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Project Specific Discussions => Filters/Equalizers => Topic started by: OneRoomStudios on August 29, 2019, 12:32:11 PM

Title: The People's Equalizer
Post by: OneRoomStudios on August 29, 2019, 12:32:11 PM
**If this should be in Drawing Board, feel free to move**

 I've been thinking about EQ's a lot lately. There aren't a lot of DIY projects available for EQs, and the ones that are out there tend to fall into one of three categories: passive LC with tube/DOA make up (Pultec-style), active inductive (Neve/API style), or gyrator. There are a few other interesting projects here and there too, but they often suffer from the same issues as the others I mentioned - they're usually hard to get a hold of, discontinued, expensive to build, or not something I'd want to bother with.

I've been wishing for an EQ project that was easy to tune to my tastes (not dependent on inductors), could be built with either cheap parts or fancy parts as budget permits, and most of all, is both great-sounding and highly useful. I've been tossing around ideas for a while and have come up with what I think is a good concept, but I'd like to open the project up to others here to solicit feedback, and also to get some help with the PCB layout so we can have a board that accommodates lots of options.

The idea is this - an EQ that could be built with either That Corp line receivers /drivers, and IC opamps, for a cheap/quick build, or with transformers, discrete buffers, and DOAs for more 'color.' You can think of it as an EQ that could grow with you and your budget. The topology I'm thinking consists of active HP/LP filters, Baxandall shelving filters, and a Wien Bridge mid-band. This gives you a pretty flexible EQ, with a minimum of components and hard-to-get parts.

Here are some details as I'm thinking of them:

I'm partial to the HP/LP filters used in the Urei 539 (http://www.jblproservice.com/pdf/Vintage%20JBL-UREI%20Electronics/UREI-539.pdf (http://www.jblproservice.com/pdf/Vintage%20JBL-UREI%20Electronics/UREI-539.pdf)). Both of them are wrapped around a single opamp (not including the buffer driving them), and I like the 1 or 2 pole switch on the LP - very useful. With 150nF caps and a dual 100K pot, the HPF would range from around 10Hz to around 320Hz, which feels pretty good to me. With 1nF caps and a dual 20K pot, the LPF would range from around 25KHz (closer to 30KHz in single-pole mode) down to around 6KHz - also pretty nice.)

The Baxandall shelves can also both be wrapped around a single opamp, like Rod Elliot does here: https://sound-au.com/dwopa2.htm#s7 (https://sound-au.com/dwopa2.htm#s7). It's easy enough to calculate a handful of different cap values for each band to provide 4-6 frequency selections (for example, the high shelf could use a 10K pot, 1K5 resistors, and 1n2, 1n8, 2n7, and 3n9 caps to provide 16KHz, 10KHz, 7KHz, and 5KHz shelves).

The Wien Bridge mid-band is a simple pseudo-parametric  choice for the mid frequencies that only needs a single opamp. You could even cascade more than one if you wanted more bands. Rod Elliot has another nice example here: https://sound-au.com/articles/eq.htm#s7 (https://sound-au.com/articles/eq.htm#s7). With 3n3 caps and 100K and 10K pots, you'd get +/- 12dB sweepable between 430Hz and 4.8KHz.

Building something like this with integrated line receivers/drivers and IC opamps would be very inexpensive, and the ability to build it with transformers, DOA's (3 needed), and discrete buffers would make it a relatively simple-to-build discrete EQ.

I mocked up an example front panel for those who need a visual.
(https://ibb.co/b7bbCmT)

What would make this especially enticing is if a PCB could be made to have footprints for both the monolithic ICs, and DOAs/various transformers - a universal/upgradable board.

Thoughts?
Title: Re: The People's Equalizer
Post by: shabtek on August 29, 2019, 04:39:45 PM
You may be describing “perf board “
Title: Re: The People's Equalizer
Post by: OneRoomStudios on August 29, 2019, 05:12:37 PM
You may be describing “perf board “

Ha!

