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I'm thoroughly embarassed... But I need some help, 3-band EQ
Old 19th September 2012
  #1
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samwell7's Avatar
 
🎧 10 years
I'm thoroughly embarassed... But I need some help, 3-band EQ

I'm currently studying electrical engineering (3rd Year) and have a year long class called 'Engineering Design' in which me + a group has to make a 'product'.
For this we are making a basic audio-spectrum analyser using Analog Devicess Blackfin 548 with a passive analog 3-band EQ (to show people how music works, sort of).
The Software side is basically finished, but for some reason this super basic 3-Band Shelving EQ/Filter is giving me hell, it just will not work the way any of the simulations say it will (the attenuation it provides is minute).

The circuit is as follows

with 5K logarithmic potentiometers to the output,
Do I need bigger potentiometers to attenuate the outputs of each band more? or is there a flaw in my design I'm failing to pick up on.

Incase the picture isn't clear enough the components used are, 4x100 ohm resistors, 2x10uF caps & 2x470nF caps (16v & 30v).

Sorry for such a noob question! Hopefully I get some light on the matter
Old 19th September 2012
  #2
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Richard Crowley's Avatar
 
🎧 10 years
How did you arrive at the value of those 100 ohm resistors? They seem too low by a couple orders of magnitude. Is there some reason you didn't use a more conventional circuit? One with the variable resistors used as potentiometers to cut/boost vs. using them as rheostats? Was there a requirement that you design your own circuit vs. using one of the popular published circuits? For example Google returned over a million image hits for: 3-band eq circuit This was the very first hit:

HeadWize - Project: Designing A Pocket Equalizer For Headphone Listening (A HeadWize Design Series Paper)

Old 19th September 2012
  #3
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🎧 10 years
I'm not sure how you got an active Baxandal topology from that passive schematic.

On obvious shortcoming of that passive circuit, is if multiple output pots are turned to low impedance, they will load each other down.

JR
Old 19th September 2012
  #4
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🎧 10 years
I'm with rcrowley! That passive circuit won't work. At least the active circuit will. And it sounds like this part is the lesser part of the project, so just make something that will work, and study it.
Old 19th September 2012
  #5
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🎧 15 years

As to why your circuit didn't match your results:

Did you check the impedance of the source? Did you acount for the load impedance?




-tINY

Old 19th September 2012
  #6
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🎧 10 years
Quote:
Originally Posted by rcrowley ➑️
How did you arrive at the value of those 100 ohm resistors? They seem too low by a couple orders of magnitude. Is there some reason you didn't use a more conventional circuit? One with the variable resistors used as potentiometers to cut/boost vs. using them as rheostats? Was there a requirement that you design your own circuit vs. using one of the popular published circuits? For example Google returned over a million image hits for: 3-band eq circuit This was the very first hit:

HeadWize - Project: Designing A Pocket Equalizer For Headphone Listening (A HeadWize Design Series Paper)

I did the 'choose an arbitrary R value' so I could use the formula to find a C value, it has variable resistors on the output which I thought were going to cut/boost the output, I.e potentiometer on 0 ohm equals full output and at Max resistance I was hoping it would absorb the full voltage on that branch, unfortunately we had to design everything ourselves and weren't able to use any pre-existing kits, which I know would be millions of times easier and most likely function better, so you think the resistors in the RC branches need to be bigger? E.g 1-10k?
Thankuou for your feedback

Sent from my Galaxy Nexus
Old 19th September 2012
  #7
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🎧 10 years
Quote:
Originally Posted by Radardoug ➑️
I'm with rcrowley! That passive circuit won't work. At least the active circuit will. And it sounds like this part is the lesser part of the project, so just make something that will work, and study it.
Can't use an active system because it will need an extra power-supply and we have to use the power supply that came with the blackfin board for the blackfin board so I don't want 2 power plugs coming out of it haha. Quite limited :/

Sent from my Galaxy Nexus
Old 19th September 2012
  #8
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🎧 10 years
Quote:
Originally Posted by JohnRoberts ➑️
I'm not sure how you got an active Baxandal topology from that passive schematic.

On obvious shortcoming of that passive circuit, is if multiple output pots are turned to low impedance, they will load each other down.

JR
I had noticed that!
Seems like this thing won't work the way I wanted it to

Sent from my Galaxy Nexus
Old 19th September 2012
  #9
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🎧 10 years
Quote:
Originally Posted by tINY ➑️

As to why your circuit didn't match your results:

Did you check the impedance of the source? Did you acount for the load impedance?




