Guitar Audio Filter For Frequency Detection - Electronic Circuits Projects Diagrams Free

transistance · 8th July 2008, 12:24 PM

here is the filter that I'm having problem with. This thread is as a follow up from "2 x TL071 = TL072??". I can not get any audio after the comparator, am i not supposed to? i remember audioguru tellin me i should get a very distorted sound, yet that was with schmitt trigger.

also the original design had a 16u cap for C5, is replacing it with a 10u going to make a huge difference? (btw just to make it clear C4 and C5 are electrolytic)

thanks in advance,

P.S. sorry for the crappy schematic, i had to use pspice student version at work...

transistance · 8th July 2008, 03:09 PM

what's the reason behind supplying 2.5Vdc to positive inputs of the TL071's btw? This circuit was redesigned with reference to nilS thegimmick btw.. and he referenced it off some german guy...

ericgibbs · 8th July 2008, 03:40 PM

Quote:

Originally Posted by transistance

what's the reason behind supplying 2.5Vdc to positive inputs of the TL071's btw? This circuit was redesigned with reference to nilS thegimmick btw.. and he referenced it off some german guy...

Replied to your PM.

transistance · 8th July 2008, 03:46 PM

i'll paraphrase ericgibbs' pm here so ppl like me have an idea:

ericgibbs:
"R2/R3 divide the +V1 into half [the centre point of the +V supply] By applying this Vss/2 to the TL opa NI inputs , this sets the operating point of the opa's OUTPUTS to V1/2. This means without an audio signal the opa outputs sit at +2.5V, so any audio signal will drive the opa down towards 0V and up towards +V1.. This means the signal will be symetrical about +2.5V,,, this is what you want in a 'ac' amplifier, when its only got a single power supply line [+5v].

The C4 is just noise decoupling of this centre point voltage.

C5 will act as a filter with with R9/10. The LM393 comparator has positive feedback R12, so R9/10 C5 and R12 work together to determine at which voltage point on the audio signal, the signal is squared up by the LM393.
"
This clears a lot of stuff up for me; except one

. If anyone could explain how the comparator and that part of the circuit works? with a little equations incorporated if possible.

your input is greatly appreciated,

(i feel like an information leech, i haven't been able to really help anyone yet. but it'll happen as soon as i get this project done, I'm going to post a huge tutorial on automated guitar tuning with servos)

audioguru · 8th July 2008, 03:52 PM

Why are there so many threads about this circuit. I wish it is in a single thread so I can see the original schematic. I searched and found the original schematic.
1) the value of C3 is 453 times too high.
2) The minimum supply voltage for a TL071 is 7V. This circuit has only 5V. The opamps might not work.
3) A guitar pickup is designed to be loaded with 1M to 3M. Usually a vacuum tube or FET is used. This circuit has 4.7k which is like a dead short and will reduce the output from the pickup a lot.

If the capacitor value is corrected, if the opamps work and if the pickup has enough output then of course the output of the comparator is a square-wave. The comparator is a schmitt-trigger.

The value of C5 is fine if it is 10uF.

transistance · 8th July 2008, 04:00 PM

lol.. kk that was a typo, but i didnt know about the 7v supply, I'm going to try that soon.. thx audioguru

ericgibbs · 8th July 2008, 05:18 PM

Quote:

Originally Posted by transistance

lol.. kk that was a typo, but i didnt know about the 7v supply, I'm going to try that soon.. thx audioguru

By posting my reply to your PM, without posting your PM question to me, has made my reply look nonsense, as its out of context..

If you must post members replies to your PM's, please also post the question you asked in the PM.

audioguru · 8th July 2008, 05:41 PM

Eric explained how the comparator works:
A comparator switches its output low when the voltage on its (+) input is lower than the voltage on its (-) input.
The output goes high when the voltages are reversed.