There wouldn't actually be that much to making it adaptable. There are already lots of kits that allow for either discrete or monolithic opamp (JLM Baby Animal is a good example), and there are kits out there that allow for either discrete/transformer output or balanced line driver (see DIYRE EQP-5). The rest of the circuit would be worth having on a PCB for easy assembly and modification. I just don't have the footprints/layouts for this kind of thing handy, so was hoping someone could help out with that part.
Title: Re: The People's Equalizer
Post by: abbey road d enfer on August 30, 2019, 09:59:07 AM
Ha!

There wouldn't actually be that much to making it adaptable. There are already lots of kits that allow for either discrete or monolithic opamp (JLM Baby Animal is a good example), and there are kits out there that allow for either discrete/transformer output or balanced line driver (see DIYRE EQP-5). The rest of the circuit would be worth having on a PCB for easy assembly and modification. I just don't have the footprints/layouts for this kind of thing handy, so was hoping someone could help out with that part.
I don't understand what it is you're stumbling on. Do you have a PCB software? They all offer the possibility to create your own devices and footprints.
Now you can't provide footprints for all types of xfmrs, but you can choose a couple of candidates.
Title: Re: The People's Equalizer
Post by: OneRoomStudios on August 30, 2019, 10:27:50 AM
I don't understand what it is you're stumbling on. Do you have a PCB software? They all offer the possibility to create your own devices and footprints.
Now you can't provide footprints for all types of xfmrs, but you can choose a couple of candidates.

I have and use Eagle CAD, but I figured that some other folks might have the footprints handy (preferably in Eagle format) for DIP 8/DOA and for multiple transformers (as JLM does). Obviously not all transformers would be able to fit, but it'd be nice if it could fit the usual suspects from Cinemag/Jenson/Lundahl, etc. I also thought some folks might have input on the topology or concept.

Is this really that strange of an idea? It seems like this place used to be more open to this kind of collaborative idea in the past. Maybe I'm barking up the wrong tree.
Title: Re: The People's Equalizer
Post by: abbey road d enfer on August 30, 2019, 10:52:06 AM
I have and use Eagle CAD, but I figured that some other folks might have the footprints handy (preferably in Eagle format) for DIP 8/DOA and for multiple transformers (as JLM does). Obviously not all transformers would be able to fit, but it'd be nice if it could fit the usual suspects from Cinemag/Jenson/Lundahl, etc. I also thought some folks might have input on the topology or concept.

Is this really that strange of an idea? It seems like this place used to be more open to this kind of collaborative idea in the past. Maybe I'm barking up the wrong tree.
I also use Eagle, and have grown weary of using third-party models and footprints. I always make my own. Good for the brains and good for the hair (not pulling it).
Title: Re: The People's Equalizer
Post by: john12ax7 on August 30, 2019, 03:16:28 PM
+1 on making your own footprints.   You do it once and then have it for all future designs.

I get the concept you are going for,  the issue that often comes up with the one size fits all approach is that everyone inevitably wants something different.  So you need to figure out how to overcome the too many chefs in the kitchen situation.
Title: Re: The People's Equalizer
Post by: abbey road d enfer on August 30, 2019, 04:04:47 PM
Is this really that strange of an idea? It seems like this place used to be more open to this kind of collaborative idea in the past. Maybe I'm barking up the wrong tree.
Possible; you may get answers you'd prefer addressing your request to the Eagle forum.
Title: Re: The People's Equalizer
Post by: Ricardus on August 31, 2019, 11:46:20 AM
So I have a friend who is a talented engineer/tech/designer. Some of you might have even heard of him. I'm always telling him about the projects I'm working on, and how I have to order unobtanium parts from surplus houses all over the place (and in other countries), and he appreciates that it keeps my happy and occupied, but his reply one day was something like, "That's not my thing anymore. My goal now is to design great sounding stuff with easily obtainable  components that will likely still be available 20 or 30 years from now."

So I like the idea of this People's EQ. The Volks EQ, if you will.

When I first joined this forum I was surprised there wasn't one of every kind of device, designed through collaboration and crowd sourcing. A GroupDIY EQ, a GroupDIY Preamp, a GroupDIY Compressor, etc....

