Audio Circuit(Bass,Mid,Treble)

I don't know if this is a good design or not.Circle Op-amp are 2nd order low/high pass filter.
Low pass filter range is 20-250Hz
High pass filter range is 3.5Khz-20Khz
So this is a band pass filter, I think this kind of circuits are known as Speech Filter. I think this circuit is wrong. This circuit was given to me for inspection, by a student, his teacher said your circuit is OK.Now he is asking me, if this is correct. I think this circuit isn't correct. Because this circuit only passes the frequencies from 20-20Khz. So this is kind of a speech filter, but in this circuit, there are gains for Bass,Mid and Treble.I know BASS frequencies are low. I have seen Cars with Woofers fitted in them, which rocks up the floor, as they are low in frequency and treble is for high frequencies, if I'm not wrong. The last stages are gain of them. For the amplifier, the speaker would be just a small 8-16Ohm speaker driven by 386.



Is this true also, below circuit is low pass filter of 1st order, if we cascade the same filter again, will this become low pass filter of 2nd order. If we keep on cascading same stages, will the order become higher and higher?

To be honest this circuit is wrong.
First of all, filters has positive feedback (instead off negative feedback) . And the last opamp don't have any feedback. The resistor have theoretical value. This resistor don't existed in real life and in E standard.
https://www.logwell.com/tech/components/resistor_values.html

umery2k75 said:

Is this true also, below circuit is low pass filter of 1st order, if we cascade the same filter again, will this become low pass filter of 2nd order. If we keep on cascading same stages, will the order become higher and higher?

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Yes it's truth. But to reduce the number of opamps we use this circuits
https://en.wikipedia.org/wiki/Sallen_Key_filter

I'm sorry, the last opamp do have a feedback resistor of of 1KOhm, I forget to mention it.

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I have also changes the OPamp + and - symbol, exactly as where they should be like. Now this diagram is 100% similar to what have been given to me.
I don't understand as why this circuit is used for amplifing Bass,Mid and Treble. This is a speech filter.

Hi,
The circuit consists of a low pass filter for bass, two high pass filters for mid-range / treble and an analogue summing circuit to mix the audio streams back together before feeding them to ther power amp. The filter circuits have unity gain but all the op-amps prior to the summing amp do have a gain of between 1 and about 10 so they will amplify the filtered signals independently. A speech filter will only need to respond over the range 300 to 5kHz, if this circuit works from 20Hz to 20kHz it is similar to most domestic range amplifirs so it is adequate for average quality music. Remember there are two high pass filters, so it will cover the full audio range.
I hope this helps
Bob.

This is what I know in this circuit, I need to know is there really need for the Band pass filter. The range of audio is from the MP3 player or Computer Port, I don't know what are the frequencies, they must be in the audible range from 20Hz to 20KHz. So I make this assumption, Yes the range of audio freq. is from 20Hz-20Khz.I think the band pass filter must be given the name of "MID" and the high pass filter be given the name of "TREBLE" , Am I right?

umery2k75 said:

I think the band pass filter must be given the name of "MID" and the high pass filter be given the name of "TREBLE" , Am I right?

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Yes,
and for sure, this circuit isn't a speech filter, because the high pass filter will pass all high frequency without any limitation. This circuit behave like a equalizer. (so you can adjust the gain of amplifiers that you will finally get a speech filter)
Speech filter need only a band pass filter.
And 3-band ±12dB equalizer can be done much easer

How do you run the stability analysis for the first order LPF? It seems the classical break loop method is not working here because the 63K resistor between input and OP's input is not big enough if you break the loop at op's input.

Please help and thanks in advance!