Passive Mastering EQ

[sr1200]

The plan is for them ultimately to be available via the Emporium. I ordered five prototypes because that is the minimum order. I only need two for the first customer so, assuming they work as designed, the other three will then become available.

Cheers

Ian

Count me in!

 

[sr1200]

Pardon for the double post.  Im catching up on your progress.  Out of curiosity, besides cost, any reason not do do a board for the front to connect channels rather than entertaining ribbon cable? 
Something similar to how the G-pultec front panel boards are or how Mike did the front panel board of the SB4000.

 

[ruffrecords]

Pardon for the double post.  Im catching up on your progress.  Out of curiosity, besides cost, any reason not do do a board for the front to connect channels rather than entertaining ribbon cable? 
Something similar to how the G-pultec front panel boards are or how Mike did the front panel board of the SB4000.

None whatsoever. In fact I have abandoned the idea of ribbon cable and designed just such a board that sets the 5 main level setting rotary switches on 1.6 inch centres. This should allow you to fit two sets across the width of a 19 inch panel. Boards arrived a couple of days ago. If I get time this weekend I hope to start building one and do some tests.

Cheers

Ian

 

[ruffrecords]

I finally found some  time to calculate a resistor ladder to test the boost/cut PCB I designed. The steps I decided on were:

+10, +8.0, +6.0. +5.0, +4.0, +3.0, +2.5, +2.0, +1.5, +1.0 ,+0.5, 0, -0.5, -1.0, -1.5, -2.0, -2.5, -3.0, -4.4, -5.0, -6.0, -8.0, -10

so the first 3dB are in half dB steps, the next three are in 1dB steps and the last 4dB is in 2dB steps.

I fitted these and one of the ELMA boost/cut switches to the PCB I designed and checked the right pot position occurs at each switch position. Now I can add some EQ components and check the response. if that works out OK I can add other bands and check interaction.

Cheers

Ian

 

[thomasdf]

Great! Can't wait to read more

 

[ruffrecords]

Tried some tests to measure the level at each step to make sure they were correct. Turned out to be all over the place. I had calculated the resistor ladder completely wrong. I re-did the math and fingers crossed removed the old ladder and replaced it with the new. Thankfully the steps are close to where they should be.

I then tried it with an inductor I had lying around which should give a shelving LF boost/cut but I was disappointed find the range was only 5dB each way. After a bit of head scratching i decided to measure the CDR of the inductor - is was in the hundreds of ohms. No wonder it did not work; the design needs inductors with a DCR of no more than 20 ohms.

So I tried with a capacitor to check shelving high boost/cut. 100NF gave a nice boost cut up to 10dB each way at about 16KHz. So basically it works!! Just need to get some decent inductors and try LC mid bands, then add another seitch or two and look at band interaction.

All the test were done with the input fed from a balanced  source and the output connected balanced to the test kit. So if you want  to run it completely passive with an external 10K line in as gain make it will be fine.

Cheers

ian

 

[ruairioflaherty]

Hey Ian,

When I first moved to California 6 years ago I had more free than I'd ever had before while my business ramped up.  One of the projects I did in that time was a passive eq much like yours. I had started with the idea that I wanted a Pultec which +/-3dB of gain which was totally passive and would use gain available elsewhere in my mastering chain. 

Anyway, after lots of learning and experiments I did end up building a mono 3 band circuit on a breadboard that did exactly what I wanted, a low and high shelf and a mid band with 6 frequencies, with low Q.  Like yours, all 3 bands shared the same resistor ladder (the spacing of which was not at all intuitive as you mention).  Mine was 5k series and shunt but my lowest band inductor was 10H.

I needed some parts from the proto a few years ago so it is no more and I can't find my REW sweeps but I was very happy with how it sounded and measured, though this was before I had the test gear I have now, it might have been a disaster at 23kHz and up!

I'm following your project with great interest.

Cheers,
Ruairi

 

[ruffrecords]

Hey Ruairi,

Yes, the ladder is certainly a PITA to calculate. Because the output comes from about two thirds the way down it is really two ladders in one; one for boost and one for cut. And it is not a regular ladder where you tap off a selected point; instead it shorts from the output to a tap point and that is what messes with my head.

The impedance of mine is lower than yours which means the largest inductor I need will be about 2H for a Q of about 0.6 . Even so, that is a tall order for a DCR of no more than 20 ohms. Did you use an off the shelf inductor or a custom made one?

Cheers

Ian

 

[ruairioflaherty]

I was using a bunch of different off the shelf inductors I had to hand, some Sowters intended for Pultec MEQ5 and a bunch of Carnhills from different Neve circuits.  It's a project I'd love to revisit at some point with a view to winding some inductors to suit.

 

[ruffrecords]

I was using a bunch of different off the shelf inductors I had to hand, some Sowters intended for Pultec MEQ5 and a bunch of Carnhills from different Neve circuits.  It's a project I'd love to revisit at some point with a view to winding some inductors to suit.

it is below 1KHz that is going to be the problem. For 1KHz upwards you need an inductance of 47mH and below and these can easily be obtained with very low resistances as they are in the range provided by commonly available power inductors. Inductances above that will need careful selection to make sure their DCR is low enough.

Cheers

Ian

 

[ruffrecords]

I found some nice low DCR inductors used for SMPS designs and ordered a bunch of values up to 10mH. These should be good for 1KHz and up. DCR is very good - the 10mH type is only 4 ohms so even five of them in series to make 50mH is within the 20 ohms needed for this design. So this weekend I tried a couple of the 10mH at 2KHz and 4KHz to check they work OK and that peaking boost/cut works as expected.

Results so far look promising although the boost maxes out at 8.5dB and the cut at -9dB but that should just be a matter of tweaking the resistor ladder values (it might be a test rig loading problem also). The inductors measure consistently above the stated value so some inductor selection and capacitor tweaking will be necessary to ensure a stereo version tracks properly int the frequency domain.

Next step is it build a couple more bands and see how they interact.

Cheers

Ian

 

[ruffrecords]

Today I completed some basic tests on four bands of the EQ operating at once to study the shape and interaction of the various bands. I chose frequencies of 2K, 4K, 8K and 16K. I did not have enough inductors to do 1K otherwise I would have had all five bands going.Here is the test rig on the bench:

16K boost and cut response:

8K boost and cut:

4K boost and cut:

and lastly 2K boost and cut:

Alternate boost and cut:

All boost:

And all cut:

Tis all seems to work rather well. Next step it to order some low DCR  inductors for the lower bands.

Cheers

Ian

 

[ilfungo]

I'm waiting!!!

 

[5v333]

the one with alt boost and cut looks really funny actually

have you find a source for the bigger inductors?

looks good by the way!

 

[ruffrecords]

the one with alt boost and cut looks really funny actually

have you find a source for the bigger inductors?

looks good by the way!

I have asked Sowter to quote. No answer yet.

Cheers

Ian

 

[5v333]

What are your requirements?
Henry and dcr

 

[ruffrecords]

What are your requirements?
Henry and dcr

750mH with five taps; total DCR less than 20 ohms.

Cheers

Ian

 

[sr1200]

Ian, that looks sexy as hell.  Keep it going!

 

[5v333]

what about the taps?

 

[ruffrecords]

what about the taps?

55mH, 100mH,  200mH, 300mH, 500mH and 750mH

DC resistance at 750mH must be 20 ohms or less.

Cheers

Ian