# Graphic Equalizer

Joined Feb 5, 2013
24
Hi,
I am currently working on a audio mixing console and part of it consists of a graphic equalizer, I will be prototyping this entire project on breadboards first. I am thinking I will need two graphic EQ's, one for each channel of output.
My idea originally was 3 band pass filters each to have a resonant frequency in the range of the Low, Mid and High, then passing through a diode and then into an ADC and then into a driver and then into the LED's on the display.
(Image 3 shows the functional blocks)

Today I have learnt that using a band pass filter is an ineffective method as the Q factor is too small and the bandwidth is too great. I have looked around and it seems an active band pass filter is the way to go based on an op amp. After the active band pass filter, I am thinking feed this through a diode to take out the negative component this would make it suitable for the ADC. (Image 2 shows a quick circuit diagram of the active filter)

However I am unsure about what the best option would be regarding the construction of the ADC. Considering I will need 3 ADC's for each bar.
1) A successive approximation counter, I have some designs it uses op amps and counters for the graphic equalizer. I think this would be difficult to construct considering my resources it would also accumulate 2 to 3 breadboards.
2) A Pic Microcontroller, after looking around on the net it seems that it can be done, it seems a much more viable option as I would only need 1 PIC for each bar. If so I will probably need to take samples at 10 to 20 hz. I am a also unsure about the output I know it will be binary format, It wont be able to go directly into the LED's. Maybe I need a logic system between this to determine the height of the bar depending on the magnitude of the binary output?
(Image 1 shows the LED Bar display)

Any help or guidance would be greatly appreciated, I am a bit uncertain of the feasibility of this project. Thank you for reading

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#### #12

Joined Nov 30, 2010
18,210
Have you ever heard of an LM3915 chip? (Some things are not easier in digital.)

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#### bountyhunter

Joined Sep 7, 2009
2,512
Hi,

I have looked around and it seems an active band pass filter is the way to go based on an op amp.
I built a ten band LED stack display using active bandpass filters. I got the design info out of the National Semiconductor Audio Radio handbook. Still available on the net. You could also use it to build a Freq Eq but I just bought one of those. Since you can get a pro equalizer for $100, makes no sense to try to build one. http://www.jr.com/marathon-pro/pe/MAA_MA_hy_EQ215/ Thread Starter #### Renegade243 Joined Feb 5, 2013 24 Have you ever heard of an LM3915 chip? (Some things are not easier in digital.) Maybe I should have mentioned this is a project for college. Thanks for the reply, I am aware that what I am trying to achieve can be performed within the functions of just a few chips. Unfortunately doing so would not gain much credit. Im having a few doubts using an ADC, maybe Im just not realizing a simpler system which could perform the functions I want. But just a quick thought I was having, was multiplexing a successive approximation converter so that I would only need one ADC to run the whole display. It has a high operating speed, so I was thinking maybe I could make use of it. Like I said just a thought. At the moment I have been working on the active filters in college, I have made all 3 and will be testing them tomorrow. Its just the ADC stage that will be up next. Thread Starter #### Renegade243 Joined Feb 5, 2013 24 I built a ten band LED stack display using active bandpass filters. I got the design info out of the National Semiconductor Audio Radio handbook. Still available on the net. You could also use it to build a Freq Eq but I just bought one of those. Since you can get a pro equalizer for$100, makes no sense to try to build one.

http://www.jr.com/marathon-pro/pe/MAA_MA_hy_EQ215/
Thanks however I dont intend on simply buying an equalizer.
I will have a little nose around for that handbook though.

Cheers

#### Audioguru

Joined Dec 20, 2007
11,249
The bandpass filter is needed only for the midrange. Use a lowpass filter for the lows and use a highpass filter for the highs.

Your "bandpass filter" is too simple. It cuts low frequencies GRADUALLY and it cuts high frequencies GRADUALLY.
An audio bandpass filter passes a BAND of frequencies then SHARPLY cuts highs and lows.

Maybe your midrange is 400Hz to 4kHz. Your simple circuit will cut 400Hz and cut 4kHz a little (-3dB).
But 200Hz, 100Hz, 50Hz and 25Hz are still there. 8kHz, 16khz, 32kHz and 64kHz are also still there.

