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Old 24th Jun 2022, 9:13 pm   #1
Bufo Bill
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Default Help with passive eq

Let's take the Alcoholics Anonymous route: Hi I'm Bill and I . . . Play the five string banjo! I know, filthy habit.
I have just built a piezoelectric pickup for my banjo, and I want to dally ever further down the path of madness and build a passive EQ for it.
I intend to use 5 band pass filters with individual volume and guitar style "tone" log pots.
What I am thinking is RC filters, from 60 Hz to 6kHz. Maths is not my strong point however and I am having trouble understanding the calculation process. Can a kind and patient soul give me some help? Is what I want to build even feasible?
Cheers from Bill.
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Old 24th Jun 2022, 11:32 pm   #2
fetteler
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Default Re: Help with passive eq

Hi Bill,

For a start, I know all the jokes about banjos but IMHO they are a fine instrument and you have nothing to be ashamed of

As for your idea about passive EQ well, I'm not so sure. A simple passive tone control is certainly possible but these transducers are such high impedance that my own approach would be to make it active. Multi band EQ on guitars is always done this way as far as I know... well it certainly is on the two I have owned in my time.

Now, have you heard the one about the definition of perfect pitch?

Steve.

PS Check this out and have a think about adapting something like it to your needs:
https://www.ebay.co.uk/itm/254848251...Cclp%3A2047675
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Last edited by fetteler; 24th Jun 2022 at 11:38 pm.
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Old 25th Jun 2022, 12:08 am   #3
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Default Re: Help with passive eq

As Steve has mentioned. I think you will need go active. Sure you can use passive controls, but without some gain the bandwidth of simple RC networks will be too broad in range to do much.
A guitar is really only treble cut, there is no way to boost any frequencies without having some gain.
With gain, you can control feedback ( one way ) and make the filters much sharper.
Five bandpass filters for a passive system is really not very easy to do.

Joe

p.s. I have LOTS of bluegrass LP's.
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Old 25th Jun 2022, 5:34 am   #4
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Default Re: Help with passive eq

You're heading into graphic equaliser territory. You can either do them with L-C resonators, or else as active bandpass filters to avoid inductors. As the frequency ranges of strings overlap, you won't get string by string separation of adjustment, the best you can do will be to have EQ bands centred on the mid range of each string.

Pick up a graphic equaliser and have a play with it to see if you like what it does. They're cheap, the hifi world really looks down on them. Combine one with a banjo to blow a right raspberry at detractors in general

David
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Old 25th Jun 2022, 11:39 am   #5
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Default Re: Help with passive eq

I don't know about centering the bands on each string as the notes of the open strings are only a small fraction of the notes that will be played. Also there is a huge percussive element to the banjo sound and the frequencies that you would tweak to adjust that component of the overall timbre are probably fairly unrelated to the open string notes.

Having a play with a graphic eaqualiser is a good idea to start with but it's probably even more useful to have a go with a preamp/equaliser designed for a stringed instrument like a guitar. Lots of these pedal type accessories are available inexpensively and you may find that they are all you need and there's no need to re invent the wheel so to speak.

Steve.
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Old 25th Jun 2022, 12:54 pm   #6
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Default Re: Help with passive eq

Hello,

Alright this is not passive, but there is the Wien Bridge Parametric Equaliser.

I’ve used this in conjunction with a Baxandall tone control circuit to give HF and LF EQ centred on 'say' 500Hz and the Parametric Equaliser is used to give a ’sweepable’ middle control. With the right values you can get a middle control that sweeps between a couple of hundred Hz and a couple of KHz.

Link to some info on the Elliott Sound Product site

https://sound-au.com/project150.htm

Regards
Terry
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Old 25th Jun 2022, 4:14 pm   #7
Bufo Bill
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Default Re: Help with passive eq

Okay, I can go with active eq.
Fettler has a point that has been troubling me too. The pickup is taped with duct tape onto the Banjo "skin" (we call it the "head" of the banjo). Part of my technique is to beat the head with my thumb as I pick the 5th string (the Banjo Community looks at this like if you ordered lobster thermistor then asked for brown sauce). This is why I chose a low starting point in my frequency range, and as pointed out this pickup solution is one reason why you can't have a channel for each string.
I will have a look at cheap graphic eq's and guitar oriented preamp and eq combos, but the point of this exercise is the "journey" of the design and manufacture of a unit rather than the "destination" of annoying the neighbours even more loudly. I will check out the links, have a ponder and get back to you.
Cheers from Bill.
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Old 25th Jun 2022, 6:05 pm   #8
Cruisin Marine
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Default Re: Help with passive eq

I reckon you won't get away with a passive eq.

There are plenty of three band designs you could modify for your preferred frequencies, there is much info. in the National Semiconductors Audio handbook, three band on page 2-49, a full ten band on page 2-55 full details on design are in that fine book.

Here is a pdf for download
https://ia800103.us.archive.org/3/it...ioHdbk1977.pdf

I highly recommend getting a hard copy of that book, I have used mine many a time over the years.

Last edited by Cruisin Marine; 25th Jun 2022 at 6:11 pm.
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Old 25th Jun 2022, 7:30 pm   #9
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Default Re: Help with passive eq

Terry's suggestion is interesting and the article looks to answer some of the design queries I have.

