Graphic Equalizer Project. - Page 1
Forum section: electronics / projects

author: Ronaldo95163
author_rank: Contributor
timestamp: September 07, 2018, 06:44:07 pm
content: Hey guys so i'm gonna be working on a 15 band graphical equalizer that includes a digital interface and i'm still in the research phase of the project.From what i've gathered, a Multiple feedback bandpass filter is what's commonly used for the bands of the EQ. However as per the requirements of the project, each band must have +/- 10dB gain adjustments. So I had planned to use an inverting amplifier at the end of each band pass stage for gain adjustments (inverting since the output of the MFB filter is inverted). Then all of the outputs of each inverting amplifier be summed using a summing amplifier.The digital interface requires me to use digital pots (requirements of the project) to replace the analog pots you normally see on a graphic EQ. My plan was to utilize the digital pot in the feedback loop of the inverting op amps which would be controlled by an arduino for gain adjustments. However that's the least of my worries right now. I plan on getting a fully working analog system before I integrate the digital aspect. It seems easier that way for me.I came across one of these write ups where someone designed a 5-Band EQ using MFB filters as well.https://www.transkommunikation.ch/dateien/schaltungen/diverse_schaltungen/audio_circuits/5-Band%20Graphic%20Equaliser.pdfWhat i've noticed is that instead of using an inverting stage at the end of each MFB filter stage, the person used 4.7k pots at the end of each MFB stage and then fed all the outputs into a SINGLE inverting amplifier stage. This looks like a much cleaner way of doing it.However they didn't provide a rationale for why they used these size potsLike what gain adjustments would this allow for?How did they go about calculating the size of the pot to provide a specific range of gain or attenuation for each band (This is what has been puzzling me the most).I have a few other minor questions regarding some stuff in their design but those were the major things so far that i'm having trouble understanding.Thanks alot in advance guys.Will keep you posted along the journey of the project...i'm really looking forward to it

author: JS
author_rank: Frequent Contributor
timestamp: September 07, 2018, 06:55:41 pm
content: That design is just wrong, first the input stage bias is wrong, sitting at about 2V instead of 6 as it should, then the first cap will be biased backwards.Next, dividing in frequency with jelly bean components and then summing and expecting things to sum nicely is just wrong, it won't happen, even if you select and trim the drift will make it go away. A flat EQ should have no effect in the signal. Also, the filters will add some distortion around the cross frequency, depending on the quality of the caps mainly. Then, if you are just adding a portion of the needed signals instead of passing all the signal through the filters you have a much cleaner solution.To archive that a different topology is used, having a straight signal path and then adding or subtracting the filters, then, to deal with zero tolerances at pot mid point you can just disconnect the band at 0 or mute it's output.There are several professional audio schematics out there, many, if not most, if not all pro grade graphic eqs use this approach, is not more complex, not even in the parts count, but it is way better.JS

author: Benta
author_rank: Super Contributor
timestamp: September 07, 2018, 07:23:59 pm
content: Before proceeding, I strongly encourage you to use a bipolar supply for the complete circuit. Your "virtual ground" is not going to work well, you need a firm signal ground for audio. This also gives the best PSRR for the amps.Concerning the architecture, I recommend that you start reading up on frequency/phase responses for filters. This is not trivial, simply adding/subtracting amplitude responses will NOT work.

author: djacobow
author_rank: Super Contributor
timestamp: September 08, 2018, 01:07:26 am
content: Go "software defined."Why not ADC => FFT => apply filter shape => IFFT => DAC?With today's compute hardware, grunty FFT is cheap. And you learn DSP in the process!

author: Ronaldo95163
author_rank: Contributor
timestamp: September 08, 2018, 02:13:08 am
content: Thanks for the tips guys.The thing with it is it's a school project so i've gotta work with whatever's at my disposal from the school (Which are jellybean components) after doing my research and coming up with designs etc.This is what I got to work with (See attached image)I had the same queries with my teacher abt the power supply issue. When we meet again i'll clarify exactly, because it didn't have any mention of it in the outline of the project. A bipolar power supply would be the most feasible thingQuote from: JS on September 07, 2018, 06:55:41 pmWhat was their reasoning for going with that design decision?Again the article is a bit vague and they didnt justify alot of their steps.

