (like most guitar circuits) but Digikey still has 881 in stock for $.30US each.

I would use a 2N5484 FET which has a little more current but should work fine in the simple circuit.

Many FETs have a current that is much too high and the circuit will need re-design for them to work properly.

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Uncle $crooge

 

Ok, here is the deal.. I took my circuit to friend of mine with an oscilloscope, he has been an electronic hobbyist since he was 9 and an electronics engineer for about 10 years now. We checked the circuit stage by stage, there is no phase reversal in the circuit, thus no hardware fix needed. The second order harmonic just manages to pass through at lower frequencies and makes the comparator switch states.

My plan is either to have a low pass filter for each string according to proper bandwidth, or use a second PIC that will do some simple software filtering (e.g. do not stop timer if timer is less then expected value on that string).

Any simple ideas that might be helpful?? (please no software FFT)

or how about switched-capacitor filters??

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transistance is futile...

 

If you check EPE they did a PIC based guitar tuner a couple of years back, and they used a variable filter driven by the PIC.

I'm presuming this is for a college assignment?, as you can buy far better commercial ones for very little money.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

Nigel it has been longer than 4 months that I am working on this project, if it was for college I would be screwed by now...

It's a guitar embedded tuner with multiple tuning schemes, according to scheme chosen the PIC will control 6 servos on the headstock of the guitar to tune it to the scheme user input.. if i get it done, it will just be awesome!

I am in no hurry for a deadline, I'm just impatient by nature...

I have checked the EPE one when I first started the project, i just couldn't make much out of it, I just skimmed through the code (seemed too complicated), didn't see the schematic thus didn't know they had an controlled external filter.. I thought what they did was software filtering.

I already have a separate tuner and a boss gt-8 multi-fx floor pedal with embedded digital tuner. I'm just doing this because I want to learn something and I like challenge.

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transistance is futile...

 

Maybe you could add an automatic-gain-control circuit so that the level of the signal to the comparator is just the peaks of the fundamental frequency so that the comparator will not be triggered by harmonics.

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Uncle $crooge

 

Have you built the mechanical hardware yet?, obviously you couldn't use radio control servos for it, as they don't rotate far enough.

Do you really use different tuning schemes so much?, the usual method is to have two guitars! - my daughter plays bass , and has written various songs that require different tunings, as well as playing covers that do - but she usually just retunes on the last note of the previous song. She has pretty good pitch, and can retune a string as the previous note dies away - pretty impressive to watch. In one of her previous bands the guitarist used to pass her the guitar to tune - it was faster Melissa doing it by ear than using a tuner, even including the time passing the guitar over.

No, it's external controlled by the PIC - I would suggest you try something similar, and it may be worth buying the back issue(s).

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PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

I have modified sub-micro servos for full rotation before, it will work.

Yes. I play a wide variety of modern rock, metal and electronic genres.

You must be proud. When I have a kid, I'm going to make sure he/she is raised as a dedicated and far-better-than-me musician. I started playing when I was 15 (a little late imo); I'm a decent guitar player, and a pretty good electronic music composer. When it comes to perfect pitch for tuning though, I don't have the trained or innate ears for that.

This project is for the less fortunate like me...

I think that project is a little ahead of my skills, especially since it has been a long time since i coded any assembly. Maybe, I will look into it when I decide to improve my system.

Are EPE issues electronic or prints?

Anyways, here is the latest version of my circuit, it's a little bit more complicated. (Sorry for that...)

I calculated the ranges of each string in different tunings to develop as little number of filters as possible. I came up with a 3 low-pass filter design that will filter for 2 strings each while attenuating the 2nd and higher order harmonics.

I greatly appreciate all your inputs,

Attached Images

Multiple Low Pass.jpg (93.6 KB, 10 views)

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transistance is futile...