There's so much talent up here it's amazing. Would love to see something like this.

Title: Re: The People's Equalizer
Post by: RuudNL on August 31, 2019, 11:53:28 AM
Some time ago I developed a very simple equalizer: shelving low with variable turnover frequency, four overlapping full parametric filter sections and shelving high with variable turnover frequency. All + or - 15 dB. Also a gain control +/- 12 dB in case you gain or lose level due to heavy equalization... All pretty cheap parts, no 'exotic' integrated circuits but 'good old' TL074 etc.
Works perfectly. It can make right what is wrong. (And also wrong what is right... :D)
Baxandall circuits and schematics for parametric sections you can find online.
This thing has proven to be a problem solver!
Title: Re: The People's Equalizer
Post by: Ricardus on August 31, 2019, 12:07:13 PM
Very nice.
Title: Re: The People's Equalizer
Post by: RuudNL on August 31, 2019, 12:33:47 PM
This is what I used for the parametric sections. The only 'difficult' part is the dual reverse-log 100 K potentiometer.
(You could also use a rotary switch with resistors of course, if you prefer repeatability.)
Title: Re: The People's Equalizer
Post by: Ricardus on August 31, 2019, 04:34:55 PM
**If this should be in Drawing Board, feel free to move**

 I've been thinking about EQ's a lot lately. There aren't a lot of DIY projects available for EQs, and the ones that are out there tend to fall into one of three categories: passive LC with tube/DOA make up (Pultec-style), active inductive (Neve/API style), or gyrator. There are a few other interesting projects here and there too, but they often suffer from the same issues as the others I mentioned - they're usually hard to get a hold of, discontinued, expensive to build, or not something I'd want to bother with.

...SNIP...

What would make this especially enticing is if a PCB could be made to have footprints for both the monolithic ICs, and DOAs/various transformers - a universal/upgradable board.

Thoughts?

 Well if you're asking for ideas, for me an ideal channel EQ would be 4 band. Low, low-mid, high-mid, high, and maybe  a high pass.

Top and bottom bands can be switchable between shelving and peaking.
Title: Re: The People's Equalizer
Post by: alexc on September 02, 2019, 01:31:11 AM
My opinion is ....

- there is a need for a stereo unit, one set of controls for both channels

For each channel of the stereo [only] pair, a number of buffered 'filtered sections' in-line (series)  with each other  ...  quality chips are  great for this purpose.

Done with some 'alps' pots for gain, some suitablely modest 'decked rotary' switches for freqs,   and a simple toggle for each band 'in/out' 

..  with an overall relay bypass (for the stereo channels)  and of course, balanced inputs and outputs. One might add a traffo O/P option if one so desires .... 

I like a DRV chip bal output :)  as well as a properly driven traffo output [ both types available at same time]

Further, I like the 'Bax type of eq'  circuits done in the style of 3-band  but  with seperate 'hp and lp' filter sections.   :)

What's not to like? for the People  8)
Title: Re: The People's Equalizer
Post by: Phrazemaster on September 08, 2019, 09:54:07 PM
.
Is this really that strange of an idea? It seems like this place used to be more open to this kind of collaborative idea in the past. Maybe I'm barking up the wrong tree.
I think it’s a great idea. Don’t give up. Forums are weird; sometimes they behave the exact opposite of what you think they should.

Also the culture here is generally fierce independence, even in terms of sharing, so I think less collaboration occurs because of that.

I’m hopeful someone will step up and want to offer ideas and help for you.