author: JS
author_rank: Frequent Contributor
timestamp: September 08, 2018, 03:23:43 am
content: As Benta said, it won't sum nicely, while there's a way to make it work it's still not good for eqs, it's used in some other multiband processing but not for this.Also 15 bands is quite a lot for a single stage, so you usually use two or three and interlive bands in each stage, not putting consecutive bands together to minimize interaction. Here we are just optimizing the design for better quality, respinse and usability.I don't know why they chiose that, looks like it might work, it's a first approach to solve the riddle but it's not a good solution at all.JS

author: Ronaldo95163
author_rank: Contributor
timestamp: September 08, 2018, 07:27:56 pm
content: Saw someone used a similar approach to what I was thinking about on youtube. Doesnt sound too badwish they did more follow up vidsI'm kinda worried myself as to how it would work considering the jellybean components we have at our disposal.

author: oPossum
author_rank: Super Contributor
timestamp: September 09, 2018, 01:40:17 am
content: Every graphic EQ that I have seen a schematic of used gyrators, or in a few rare cases inductors.A 'textbook' graphic EQ using gyrators...

author: JS
author_rank: Frequent Contributor
timestamp: September 09, 2018, 02:28:58 am
content: Quote from: oPossum on September 09, 2018, 01:40:17 amInteraction between bands in that circuit will be noticeable, if splitting the same circuit as IC1/2 into two or three identical ones in a chain with interleaved bands in each stage will make it more easy to use, as moving one band will only move that band and leave the effect of the consecutive bands untouched.Some pro gear use band pass filters instead of gyrators, giving the designer more control over the shape of each band, but using SVF for each stage gets messy pretty quickly in a 15 bands EQ for a school project.JS

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: September 09, 2018, 04:00:47 am
content: I'd think you'd want a virtual ground on the output amp so you can sum all the filters properly, IOW, ground the non-inverting input. Such a scheme is well behaved because you can then analyze the filters separately.

author: boB
author_rank: Frequent Contributor
timestamp: September 09, 2018, 04:59:22 am
content: oPossum's gyrator circuit is a very good way to go. Much more than 10 bands done that way though and you may very well run into some problems as JS mentioned. I seem to remember the problem was more noise but interaction may be another issue.The "Q" and center frequency of the filters should be done correctly as well. If you say, boost or cut all of the sliders, the overlap of two adjacent filters should be at their -3dB point so it comes out flat response.I know there are 10, 20 and 30 band ISO center frequencies (and Q ?) but I don't know if there is or was one for 15 bands.These days, I kind of like the DSP method idea.boB

author: JS
author_rank: Frequent Contributor
timestamp: September 09, 2018, 08:01:43 am
content: There are 15 bands eqs, 31 being the third of octave evenly spaced bands covering the whole audio range, 15 is not so hard to see as half that many bands, the frequencies would be different in each decade, as expected but covering the whole range two thirds of octave at the time. I think it's more common than 20 bands.I don't think they need to sum perfectly at full boost our cut, would be nice but in analog design is pretty hard to do and not to speak with gyrators where the q would bet different for boost and cut.JS

author: Ronaldo95163
author_rank: Contributor
timestamp: September 10, 2018, 03:00:20 pm
content: Ive come across the gyrator approach but didnt really give it much consideration for the same reason of it being 15 bands and they may overlap.And yeah I also couldn't really find any standards for Q and center frequencies for 15 bands more than just some pictures of software 15 band EQs which id post later.Also wouldn't it better to not use the virtual ground approach on the op amps...even the summing one? To me it seems better not to to get the full swing of the op amp on the output.I derived the transfer function yesterday for the MFB bandpass filter which I could use for designing once I figure out what center frequencies to use.And yes the overlap should occur in the -3dB ranges...hoping this works out nice for me as it did for those folks in the vid lol

author: Lee Leduc
author_rank: Regular Contributor
timestamp: September 10, 2018, 04:04:08 pm
content: Here's some info you may find useful."Designing with the LMC835 Digital-Controlled Graphic Equalizer" Chip is obsolete but documentation has some good design info on selecting "Q". Application note attached.Elliott Sound Products has about 10 EQ design projects.http://sound.whsites.net/projects-0.htm#equRane has a good free tech library of EQ, and other professional audio processing things.https://www.rane.com/library.html#gpm1_1

author: Ronaldo95163
author_rank: Contributor
timestamp: September 11, 2018, 04:25:50 pm
content: Woa thanks man some really good info thereEspecially for the band pass filterI managed to find out that its the iso 2666 standard that eqs center frequencies are based on15-17 band eqs should use bands with 2/3 octave spacing.The freqs for a 15 band eq according to the iso standard are25 40 63 100 160 250 400 630 1k 1.6k 2.5k 4k 6.3k 10k 16kCouldn't find anything being mentioned about recommended Q for a 15 band how ever.