R12 provides positive feedback together with R10 for a snap-action Schmitt-trigger.

transistance · 10th July 2008, 12:51 PM

I rebuilt the circuit with +7V power supplies to LM393 and TL071's which got my circuit working!! But I'm getting some inconsistencies; such as a 100-120Hz and sometimes 600-620Hz square wave when there is no input. The range that I'm interested in is 50-380Hz roughly.

First of all, I'm thinking that this circuit is a 3rd order low pass filter, so how is a 600+Hz signal is even passing through to output?

Secondly,when I get the 100-120Hz noise it's a big problem because it's in the range of frequencies that I'm trying to analyze. When I generate ~82Hz (low E) recorded guitar signal, the oscilloscope sees about 160Hz.

FYI - I do not own a proper oscilloscope, I use my laptop and Scope software, with coaxial cables coming out from headphones as signal generator and mic as scope. The coaxial cable had solved my noise problems previously coming from my magnetically crowded room.

What can I be doing wrong? My friend suggested that I move my circuit to a PCB since the protoboard's grid might collect a lot of EM, but I'm still in the prototyping phase!

thanks in advance,

-I would like to thank audioguru and ericgibbs for their help to get me up to this point.

ericgibbs · 10th July 2008, 02:39 PM

hi,
Got your PM.

What power rail decoupling do have on your project board.?

Is it possible to post a picture of the project board, so that we can get some idea on how its laid out.

Whats the mains frequency in Istanbul.?

EDIT: did you get the 10uF/16uF

transistance · 10th July 2008, 02:48 PM

Quote:

Originally Posted by ericgibbs

hi,
Got your PM.

What power rail decoupling do have on your project board.?

Is it possible to post a picture of the project board, so that we can get some idea on how its laid out.

Whats the mains frequency in Istanbul.?

EDIT: did you get the 10uF/16uF

I'm not too familiar with what power rail decoupling is supposed to be like exactly. I have one rail fed directly from a power supply ~7-8V, other rail going through a 7805, grounds are connected to each other. Is power rail decoupling like a capacitor between ground and power to get rid of AC? I think I just placed a 10uF electrolytic for the 5V supply, and nothing for the 7V.. oops i guess?? What values should I use?

I will post a picture when I get home. I will this time... like for real!

Istanbul's electricity is distributed at 220VAC @ 50Hz.

I went with the 10uF electrolytic instead of 16uF.

audioguru · 10th July 2008, 05:07 PM

The cable from the gutar to the circuit must be shielded audio cable to avoid mains hum pickup.

I have never seen a second-order lowpass filter like yours. It might be a simple bandpass filter if it was wired a little differently.
Here are two second-order lowpass filters:

transistance · 10th July 2008, 06:32 PM

Just to clarify things; here is what I have, and what I'm experiencing with my circuit:

Updated schematic
Image of circuit
Static noise my computer stopped analyzing to the right of the screen here because it slowed down due to print screen command
Noisy output 81.81 Hz btw, sorry for the crop

audioguru · 10th July 2008, 07:38 PM

There are a few guitar runer circuits on the internet. One has a real Sallen and Key Butterworth lowpass filter and a 1M input impedance.

I think your circuit has an input impedance that is far too low and I don't think the filter is correct.

transistance · 11th July 2008, 06:05 AM

something like this ^^ u say? This one is for 5 string bass it says but, I'm gonna check out the frequencies attenuated maybe, it can be used for a 6 string electric guitar...

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8th July 2008, 03:09 PM	(permalink)
transistance	what's the reason behind supplying 2.5Vdc to positive inputs of the TL071's btw? This circuit was redesigned with reference to nilS thegimmick btw.. and he referenced it off some german guy... __________________ transistance is futile...