That National Semiconductors book looks great so thanks for that suggestion @Cruisin Marine.
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Old 27th Jun 2022, 3:46 pm   #10
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Default Re: Help with passive eq

Well, there seems to be a surprisingly good result here.
The pickup is not at all what I expected from hearing people from the "Cigar Box" guitar community talk about these piezos.
The sound reproduction is excellent and doesn't need to be "tamed" with EQ at all.
I had a look at what was on offer on EBay, and some of the EQ pedals look to be very similar to what I had in mind.
I would still like to build one with a tone pot and boost and cut for each channel sometime soon however, the idea being to use the tone pots as low pass filters, just like on an electric guitar and then boost or cut the resulting signal. I'm not sure if that will work but it will be fun finding out!
You guys are great, your suggested reading has really inspired me. Working my way through the Semiconductors book just now.
Cheers from Bill.
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Old 1st Jul 2022, 5:05 pm   #11
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Default Re: Help with passive eq

Right, I've had a think and I'm going to build an octave equaliser, like in the National Semiconductors Audio Handbook mentioned above. It will certainly be a learning process, and hopefully a lot of fun too. I might need some help finding one or two of the small capacitors needed. I'll make a request in the appropriate section if I can't find some.
I would like to hear if you have any suggestions on how best to power such a device too.
Cheers from Bill.
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Old 1st Jul 2022, 10:16 pm   #12
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Default Re: Help with passive eq

I'm very happy to hear that you are on the road to success Bill.

Twang Twang!!!

All the best,
Steve.
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Old 2nd Jul 2022, 7:24 pm   #13
Cruisin Marine
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Default Re: Help with passive eq

I would like to hear if you have any suggestions on how best to power such a device too.
From memory, I think those LM type op-amps are designed to work off single rail, so no half rail/false zero rail line is needed.

If you use "normal" op-amps a false zero rail will be needed unless you have a + zero and - supply.
Easily done on a single rail supply with a potential divider of say two 470-560 ohm resistors bypassed with a couple of 100-470 uF caps will do nicely.
As for PSU required you will have to look up the current consumption of devices you choose to use and see whether a battery supply will suffice or not, and evaluate how "mobile" you wish to use the graphic of course.

As for voltage, I would hope 9 to 15 volts would do, check the spec of the op-amps maximum rated voltage.

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Old 3rd Jul 2022, 10:14 am   #14
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Default Re: Help with passive eq

I would like it to run off mains, as I am terrible at removing batteries to prevent leaking. I suppose SMPS would be my best option, I had thought about this to power the three IC's in parallel:
https://uk.rs-online.com/web/p/switc...pplies/1655098
This is totally new to me, it's quite a shock to find such a big gap in my knowledge, so feel free to say if I'm getting this wrong.
Cheers from Bill.
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Old 3rd Jul 2022, 12:59 pm   #15
Cruisin Marine
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Default Re: Help with passive eq

That should do whatever Op-amps you decide to use.
The alternative is buy a ready built second hand equaliser, Maplin used to sell one, there will be many more.
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Old 3rd Jul 2022, 2:44 pm   #16
Bufo Bill
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Default Re: Help with passive eq

Yeah, there were some nice units on ebay too, but I don't get the joy of learning that way. Or the sense of accomplishment - I reckon it's down to my fragile male ego!
You've been a great help so far, so thanks very much. I have no doubt I'll need help again before I've completed this project!
Cheers from Bill.
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Old 5th Jul 2022, 5:55 am   #17
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Default Re: Help with passive eq

If you put one of those SMPSs inside your unit you have to take care about where the mains voltages are. It's usually easier to use a plug-in PSU -- which is why so much low power equipment comes with those nowadays.
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Old 5th Jul 2022, 5:05 pm   #18
Bufo Bill
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Default Re: Help with passive eq

Quote:
Originally Posted by RichardGM View Post
If you put one of those SMPSs inside your unit you have to take care about where the mains voltages are. It's usually easier to use a plug-in PSU -- which is why so much low power equipment comes with those nowadays.
Thank you Richard, it's been something I've been pondering myself today. Not having a separate power unit would be a bit of a boon for me, but safety first! I had wondered about containing all the mains and SMPS within a plastic project box WITHIN the unit, but I am worried about heat.
Any thoughts, you chaps?
Cheers from Bill.
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Old 5th Jul 2022, 8:54 pm   #19
Cruisin Marine
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Default Re: Help with passive eq

Bill, you can run it all on a single rail supply.
I was inferring to this (albeit badly) above.

If you use a single rail supply, you will need a half rail the equivelant of the zero volts rail for the op-amps when they run on a plus and minus voltage supply.

All that is needed is a simple voltage divider of two resistors which need to be de-coupled, hardly any current will be needed, it is so simple.

It would enable you to use a single rail external wall wort type supply.

I would personally get a secondhand graphic and use that, especially as the metalwork for mounting faders would be some work indeed for a multi-channel graphic.
Even if you decide to rip the guts out of the original, the metalwork will already have been done, and the cost will be cheaper than a new build too.

If you get the Maplin one(s), I am sure there is plenty of info on those that you can get hold of.

Last edited by Cruisin Marine; 5th Jul 2022 at 9:06 pm.
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Old 5th Jul 2022, 10:05 pm   #20
Bufo Bill
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Default Re: Help with passive eq

Yes, I see. A voltage divider and wall-wort would be much easier. I was hoping to avoid using a WW, but that seems the best solution. I'll mull it over.
Cheers from Bill.
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