Hallo there

I am seeking help with this problem. We supose to conect a circuit with use of TL071,2 opamps (which will be powered by +-15V) to an ipod or some simillar mp3 player with output around 1V. Now the signal is divided into bass, midle, treble using low, band and high pass filters and amplified to light up LED for each part of the frequency (0 - 500, 500 - 5000, 5000- ... Hz). I have been trying to conect it but not succesful. I could cut of the frequency but than i tryed to amplifi it and the frequency cutoff went from 500 Hz to 10 kHz. Also we supose to thing and improve the rolloff for the frequency cut off, but clearly this is for me write now far far a way .

Thanks for looking and help

 

It is simple to make a lowpass, bandpass and highpass filters circuit.
It is easy to make 3 LED drivers that show the output levels of the filter circuits.
Why don't you post your schematic and parts list so we can see what is wrong with it?

 

Hi Audioguru
Sounds to me that this is really easy for you. Here are the circuits i tryed how ever none of them did what i wanted and i actualy ended with trial error style but without succes. The two poleactive Low pass filter was either doing right gain but too hihg frquency cut off or the other way around. Although i did not get to try the hihg pass or the band pass since i was not able to solve the low pass at first stage.
The scan 001 was the main idea, but without the roll off improvement.
The scan 002 was the one i have been trying for some times to get work.

Thanks for looking and help
ricky0066

 

The single-pole rolloff does not separate the frequencies much:
1) At 500Hz the lowpass and mid frequency outputs are 0.71 times their max outputs.
2) At 1khz the lowpass output is 0.5 times which is a very small drop of level.
3) At 2kHz the lowpass output is 0.25 times which is another very small drop of level but the frequency is not low anymore.

At 5kHz and at higher frequencies the mid frequency output is dropping slowly and is almost at maximum when the high frequency output is also almost at maximum.

The slopes of the single-pole filters are so gradual that the 500Hz to 5kHz mid frequencies filter drops the level of all mid frequencies.

The two-pole filters are much better and should work fine.
The 3-pole filters are even better.

 

Hi

Ok the theory is than clear and i am going to use the two pole as shown in the second picture, which i actually tryed, but i still have the problem of not getting the values of the components right. Although can you please let me know if the circuits in the second picture are rigth ?

Thank you very much
ricky0066

 

Look in Google at Sallen and Key lowpass Butterworth filter.
The resistors have the same value, the opamp is a follower with its (-) input connected to its output and the second capacitor has double the value of the first capacitor. Use two capacitors in parallel to make the doubled one. The simple formula for the values for a certain frequency is shown.

The highpass filter has the resistors and capacitors swapped. The capacitors have the same value and the second resistor is half the value of the first resistor. Use two resistors in parallel to make it.

Your resistors do not have the same value in your lowpass filter and your capacitors do not have the same value in your highpass filter.

Each Sallen and Key filter must be fed from a very low impedance like the output of an opamp.

 

Hallo there

I wish i can say i was succesfull, but ... not yet. I will give it a go one more time troughout saturday and than i do not know. I have used the theoretical values and the double capacitor bud it did not had any sort of cutting frequency and the LED was flashing continously with no respoce to frequency changes at all.
However i am still not giving up the only think which makes me very is that you find it easy to solve this

 

The filters circuits cannot drive the LEDs. The filters circuits drive transistors that drive the LEDs.
Please post you entire schematic.

 

I will post it today once i will get home.
Thanks

 

Ok Here it is

As i siad i did not get further than the loww pass part. I have been thinking about introduce diodes between the cut off part and the gain part. But i am not sure if this will be the solution.

Thanks ricky0066

 

Sorry the circuit

 

The circuit secon times. ??

 

Most of these music displays have an active rectifier circuit after the filters then a peak sample and hold circuit for the LEDs.

 

Take a look at chapter 12 - Special Effects

It has several schematics showing the back half of a color organ.

 

Nobody makes a "6V" LED. A red one is about 2V and a blue or white one is about 3.5V. They need their current limited with a series resistor.

Your opamps are in a filter circuit that should not have its output shorted with an LED. The opamp can drive a transistor that drives the LED.

Your filters are not correct. A Butterworth lowpass filter is made with equal resistor values and equal capacitor values and the opamp has a gain of about 1.6. Or the feedback capacitor can have double the value of the capacitor to ground then the opamp has a gain of 1.

You need a rectifier circuit to drive each LED. Or bias the opamp off then half-waves of the signal turn it on and also turn on the transistor and LED it drives.

 

I hope the scan will be there now

 

Hallo there

I am not suppose to use transistors, but i can use more opamps. I just connected this cirquit. The theoretical gain suppose to be 11 but there is just 1 and the theoretical cut off frequency is 12Hz but real is around 120 Hz. What is wrong with me ?? It loooks like i will never get it done

 

You do not need a dual polarity power supply if you bias the opamps correctly.
Opamps like the MC34071, MC34072 and MC34074 operate from a single polarity supply and their inputs work at voltages as low as 0V so you can bias them so that the LEDs are almost lighted without music.

I think your frequency problem is caused by the gain you have in your filters.
Use simple Butterworth filters that have a voltage gain of 1. Then add one opamp ahead of them to add some gain.

 

sounds like a plan i am going to do it now

 

I am restricted to use only TL071 or 2
the filter on its own works fine it does cut off the frequency around 500Hz,
but than i conect the gain and it is moved to cca 1000 Hz and the gain instead of been 3 is 2 only. So no light yet.

Any idea pls.
Thanks