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Graphic Equalizer Project.
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Ronaldo95163:

--- Quote from: oPossum on September 16, 2018, 03:15:28 am ---
--- Quote from: Ronaldo95163 on September 15, 2018, 10:20:06 pm ---He placed pots on the end of each bandpass filter output, but I don't understand them...are the outputs going into the wipers?
Normally you wire up a pot between a voltage source and ground and the wiper gives the output voltage...I just can't seem to wrap my mind around this topology for boosting/cutting.

--- End quote ---
The pots distribute the output current of the filters to an in-phase or out-of-phase summing node. Analyze the circuit with the pot centered, full boost, and full cut to understand it better.


--- Quote ---I also have no idea how he determined the size of the pots to give him a specific gain...or is this determined by the previous circuit that he said provided a maximum boost/cut of 12dB

--- End quote ---

U2B has global influence on the boost/cut range. With the resistor values shown the gain is 3.9. That is 11.8 dB. Change the gain there if you want something other than 12 dB.

--- End quote ---

http://rane.com/pdf/constanq.pdf

Managed to find the paper that the design was based off of but it didnt go too much into the workings of the circuit...more the analysis.

I got that for the cct the gain would be ~12dB cause 20log3.9 ~ 12dB...what he said though that confused me was that he said a value of 10k would give roughly 5dB...how could this be since 20log(1) = 0dB. I can't seem to understand how U2B has an influence on the gain/cut range either.

I found this write up on boosting or cutting a signal with a single pot (http://www.ecircuitcenter.com/Circuits/op_bandcut_boost/op_cutboost.htm)

Will try to apply it to this circuit and report back with any questions/findings

This project is exposing me to alot of really new stuff  :scared:
Ronaldo95163:
Woa took a break for a while had some other stuff going on...got around to doing some simulations, here's what they look like for a Q of 4 for the 15 bands.

Ronaldo95163:

--- Quote from: oPossum on September 16, 2018, 03:15:28 am ---
--- Quote from: Ronaldo95163 on September 15, 2018, 10:20:06 pm ---He placed pots on the end of each bandpass filter output, but I don't understand them...are the outputs going into the wipers?
Normally you wire up a pot between a voltage source and ground and the wiper gives the output voltage...I just can't seem to wrap my mind around this topology for boosting/cutting.

--- End quote ---
The pots distribute the output current of the filters to an in-phase or out-of-phase summing node. Analyze the circuit with the pot centered, full boost, and full cut to understand it better.


--- Quote ---I also have no idea how he determined the size of the pots to give him a specific gain...or is this determined by the previous circuit that he said provided a maximum boost/cut of 12dB

--- End quote ---

U2B has global influence on the boost/cut range. With the resistor values shown the gain is 3.9. That is 11.8 dB. Change the gain there if you want something other than 12 dB.

--- End quote ---

This is my analysis in the attached pdf.
So the AH(s)Vb term is what sets the overall gain? (Vb is the output of the buffer at the input stage.

H(s) is the transfer function of the MFB filter....derived that one in a notebook so its a pain to scan...its correct though as I checked it against the actual thing.
Ronaldo95163:
I built up the entire circuit on multisim...it works...Except for the gain. Im getting about 1.6dB or so on full boost/cut and I cant seem to figure out why. I set the gain of the op amp that outputs sig as 3.16 which should set the gain of the circuit to 10dB. Confusing as to why this is happening  :-//... Checked all 15 filters separately and those seem to be good (Designed them for a pass-band gain of 1 (0dB) ), used a multimeter in simulation to even verify that the output of that op-amp is 3.16 times the input...not sure why I can't get +/-10dB of boost/cut on the bode plot.

Otherwise everything works as it should for varying the pots.
Ronaldo95163:
Been Busy with exams etc, still been working on it but haven't been posting much, I'm still having the issue with the gain.

This is what I get for:
25Hz pot @75%
40Hz pot @25%
6Hzpot @50%

Notice the almost flat response for 63Hz but it's not at 0dB. I'm pretty sure it has to do with that R6/R5 op-amp that is supposed to determine the gain

"There is one thing of special note in this circuit. R6 (39k as shown) determines the maximum amount of boost and cut, and if you wanted to, you can make it variable. With the filter circuits shown below, 39k allows a boost and cut of 12dB - which is about right in most installations. A value of 10k will allow a maximum of a little over 5dB. Any value between these limits will provide the optimum for a given environment, and this can be preset. This is a very useful feature, and one that I believe is unique to this circuit. "

Still can't understand this, well I understand the 12dB part....20log(3.9) = 12dB...but how in the world does a value of 10k give you 5dB?
Is it something i'm missing in my analysis  :(


The weird thing is that for Full boost or cut, a value of 39k gives me more or less the +/-10dB which makes no sense to me either  :-//
 
I'm honestly stuck now...not sure what else to do or what could be the problem  |O
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