Best,

Mike
Title: Re: The People's Equalizer
Post by: OneRoomStudios on September 09, 2019, 04:02:03 PM
Thanks to everyone for the ideas and encouraging words! I've been thinking about the design some more, and it seems like the easiest and most universally-appealing design might be:

-Stereo with one set of controls (you can always use just one channel if you need a mono EQ)
-Four bands plus filters set up as:
Hi-Pass, Low Shelf (Baxandall), Low-Mid (Wien Bridge), High-Mid (Wien Bridge), High Shelf (Baxandall), Low Pass
-While it would be nice for the high and low bands to be switchable between peaking and shelving, the combination of shelving and hp/lp filters can result in peaking, and it keeps the circuit simpler (plus, I like the sound of Baxandall shelves)
-This would require 4 opamps per channel (not including line drivers/receivers). 8 opamps for a stereo unit might be a few too many to use DOAs, but the option could be there.
I still think the option to use transformers or IC receivers/drivers would be nice

I'd be happy to start piecing together a schematic if someone were willing to take on the pcb layout (I don't have the time to do both).
Title: Re: The People's Equalizer
Post by: Ricardus on September 10, 2019, 09:22:29 PM
What about the stereo unit as you described, and then eventually a single channel unit as well? I'm not a design guy but a single channel unit should be easy to carve out of a stereo unit, yes?
Title: Re: The People's Equalizer
Post by: ruffrecords on September 11, 2019, 07:27:40 AM
I created something similar back is 2012. the universal passive EQ. There seemed to be lots of interest, I had some boards made but hardly anyone bought them. I eventually more or less gave away the ones I had left and open source the PCB layout.

https://groupdiy.com/index.php?topic=50484.msg639985#msg639985 (https://groupdiy.com/index.php?topic=50484.msg639985#msg639985)

Cheers

Ian
Title: Re: The People's Equalizer
Post by: OneRoomStudios on September 11, 2019, 04:52:08 PM
I created something similar back is 2012. the universal passive EQ. There seemed to be lots of interest, I had some boards made but hardly anyone bought them. I eventually more or less gave away the ones I had left and open source the PCB layout.

https://groupdiy.com/index.php?topic=50484.msg639985#msg639985 (https://groupdiy.com/index.php?topic=50484.msg639985#msg639985)

Cheers

Ian

Thanks Ian,  that's definitely a cautionary tale.  I didn't know about that project (must have missed it somehow).  That's exactly the reason why I'm not too keen on doing all the PCB layout/ordering myself. I've had to eat the costs of PCBs and front panels before, not to mention the time spent in design. I was hoping there would be enough other interested people to help hoist this project into reality, but there may not be.
Title: Re: The People's Equalizer
Post by: Ricardus on September 11, 2019, 08:25:31 PM
When guys order PCBs up here, like for the Bo Hansen DI or these other projects, how many boards do they usually order?
Title: Re: The People's Equalizer
Post by: ruffrecords on September 12, 2019, 04:10:47 AM
When guys order PCBs up here, like for the Bo Hansen DI or these other projects, how many boards do they usually order?

My own experience is mostly a couple.

Cheers

Ian
Title: Re: The People's Equalizer
Post by: Ricardus on September 12, 2019, 09:34:24 PM
My own experience is mostly a couple.

Cheers

Ian

Nonono.

 I mean the people who stock them.
Title: Re: The People's Equalizer
Post by: Ricardus on September 12, 2019, 09:37:47 PM
My thoughts on this EQ and other projects like this, is I would definitely buy some if it fit my needs. A stereo EQ with one set of controls is the kind of thing I would probably only build one of, TWO MAX.

 But individual "channel" EQ units I would build many many. Like 8 or more (in time).

When I discovered the Bo Hansen DI I initially ordered two boards. I loved the sound so much my goal was then to build 2 with every supported (on the board) transformer flavor, which would have been a total of 6. Which I did. Then I became so obsessed with making another pair with Carnhill traffos, I did that too. That made 8.

I'm looking for a nice channel EQ option, and I am hoping to find it up here one of these days.
Title: Re: The People's Equalizer
Post by: ruffrecords on September 13, 2019, 03:02:54 AM
Nonono.

 I mean the people who stock them.

Do you mean how many boards to sellers usually order from the PCB manufacturers?

Cheers

Ian
Title: Re: The People's Equalizer
Post by: Ricardus on September 13, 2019, 10:45:30 AM
Do you mean how many boards to sellers usually order from the PCB manufacturers?