#### #12

Joined Nov 30, 2010
18,210
If you're going to ADC an audio signal, use a precision rectifier (made with an operational amplifier). Without that, you will lose the first .6 volts, and that makes a difference in audio!

#### bountyhunter

Joined Sep 7, 2009
2,512
Maybe I should have mentioned this is a project for college.
Here is a complete 10-band grahic EQ design senior project including schematics:

http://digitalcommons.calpoly.edu/cgi/viewcontent.cgi?article=1118&context=eesp

I have no idea why anybody would use ADC's, but maybe that's me.

From the above (very complete) 69 page dissertation on the freq eq using ADC's..... an object lesson on trying to use dig hardware when analog would work just fine:

VII. Conclusion and Recommendations
In the final product, an annoying buzzing and clicking noise emanates from the speakers. The constant, high switching rate of the digital circuitry causes this interference. The quick switching between high and low states generates both ripple in the ground levels and electromagnetic interference. The PCB design separates the analog, microcontroller and display driver ground connections in an effort to minimize this type of interference. However, all ground wires connect to the same place in the power supply (hence the term common ground), so audible high frequency digital interference still exists in the output waveform.

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#### thatoneguy

Joined Feb 19, 2009
6,359
I'm confused.

Is the OP Attempting to build a Visual Audio Power Spectrum Anaylizer, or an equalizer, or both components combined in the project?

Pioneer had a pretty cool version 25 years ago that used green/yel/red bar graphs, 8 of them, with up and down buttons. When you hit a button, the display would switch from spectrum display to EQ settings, which you could adjusts in steps of 1/10th using the up/down buttons on each of the 8 bands, the music would continue playing during adjustment. After about 5 seconds with no button press, the display returned to spectrum display. It was ahead of it's time.

Joined Feb 5, 2013
24
The bandpass filter is needed only for the midrange. Use a lowpass filter for the lows and use a highpass filter for the highs.

Your "bandpass filter" is too simple. It cuts low frequencies GRADUALLY and it cuts high frequencies GRADUALLY.
An audio bandpass filter passes a BAND of frequencies then SHARPLY cuts highs and lows.

Maybe your midrange is 400Hz to 4kHz. Your simple circuit will cut 400Hz and cut 4kHz a little (-3dB).
But 200Hz, 100Hz, 50Hz and 25Hz are still there. 8kHz, 16khz, 32kHz and 64kHz are also still there.
So I should tighten up the the two lower and upper frequences of the active band pass filters so they are closer together. I only require the magnitude of a small range of frequencies in the audio signal to represent the height of a bar. I will adjust the resistor and capacitor values to suit this as soon as I can.

Thanks

Joined Feb 5, 2013
24
Here is a complete 10-band grahic EQ design senior project including schematics:

http://digitalcommons.calpoly.edu/cgi/viewcontent.cgi?article=1118&context=eesp

I have no idea why anybody would use ADC's, but maybe that's me.

From the above (very complete) 69 page dissertation on the freq eq using ADC's..... an object lesson on trying to use dig hardware when analog would work just fine:
Thanks for that, using an ADC was only an idea I was considering it as I needed to convert a small range of a frequencies in the audio signal into a digital form taking only the magnitude of the frequencies. Also if the ADC is do-able in this application it would score me some good marks. I had a quick skim through the dissertation you linked I dont think I will be able to make a final project that in depth as I am not at university level (17 yo). I will have a proper read soon as it is very detailed and helpful though.

Joined Feb 5, 2013
24
If you're going to ADC an audio signal, use a precision rectifier (made with an operational amplifier). Without that, you will lose the first .6 volts, and that makes a difference in audio!
I will work on some full circuit schematics as soon as I can and I will post it to see what you guys think. I do intend to rectifty the signal to recieve only the postive component and I will probably be using an op amp.
Thanks

Joined Feb 5, 2013
24
I'm confused.

Is the OP Attempting to build a Visual Audio Power Spectrum Anaylizer, or an equalizer, or both components combined in the project?

Pioneer had a pretty cool version 25 years ago that used green/yel/red bar graphs, 8 of them, with up and down buttons. When you hit a button, the display would switch from spectrum display to EQ settings, which you could adjusts in steps of 1/10th using the up/down buttons on each of the 8 bands, the music would continue playing during adjustment. After about 5 seconds with no button press, the display returned to spectrum display. It was ahead of it's time.
Oh.. I see where you are coming from, maybe Ive not thought about this project as well as I thought. and I do intend for a graphic equalizer. I will have to re-think this. ..