-tINY

Me and the guys had got into this discussion just yesterday as it works better playing out of a Samsung phone than an iPod, to account for the input impedance do I need to use bigger resistors, as was quoted before?

Sent from my Galaxy Nexus
Old 19th September 2012
  #10
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🎧 10 years
Designing a PASSIVE 3-band EQ circuit is probably a bigger challenge than the audio spectrum analyzer!

Throw a 9V battery in there and get on with your life. You will spend more time trying to do this passively than it is worth.
Old 19th September 2012
  #11
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🎧 15 years

Assuming a 100ohm drive impedance and a 10k load impedance, this is what I get for sweeping the mid 1.25k to 5k with other controls set to 2.5k.

Don't they teach AC circuits anymore?




-tINY

Attached Images
File Type: jpg broke_brax.jpg (69.0 KB, 5699 views)
Old 20th September 2012
  #12
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Jazz Noise's Avatar
 
🎧 5 years
Are you telling us that the power supply that came with your project cant draw another 10mA?

Get a TL074, do a Baxendall or do a seperate buffer and filter per stage. You're actually not as limited as you think.
Old 20th September 2012
  #13
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🎧 10 years
You must crawl before you walk...

Make the passive EQ work first. (this is a learning exercise, not high performance design).

You must account for the total impedance at every node...

If a computer simulation works and the breadboard doesn't, at least one of those two is wrong.

JR
Old 20th September 2012
  #14
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🎧 10 years
Think I might just have to just use a 9v battery with a good mAH rating or even a couple in parallel so at least it lasts a while and do it active with some op-amps, it does work passively just not as well as it did on the (ideal) simulation, due to my ignorance of the input and output impedance amongst other things.
The power supply can draw an extra 10mA its just there's not much I can do to the board to get it from the blackfin to the power supply as we aren't allowed to change the board itself, which is fair enough because they're expensive haha.
So much thanks to everybody who replied to this post!

Sent from my Galaxy Nexus
Old 20th September 2012 | Show parent
  #15
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🎧 15 years
Quote:
Originally Posted by samwell7 ➑️
Me and the guys had got into this discussion just yesterday as it works better playing out of a Samsung phone than an iPod, to account for the input impedance do I need to use bigger resistors, as was quoted before?

Sent from my Galaxy Nexus


First thing to do is measure your output impedance - you can do this with a couple of resistors and a dual-trace scope (or maybe a good 10kHz bandwitdth RMS voltmeter and a simple tone that's repeatable).

Measuring you input impedance at the other end is easy if you have an AC source (or a power amp and a recording of tones - my iPhone metronome has a tuner that would work with an amp). You still need one resistor and two measurements...

Once you know the drive and input impedances, the simulator usually works pretty well...



-tINY

Old 21st September 2012
  #16
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🎧 10 years
Quote:
Originally Posted by tINY ➑️


First thing to do is measure your output impedance - you can do this with a couple of resistors and a dual-trace scope (or maybe a good 10kHz bandwitdth RMS voltmeter and a simple tone that's repeatable).

Measuring you input impedance at the other end is easy if you have an AC source (or a power amp and a recording of tones - my iPhone metronome has a tuner that would work with an amp). You still need one resistor and two measurements...

Once you know the drive and input impedances, the simulator usually works pretty well...



-tINY

Thanks a lot. I'm going to do this at uni when I get some time, the only thing is this is supposed to be a 'marketed plug'n'play' type instrument, is there some sort of industry standard input impedance that's used?

Sent from my Galaxy Nexus
Old 21st September 2012
  #17
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1 Review written
🎧 15 years

No standards that I am aware of - but the apple may have a design guideline.

Input impedance will be 10-50K for line level consumer stuff. One would hope for an output impedance less than 100 ohms for line level out of a phone - headphone out may be lower than 10 ohms...




-tINY

Old 21st September 2012
  #18
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WHAT is supposed to be a "marketed plug'n'play' type instrument"? The spectrum analyzer? or the 3-band passive EQ? I'm really confused what the requirements are here.
Old 21st September 2012
  #19
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🎧 10 years
Sorry if I was a bit vague with the whole thing, for the class we have to basically pretend we are a real company and that we are making a product to sell, we were given 2 options, we had to write a proposal for a fully self designed project and we also had to write a proposal for one of the given project outlines. We ended up not getting our original idea so we had to make a spectrum analyser using the blackfin board (which was a school outlined project), gah writing this down is making me hate this class even more haha.
The spectrum analyzer as a unit is a marketed plug n play thing, basically it is supposed to be a consumer-level low price unit for people who enjoy music etc to put inline with their amp or other signal chain as an extra novelty.
It includes a 3-band EQ to 'show' people how the audio spectrum/frequencies sound (our project needed some sort of educational angle to be approved).
I.e a complete noob can turn the bass knob down and look at everything below 200hz drop and be like "Oooh that's what music does" haha.
We also have to present to a board of professors and our peers and then have a 'trade fair' day where we set up a stall and demo our project to people.
Hope this explanation helps further,
Also thanks tiny for the impedance help!
do you guys think powering the EQ from a couple of high mAH rated 9V batteries in parallel will suffice? Or will they run flat quickly?