Cheers

Ian

Yes. To get the best prices and such.
Title: Re: The People's Equalizer
Post by: ruffrecords on September 13, 2019, 05:27:26 PM
Yes. To get the best prices and such.

Depends on the size of the board and the expected demand. Most boards I get in batches of just 10 (passive EQ and tube mic pres). The only board I consistently sell more of is the little DB25. I now order a batch of 50 per year.

Cheers

Ian
Title: Re: The People's Equalizer
Post by: cpsmusic on April 14, 2020, 08:14:06 AM
I'm interested in this too although more from a user perspective as opposed to a designer's.

The more I though about the "Volk EQ" the more I thought that the best place to look for inspiration would be a well-thought-out channel strip EQ.

Cheers!
Title: Re: The People's Equalizer
Post by: Ricardus on April 14, 2020, 11:15:10 AM
I'm interested in this too although more from a user perspective as opposed to a designer's.

The more I though about the "Volk EQ" the more I thought that the best place to look for inspiration would be a well-thought-out channel strip EQ.

Cheers!

 Yes, exactly.
Title: Re: The People's Equalizer
Post by: fragletrollet on April 16, 2020, 12:25:24 AM
I love the idea. Personally I'd love more diy stereo eq's available, preferable with switched frequencies with resistors on the main or aux pcb. I can sacrifice customizability for less wiring/ease of build, as (imo) there's a lot of more open ended pcb's that allow more customizable setups. This way you could make a standardized frontpanel layout that the pcb slips into.

Hp filters a must, low pass is also nice so I'd vote for both. Baxandall shelves are also great, and two midrange (low mid and high mid, but even better with the midrange bells can overlap frequencies, but not a must) would make it a powerhouse.

I've been eyeing the jlm freq500, that's the closest thing I've found to your idea (but mono). I work with a lot of stereo sources so a stereo solution would be great.

I also love high density modules, as space is scarce, and I want to have as many processors as close as possible to my mixing position (reaching for processors in separate racks furter in the back/side of the studio makes it hard to trust your settings, as you have to go back and forth from the eq and the monitoring sweets pot.

Ssl had a stereo version of one oftheir xrack eq modules I believe; I might be unrealistic but to fit something like this into the api500 module width would be spectacular. But a double wide module would maybe more feasible (especially if using transformer based i/o).

Can't really help with anything more than a thumbs up and hoping to see some progress on the project! Will be following with great interest.
Title: Re: The People's Equalizer
Post by: OneRoomStudios on April 16, 2020, 02:02:42 PM
Ok, it seems like there might actually be some interest here. I mocked up a spice model, and it looks promising (see attached). This is assuming it was fully discrete,  built with 2520s, and had transformer i/o (10k/10K in, 600/600 out). It would be easy to add options for THAT IC i/o and IC opamps though, which would make this a very cheap build.

The first section is the Low-Mid semi-parametric band (260Hz - 2k8Hz variable, +/- 12dB), the second is the High-Mid semi-parametric band (718Hz - 7k8Hz variable, +/- 12dB)  . The third section is the Bax shelves (I plan to add switches to select different caps for different roll-over points, high and low), and the fourth section is the HP/LP filters, with a switch on the LP filter to select 6dB/oct or 12dB/oct. The HP is fixed at 12dB/oct. The final stage is for makeup and output drive (would be altered for THAT corp line driver output).

I didn't include all the switching (each section should be able to be bypassed and frequencies should be able to be selected for the Bax shelves), but it's pretty solid. With everything in and set to "0," it's within +/- 0.5dB from 60Hz to 12K. If you bypassed the filters it would be basically flat.

A few things:

For maximum flexibility, I think it makes the most sense to make any PCBs mono. If you want stereo, buy two and find the appropriate 2-gang/4-gang pots or switches. It might also make sense to leave pots off-board so that people can use switches if they want, or whatever footprint pots they want. That would make the wiring a little more annoying though. As always, there will have to be some kind of balance between simplicity and flexibility.

I doubt 500 series is in the cards - too many controls for the front panel.