Thanks though

#### Audioguru

Joined Dec 20, 2007
11,249
Do you understand that your bandpass filter is extremely simple with gradual slopes because it uses only a single RC to cut low frequencies and uses a single RC to cut high frequencies? It is not really an active filter, instead it is just a highpass, a buffer opamp and a lowpass.

I show Sallen and Key lowpass and highpass 2nd order Butterworth active filters. They have much sharper slopes. Use then to make the midrange bandpass filter.

Then You can use another Sallen and Key lowpass active filter for the bass and another Sallen and Key highpass active filter for the treble.

3rd-order active filters have even sharper slopes.

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Joined Feb 5, 2013
24
Hi everyone, apologies I think I have misunderstood the definition of graphic equalizer.
I was under the assumption that a graphic equalizer represented the magnitude of particular frequencies were instead it represents the frequencies in the audio signal.

However I do intend to design it so that the display will change when treble, bass and volume etc is adjusted.

Joined Feb 5, 2013
24
Do you understand that your bandpass filter is extremely simple with gradual slopes because it uses only a single RC to cut low frequencies and uses a single RC to cut high frequencies? It is not really an active filter, instead it is just a highpass, a buffer opamp and a lowpass.

I show Sallen and Key lowpass and highpass 2nd order Butterworth active filters. They have much sharper slopes. Use then to make the midrange bandpass filter.

Then You can use another Sallen and Key lowpass active filter for the bass and another Sallen and Key highpass active filter for the treble.

3rd-order active filters have even sharper slopes.

Yes I do understand the construction and theory of a simple band pass filter. Thank you for the link, may I ask what IC's A1 etc represent? Are they just standard op amps? & also would you recommend using a TL082 dual op amp instead of the standard 741?

#### Audioguru

Joined Dec 20, 2007
11,249
A 741 opamp is about 45 years old. It has noise (hiss) and has trouble above 9kHz (we can hear to 20kHz).

I recommend using more modern TL071 single, TL072 dual or TL074 quad audio opamps which are TL08x selected for low noise. They work well up to 100kHz.

The equalizer circuit I copied is used to drive rectifiers then it will light LEDs.
If it is used as an equalizer then the midrange input should be fed from an inverting opamp. The way it is shown now the filters cause phase shift which causes cancellation of frequencies between lows and mids and between mids and highs.

The filters have a gain of about 1.6 for their positive feedback which results in a Butterworth response. Butterworth has a very flat response than a sharp drop at the cutoff frequency. The resistors are the same value and the capacitors are also the same value in these filters.

#### thatoneguy

Joined Feb 19, 2009
6,359
Hi everyone, apologies I think I have misunderstood the definition of graphic equalizer.
I was under the assumption that a graphic equalizer represented the magnitude of particular frequencies were instead it represents the frequencies in the audio signal.

However I do intend to design it so that the display will change when treble, bass and volume etc is adjusted.
What you are building then, is a simple "Color Organ". Search google for that and there are nearly as many ways to make one as there are different ways to use resistors.

#### tubeguy

Joined Nov 3, 2012
1,157
An easy display would be to use bandpass filters for bass, mid, treble, or whatever frequency bands you want to display with these feeding the LM3915 LED drivers mentioned earlier.

#12

Joined Feb 5, 2013
24
A 741 opamp is about 45 years old. It has noise (hiss) and has trouble above 9kHz (we can hear to 20kHz).

I recommend using more modern TL071 single, TL072 dual or TL074 quad audio opamps which are TL08x selected for low noise. They work well up to 100kHz.

The equalizer circuit I copied is used to drive rectifiers then it will light LEDs.
If it is used as an equalizer then the midrange input should be fed from an inverting opamp. The way it is shown now the filters cause phase shift which causes cancellation of frequencies between lows and mids and between mids and highs.

The filters have a gain of about 1.6 for their positive feedback which results in a Butterworth response. Butterworth has a very flat response than a sharp drop at the cutoff frequency. The resistors are the same value and the capacitors are also the same value in these filters.
Thank you, I will probably be using the TL074 Quad Op amps as I will be using alot of OP's.