**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). 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. 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. 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.
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?
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). 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. 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. 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.
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?