Thoughts?
Title: Re: The People's Equalizer
Post by: Ricardus on April 16, 2020, 02:40:34 PM

For maximum flexibility, I think it makes the most sense to make any PCBs mono. If you want stereo, buy two and find the appropriate 2-gang/4-gang pots or switches. It might also make sense to leave pots off-board so that people can use switches if they want, or whatever footprint pots they want. That would make the wiring a little more annoying though. As always, there will have to be some kind of balance between simplicity and flexibility.

I doubt 500 series is in the cards - too many controls for the front panel.

Thoughts?

 Mono PCBs sounds great to me.

As for the rest, I wouldn't mind either way. Pots/switches mounted on the PCB or not.

The only suggestion I would make would be to make it as future-proof as possible. Choosing components that hopefully won't become obsolete and unobtanium in a short time. (so if we could make it entirely out of 555 timers that would be awesome)

I know that's a difficult guessing game, but just a thought.
Title: Re: The People's Equalizer
Post by: OneRoomStudios on April 16, 2020, 03:39:45 PM
The only suggestion I would make would be to make it as future-proof as possible. Choosing components that hopefully won't become obsolete and unobtanium in a short time. (so if we could make it entirely out of 555 timers that would be awesome)

I know that's a difficult guessing game, but just a thought.

The goal is to design it so you can use almost any standard transistors/opamps. The real issue is that by making it through-hole, it's already using obsolete parts, and making it surface-mount wouldn't be very DIY friendly. It's getting harder and harder to find through-hole transistors, but there are still enough out there that folks should be able to find them. I'm a fan of 2N5457 FETs, but J201's or whatever would be fine. The output pair (I used ZTX450/ZTX550, which are still available) is just a normal complimentary pair and you can use any variant you want, you just have to be careful to put the right pins in the right holes. The plan is to include footprints for standard DIP-8 opamps too, so that should be as future proof as it can be.
Title: Re: The People's Equalizer
Post by: fragletrollet on April 16, 2020, 03:58:07 PM
A few things:

For maximum flexibility, I think it makes the most sense to make any PCBs mono. If you want stereo, buy two and find the appropriate 2-gang/4-gang pots or switches. It might also make sense to leave pots off-board so that people can use switches if they want, or whatever footprint pots they want. That would make the wiring a little more annoying though. As always, there will have to be some kind of balance between simplicity and flexibility.

I doubt 500 series is in the cards - too many controls for the front panel.

Thoughts?

Nothing is impossible...  8)

https://www.thomann.de/gb/ssl_xrack_stereo_eq_modul.htm

Sure, no DOA's or TX's, but stereo 4-band in something comparable to to 500 series (width). 
Title: Re: The People's Equalizer
Post by: OneRoomStudios on April 16, 2020, 04:29:10 PM
Nothing is impossible...  8)

https://www.thomann.de/gb/ssl_xrack_stereo_eq_modul.htm

Sure, no DOA's or TX's, but stereo 4-band in something comparable to to 500 series (width).

Haha, sure. If you want this to be surface-mount and cost 1,000, then yes (those multi-deck pots are pricey).  The X-rack format is also 9 cm^2 larger than the 500 series, so all those pots and switches would be pretty squished together.
Title: Re: The People's Equalizer
Post by: fragletrollet on April 16, 2020, 06:36:03 PM
Haha yeah I`m probably being unresonable...  but there sure is a lack of pure stereo eq's in the diy market. But ofcourse mono boards are more customizable than a strictly stereo board. I dont really mind smt if it can drastically reduce the size of the module, because space is more of an issue where I live than the availability of gear  8)

Looking forward to what this manifests in, good luck and all the best! Will follow the progress with great interest  ;)
Title: Re: The People's Equalizer
Post by: abbey road d enfer on April 17, 2020, 02:34:55 AM
You can't have "stereo parametric EQ" and "future-proof" in the same sentence. 4-stack frequency potentiometers are always custom parts.
Wien-bridge EQ's request RevLog frequency pots, that are also hard to get.
For this reason I favour the SVF EQ/filters that uses law-steered linear taper pots. More than one opamp per stage though.
Title: Re: The People's Equalizer
Post by: OneRoomStudios on April 17, 2020, 03:04:31 PM
You can't have "stereo parametric EQ" and "future-proof" in the same sentence. 4-stack frequency potentiometers are always custom parts.
Wien-bridge EQ's request RevLog frequency pots, that are also hard to get.
For this reason I favour the SVF EQ/filters that uses law-steered linear taper pots. More than one opamp per stage though.