Sent from my Galaxy Nexus
Old 21st September 2012
  #20
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🎧 15 years
Can't you just rob some power from the blackfin PSU?

Bri
Old 21st September 2012
  #21
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🎧 10 years
Quote:
Originally Posted by brianroth ➑️
Can't you just rob some power from the blackfin PSU?

Bri
As far as I know there are no aux power points on the blackfin board and we aren't allowed to change the board in any way, or is there another way to do it? Is there a plug I can get which is male one end (sticks into the blackfin) female on the other (accepts the blackfin psu's wallwart plug) and has a cable running off the side of it like those power plugs.
The blackfin only uses 7V would that be enough?

I.e like this http://www.thecableconnection.com.au...382858EC_1.jpg
But a small AC barrel plug

Sent from my Galaxy Nexus
Old 21st September 2012
  #22
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I glanced at the blackfin pages, but didn't immediately see what sort of power supply/connector comes with the board, and I didn't want to spend hours digging through the docs <g>. Any link(s) for us to look at?

7 VDC is a tad on the low side, particularly compared to what pro audio gear uses, BUT a lot of guitar stompbox designs rely on 9V batteries, which drop down to 8.something under load.

A power splitter cord would be super easy to fabricate once you determine the exact connector specs.

Bri

edit.....does the blackfin PSU provide DC or AC power?
Old 21st September 2012
  #23
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🎧 10 years
Quote:
Originally Posted by brianroth ➑️
I glanced at the blackfin pages, but didn't immediately see what sort of power supply/connector comes with the board, and I didn't want to spend hours digging through the docs <g>. Any link(s) for us to look at?

7 VDC is a tad on the low side, particularly compared to what pro audio gear uses, BUT a lot of guitar stompbox designs rely on 9V batteries, which drop down to 8.something under load.

A power splitter cord would be super easy to fabricate once you determine the exact connector specs.

Bri

edit.....does the blackfin PSU provide DC or AC power?
Thanks for having a look, I'm at work at the moment so I can't delve into a big Google search but tonight I will have a look at the exact specs, if it helps at all it is blackfin 548 ez kit and I've attached an image.
that's promising with the stompboxes using 9V batteries


Sent from my Galaxy Nexus
Attached Thumbnails
I'm thoroughly embarassed... But I need some help, 3-band EQ-uploadfromtaptalk1348192963711.jpg  
Old 21st September 2012
  #24
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I was JUST interested enough to look at the blackfin manual, and it lists the exact power jack with a Switchcraft part number (RAPC712X).

If I was building the active filter gizmo, I'd mount the circuit board in a metal box with a panel mounted power jack which matches the power plug from the blackfin power supply. Then, a "pigtail" cable with the proper power plug would continue on from the metal box and plug into the blackfin board. iOW, the metal box would be the splitter.

Bri
Old 21st September 2012
  #25
Gear Maniac
 
samwell7's Avatar
 
🎧 10 years
Quote:
Originally Posted by brianroth ➑️
I was JUST interested enough to look at the blackfin manual, and it lists the exact power jack with a Switchcraft part number (RAPC712X).

If I was building the active filter gizmo, I'd mount the circuit board in a metal box with a panel mounted power jack which matches the power plug from the blackfin power supply. Then, a "pigtail" cable with the proper power plug would continue on from the metal box and plug into the blackfin board. iOW, the metal box would be the splitter.

Bri
Awesome! That's exactly what I was trying to explain before with the piggyback cable, I don't think the blackfin board draws much current or the EQ will draw much either so it should work.
Thanks heaps for finding that out, there is hope yet haha

Sent from my Galaxy Nexus
Old 25th September 2012
  #26
Gear Maniac
 
samwell7's Avatar
 
🎧 10 years
We have decided against using a piggyback cable from the blackfin board (no-one in the group really wants to fork out the couple of $K it will cost if something magically doesn't work).
found a low-voltage (Lm324 I believe) Op-Amp, will power this from a 9V battery as the project is due in a matter of weeks, it isn't the best option but it will (hopefully)get us by.
Thanks again to everybody for their help!
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