8th July 2008, 03:46 PM	(permalink)
transistance	i'll paraphrase ericgibbs' pm here so ppl like me have an idea: ericgibbs: "R2/R3 divide the +V1 into half [the centre point of the +V supply] By applying this Vss/2 to the TL opa NI inputs , this sets the operating point of the opa's OUTPUTS to V1/2. This means without an audio signal the opa outputs sit at +2.5V, so any audio signal will drive the opa down towards 0V and up towards +V1.. This means the signal will be symetrical about +2.5V,,, this is what you want in a 'ac' amplifier, when its only got a single power supply line [+5v]. The C4 is just noise decoupling of this centre point voltage. C5 will act as a filter with with R9/10. The LM393 comparator has positive feedback R12, so R9/10 C5 and R12 work together to determine at which voltage point on the audio signal, the signal is squared up by the LM393. " This clears a lot of stuff up for me; except one . If anyone could explain how the comparator and that part of the circuit works? with a little equations incorporated if possible. your input is greatly appreciated, (i feel like an information leech, i haven't been able to really help anyone yet. but it'll happen as soon as i get this project done, I'm going to post a huge tutorial on automated guitar tuning with servos) __________________ transistance is futile... Last edited by transistance; 8th July 2008 at 03:49 PM.

8th July 2008, 04:00 PM	(permalink)
transistance	lol.. kk that was a typo, but i didnt know about the 7v supply, I'm going to try that soon.. thx audioguru __________________ transistance is futile...

8th July 2008, 05:41 PM	(permalink)
audioguru	Eric explained how the comparator works: A comparator switches its output low when the voltage on its (+) input is lower than the voltage on its (-) input. The output goes high when the voltages are reversed. R12 provides positive feedback together with R10 for a snap-action Schmitt-trigger. __________________ Uncle $crooge

10th July 2008, 12:51 PM	(permalink)
transistance	I rebuilt the circuit with +7V power supplies to LM393 and TL071's which got my circuit working!! But I'm getting some inconsistencies; such as a 100-120Hz and sometimes 600-620Hz square wave when there is no input. The range that I'm interested in is 50-380Hz roughly. First of all, I'm thinking that this circuit is a 3rd order low pass filter, so how is a 600+Hz signal is even passing through to output? Secondly,when I get the 100-120Hz noise it's a big problem because it's in the range of frequencies that I'm trying to analyze. When I generate ~82Hz (low E) recorded guitar signal, the oscilloscope sees about 160Hz. FYI - I do not own a proper oscilloscope, I use my laptop and Scope software, with coaxial cables coming out from headphones as signal generator and mic as scope. The coaxial cable had solved my noise problems previously coming from my magnetically crowded room. What can I be doing wrong? My friend suggested that I move my circuit to a PCB since the protoboard's grid might collect a lot of EM, but I'm still in the prototyping phase! thanks in advance, -I would like to thank audioguru and ericgibbs for their help to get me up to this point. __________________ transistance is futile... Last edited by transistance; 10th July 2008 at 01:06 PM.

10th July 2008, 02:39 PM	(permalink)
ericgibbs	hi, Got your PM. What power rail decoupling do have on your project board.? Is it possible to post a picture of the project board, so that we can get some idea on how its laid out. Whats the mains frequency in Istanbul.? EDIT: did you get the 10uF/16uF __________________ Eric "Good enough is Perfect" PIC tutorials: Gramo's: www.digital-diy.net/ Bill's: www.blueroomelectronics.com/ Last edited by ericgibbs; 10th July 2008 at 02:41 PM.

10th July 2008, 07:38 PM	(permalink)
audioguru	There are a few guitar runer circuits on the internet. One has a real Sallen and Key Butterworth lowpass filter and a 1M input impedance. I think your circuit has an input impedance that is far too low and I don't think the filter is correct. __________________ Uncle $crooge

11th July 2008, 06:05 AM	(permalink)
transistance	something like this ^^ u say? This one is for 5 string bass it says but, I'm gonna check out the frequencies attenuated maybe, it can be used for a 6 string electric guitar... __________________ transistance is futile... Last edited by transistance; 11th July 2008 at 06:16 AM.