This project is best described as "semi-parametric" at best, since the Q is not adjustable.

As for the pots, I'll quote Rod Elliot:
"Resistance Vs. Frequency: Ideally, you'd use an antilog pot but unless the taper is really a reverse logarithmic type (very rare and generally considered unobtainable), you are better off with the linear taper.  Although it gets a little touchy as you get close to minimum resistance (maximum frequency), linear pots generally have better tracking than log or reverse log, and are far more predictable."

He goes on to present a table of linear pot rotation vs frequency in a Wien Bridge circuit. You can see it here: https://sound-au.com/project150.htm (https://sound-au.com/project150.htm)

In practice, linear does the job just fine, and the pots (even multi-deck) are widely available.
Title: Re: The People's Equalizer
Post by: abbey road d enfer on April 17, 2020, 07:10:58 PM
This project is best described as "semi-parametric" at best, since the Q is not adjustable.
Fully parametric or semi parametric, you still need dual pots for frequency.

Quote
As for the pots, I'll quote Rod Elliot:
"Resistance Vs. Frequency: Ideally, you'd use an antilog pot but unless the taper is really a reverse logarithmic type (very rare and generally considered unobtainable), you are better off with the linear taper.  Although it gets a little touchy as you get close to minimum resistance (maximum frequency), linear pots generally have better tracking than log or reverse log, and are far more predictable."

He goes on to present a table of linear pot rotation vs frequency in a Wien Bridge circuit. You can see it here: https://sound-au.com/project150.htm (https://sound-au.com/project150.htm)
Do you really have hands on experience of that? I have and can assure you it's not pleasant, unless you restrict the sweep range to 1:3 or 1:4, which requires switching capacitors for acceptable range.

Quote
In practice, linear does the job just fine,
This is very debatable. The same site shows that for a given rotation of about 15° the variation is +10% on the CCW side and +100% on the CW side: that's for a 1:10 sweep range. Most non-switchable midrange EQ's tend to favour a range of 1:20 or more.
Title: Re: The People's Equalizer
Post by: john12ax7 on April 17, 2020, 08:11:52 PM
Linear pots will cause large jumps with small movement.  The referenced article shows 2:1 frequency jump going 90% to 100%. Very suboptimal imo.

Dual reverse log are somewhat available,  and you can get semi custom at around 250 pcs,  not too bad.
Title: Re: The People's Equalizer
Post by: OneRoomStudios on May 15, 2020, 12:41:15 PM
I'm looking into some custom pots now. If I end up going down that road, it might be worth getting custom dual concentric pots, which would make this viable in 500 series format.

If that were the case though, I'd need to make a choice - either limit it to one variable mid band (so it would be HP, Low Shelf, Mid band, High Shelf, LP), and have room for an output volume control, or give it two mid bands, but make the gain fixed at unity while flat. There's only so much room on a 500 front panel.
Title: Re: The People's Equalizer
Post by: abbey road d enfer on May 15, 2020, 01:19:20 PM
I'm looking into some custom pots now. If I end up going down that road, it might be worth getting custom dual concentric pots, which would make this viable in 500 series format.