author: boB
author_rank: Frequent Contributor
timestamp: September 13, 2018, 06:10:22 pm
content: Here is a good little article on Q and graphic EQ design...https://www.rane.com/note101.html

author: Audioguru
author_rank: Super Contributor
timestamp: September 14, 2018, 03:30:02 am
content: The multiple feedback bandpass filters your circuit has show a bandwidth too narrow near its peak and a bandwidth too wide farther down from the peak.

author: Ronaldo95163
author_rank: Contributor
timestamp: September 15, 2018, 10:20:06 pm
content: Quote from: Audioguru on September 14, 2018, 03:30:02 amYes this is what i'd like to design for, however I still can't really find anything on selecting Q for a 2/3 octave EQ. I've read the links posted above and they mostly covered 1/2 and 1/3 octave...one mentioned 2/3 I think but didnt say anything on a Q value based on the ISO 266 standard. Q is 4 for a 1/3 octave EQ based on the ISO 266 standard btw.Also what simulation software is that you used?Quote from: Lee Leduc on September 10, 2018, 04:04:08 pmCame across this from the first linkhttp://sound.whsites.net/project75.htmHis design for boosting and cutting the various frequency bands look interesting but were a bit confusing to me particularly the adding of the signal to the CUT circuit I believe first then adding the result to the BOOST circuit.AlsoHe 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.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 12dBMy idea was after each bandpass filtering stage to add an inverting amplification stage after it and do the gain adjustments there then sum the outputs of all the amplification stages with a summing amplifier.But if this topology looks much cleaner as it eliminates the amplification stage after each bandpass filter.

author: JS
author_rank: Frequent Contributor
timestamp: September 16, 2018, 02:45:55 am
content: Be careful what you call Q in an equalizer, that's a pretty subjective topic (there are many different uses of Q in EQs, not everybody uses the same definition of Q)In fact, in a real design you should probably choose what band widths you want for each band, are they constant at any level or changes depending on boost/cut?If I were making the choice I'd probably align them to be flat at some mid level, between 3dB and 6dB, where most will be used close to and having a smoother response there seems better to me than at maximum points where raerly will get to, even less a few in a row, except maybe for cleaning up low end where you don't expect it to be heared at all but cleanin the stage monitors which can't reproduce properly the LF you are cutting away.I like symmetrical filtering, but that's not the rule, which is usually symmetrical Q (with some definition of it, like taking -3dB at maximum settings) which ends with very different filter shapes for boost and cut. There's more a discussion about this in semiparametric eqs than graph but it could apply or worth looking for someone who doesn't know about the topic and is dealing with all this.JS

author: oPossum
author_rank: Super Contributor
timestamp: September 16, 2018, 03:15:28 am
content: Quote from: Ronaldo95163 on September 15, 2018, 10:20:06 pmThe 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.QuoteU2B 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.

author: Ronaldo95163
author_rank: Contributor
timestamp: September 17, 2018, 05:26:55 am
content: Quote from: oPossum on September 16, 2018, 03:15:28 amhttp://rane.com/pdf/constanq.pdfManaged 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/findingsThis project is exposing me to alot of really new stuff

author: Ronaldo95163
author_rank: Contributor
timestamp: September 23, 2018, 04:09:00 am
content: 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.

author: Ronaldo95163
author_rank: Contributor
timestamp: October 01, 2018, 04:24:13 pm
content: Quote from: oPossum on September 16, 2018, 03:15:28 amThis 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.

author: Ronaldo95163
author_rank: Contributor
timestamp: October 02, 2018, 10:41:32 am
content: 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.

author: Ronaldo95163
author_rank: Contributor
timestamp: October 23, 2018, 04:30:00 pm
content: 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 analysisThe 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 eitherI'm honestly stuck now...not sure what else to do or what could be the problem