If that were the case though, I'd need to make a choice - either limit it to one variable mid band (so it would be HP, Low Shelf, Mid band, High Shelf, LP), and have room for an output volume control, or give it two mid bands, but make the gain fixed at unity while flat. There's only so much room on a 500 front panel.
Who needs gain control on an EQ?  :)
Title: Re: The People's Equalizer
Post by: OneRoomStudios on May 20, 2020, 12:37:10 PM
Ok, in that case, I have a rough schematic/pcb layout. Here's where I've landed feature/function-wise:

500-series, single channel

OEP transformers in and out (they're cheap and solid)
DOA output stage
IC opamps for filter stages - I decided to go with OPA2134 instead of DOAs (can't fit 5 DOA's and 2 transformers in 500 series)

4 concentric controls:

top inner knob: low pass filter, switchable between 6dB/oct or 12 dB/oct (down to 6kHz at 6dB, & 7k5Hz at 12dB)
top outer knob: Bax shelf boost/cut above 6kHz (-3dB down point at full cut)

top-middle inner knob: frequency for wien-bridge mid-band (650Hz  - 8kHz)
top-middle outer knob: boost/cut (+/-12dB)

bottom-middle inner knob: frequency for wien-bridge mid-band (145Hz  - 1K6Hz)
bottom-middle outer knob: boost/cut (+/-12dB)

bottom inner knob: high pass filter (up to 270Hz), fixed 12dB/oct
bottom outer knob: Bax shelf boost/cut below 250Hz (-3dB down point at full cut)

Plus, there'd be a bypass switch (bypassing filter stages, but still running through xfrms and output DOA).


This would (I think) result in a pretty flexible eq, with enough character to be worth building, at an affordable price. a "Volks EQ" if there ever was one.

What do folks think? The one trick would be reaching quantities for minimums on the custom pots.
Title: Re: The People's Equalizer
Post by: Ricardus on May 20, 2020, 01:55:02 PM
Quote
What do folks think? The one trick would be reaching quantities for minimums on the custom pots.

I would build the sh*t out of these.
Title: Re: The People's Equalizer
Post by: OneRoomStudios on May 20, 2020, 05:27:22 PM
I would build the sh*t out of these.

I'd call that positive feedback.

Also, I have enough room on the pcb to leave space to use either standard opamps (i.e. OPA2134) for the bands, or use Sparkos Labs SS3602 discrete opamp to make the whole thing discrete. Depending on how pricing works out on the pots and metalwork, you could probably build up a pair of these fully discrete with transformer i/o for less than most of the cheaper transformerless/IC EQs out there. I'm still waiting for word on minimums for the custom pots from TT Electronics.
Title: Re: The People's Equalizer
Post by: Ricardus on May 20, 2020, 11:10:56 PM
I've seen those Sparkos opamps, and they're a bit pricey so I'd go with standard opamps, myself. It might be fun to build one with them just to see what they sound like, but I suspect it's not necessary.
Title: Re: The People's Equalizer
Post by: SWAN808 on June 05, 2020, 06:16:44 PM
are you using 50k reverse log, log or linear pots on the high and low pass filters?
Title: Re: The People's Equalizer
Post by: OneRoomStudios on June 06, 2020, 11:57:21 AM
The plan was for dual reverse audio pots for the frequency selection. I got a quote for concentric pots with dual rev audio on the inner shaft and linear center-detent on the outer shaft.

Unfortunately, it doesn't seem like there's enough interest in a project like this to hit the minimum quantities.
Title: Re: The People's Equalizer
Post by: SWAN808 on June 06, 2020, 12:57:41 PM
The plan was for dual reverse audio pots for the frequency selection. I got a quote for concentric pots with dual rev audio on the inner shaft and linear center-detent on the outer shaft.

Unfortunately, it doesn't seem like there's enough interest in a project like this to hit the minimum quantities.

ok thanks, I was wondering as I found that Bourns do a 4 gang reverse audio taper pot which could be used in a stereo implementation...20k tho...

https://www.mouser.co.uk/ProductDetail/652-PTD904-2015PC203
Title: Re: The People's Equalizer
Post by: uptownsquash on August 24, 2020, 01:58:05 PM
Has anything come of this? i could be in for a couple?
Title: Re: The People's Equalizer
Post by: OneRoomStudios on August 25, 2020, 09:39:29 AM
Has anything come of this? i could be in for a couple?

No, unfortunately, as I mentioned above, there doesn't seem to be enough interest in a project like this to hit the minimum quantities needed for the custom potentiometers.