Passive EQ between Tube Stages

[ruffrecords]

Certainly very much of the complication with trying to make an equalizer of this style for a console is the gigantic efforts and costs if a lot of unique part values are involved. I’ve wanted a 4 pole filter and, as you can see from any calculator, making a constant-K 4 pole involves two inductor values and two capacitor values. Never mind the caps, the inductor situation gets out of hand when you’re trying to implement and purchase so many unique inductor builds.

I was messing around with values and I decided to see what would happen if instead of using the proper 4 pole design i went ahead and just put two 2-pole filters one after the other. So that is two identical caps and two identical inductors. I thought it was going to be a trainwreck in contrast to the true 4 unique value setup. Turns out, as opposed to the proper 4 pole it basically just adds a 0.5dB knee bump and then a similarly mild droop after, but then after that the curve and slope down is more or less the same nice 24dB/octave all the way. I tried this with both high pass filters and low pass filters.

So yeah, electronics engineers are not going to ever suggest this approach because it’s not clinical for any purpose and that bump can’t be removed. But it sure reminds me of resonance in any synth filter! And the fact that it’s only about 0.5dB is basically not ever going to be heard anyways. Done and done.

I think you do electronic engineers (of which I am one) a disservice. I always wanted to admonish Sheldon Cooper who, when looking down with disdain on engineers, said that physicists work to understand the basic laws of nature, by pointing out that engineers accomplish the even harder task of bending nature to our will.

In my own designs I habitually use simple two pole RC HPF where both RC sections are identical. In theory you should not do this because the second RC loads the first but in practice what you get is just two sightly different poles. If you plot the response you get a slightly different shape around the turnover frequency and you still end up with 12dB/octave ultimate slope.

Surel;y you will have heard the joke about the mathematician, the physicist and the engineer? A man placed a 50 dollar/pound note in the middle of a room. He told them they could each walk half way to the note and then stop. Then repeat that process until they got to the note.

The mathematician worked out it would take infinite time to reach the note so did not even bother to try. The physicist, who habitually worked to 5 significant figures, was still there three weeks later.

The engineer did three iterations, said "That's close enough", picked up the note and went down the pub.

This helps turn things into realistic goals. I’m going to see if I can either A, make, or B, buy custom multitapped inductors that are all identical. Just one multitap design, and use these for both the LPF and HPF…. A typical classic EQ inductor having 7 taps actually offers 7+6+5+4+3+2+1 values if you use them in series in all lengths possible, or something close to 28 unique values. The complication would be switching, but that can be worked out. 28 is a lot to work with. I was thinking having a 20 step rotary switch, but even 24 could be used.

The classic pre-existing multitapped offerings may or may not work for this approach, but besides, being able to get down and dirty with the math and create a versatile set of taps all under 800mH-ish seems like a very effective move. The EQ ones from Cinemag are quite physically small, just around an inch in diameter. I’m curious about cost of a custom selection of values in that same package so I’ve sent a message.

But then there’s this https://www.mouser.com/ProductDetail/871-B65701W0000Y038
Which is something someone on this forum pointed to, and which has a massive “AL”, and i understand has a proportionately massive tolerance swing. But, maybe these can be wound by testing each value as i go and that does away with the tolerance issue?

Now you are beginning to sound like a physicist


Maybe you are overthinking it. Can you really hear the difference between a 3 pole and a 4 poles filter?

Cheers

ian

 

[emrr]

I've heard so many times "a 2 pole filter is the steepest natural sounding filter". But no one says that anymore, as they add their 15th plugin to the DAW channel.

 

[systemtruck]

Hey I’m just happy to have learned that these constant -K poles don’t get that trashed if laid on top of each other! That’s the best news right now. Makes all of this simple to design. Great to hear about the same for RC. If you’re ok with it, that says a lot.
I may or may not be able to tell the difference between 3 pole and 4 pole on the fly, depends on how much coffee I’ve had. But I have a long standing love for 4 pole filters applied flat to live instruments not just to synths with resonance/Q, so I may as well try for it on a console I’m making from scratch haha.

Anyways, no matter what the poles end up, I have to figure out the inductors. By far the simplest way to implement them into the circuit is a multi tap with as many taps as possible. 20 taps would be just peachy but I’m not sure how much of a pipe dream that is yet. Fingers crossed that works out.
I still don’t understand why a multi tap inductor can’t be used with partial series connections though. Like say there are 10 tap points, why not connect point 3 and point 7. Seems like that’s the same thing that’s already being done on the top side, so I’m not sure why it wouldn’t work on the bottom side by bringing the ground up. If the unused top is floating free in the wind not interfering, can’t the unused bottom do that too?

 

[abbey road d enfer]

Switching complications aside.. how can that be?
Looking at any of the vintage EQ schematics, can’t we just move up the “bottom” ground/earth as well as the “top” of the selection and create a new value? Doesn’t everything floating underneath the new ground point just get nulled?

Let's say you have a tapped inductor with only 2 taps, i.e. one start S, one end E and two taps, T1 and T2.
If you want a combination of S-T1 and T2-E, you need to isolate the T1-T2 winding. How can you do that?
Shorting T1 and T2 is not an option.

 

[ruffrecords]

I still don’t understand why a multi tap inductor can’t be used with partial series connections though. Like say there are 10 tap points, why not connect point 3 and point 7. Seems like that’s the same thing that’s already being done on the top side, so I’m not sure why it wouldn’t work on the bottom side by bringing the ground up. If the unused top is floating free in the wind not interfering, can’t the unused bottom do that too?

No reason at all you can connect between any two points but I am not sure you are going to get the range of values you might expect.

Remember, on a multi-tap inductor, the inductance you get depends on the square of the number of turns between the two points. There will be a fair amount of head scratching to work out the number of turns for each tap to give you intermediate values for none standard tap combinations and as Abbey pointed out you cannot short any.

Cheers

Ian

 

[systemtruck]

Let's say you have a tapped inductor with only 2 taps, i.e. one start S, one end E and two taps, T1 and T2.
If you want a combination of S-T1 and T2-E, you need to isolate the T1-T2 winding. How can you do that?
Shorting T1 and T2 is not an option.

Correct, I was only suggesting using complete series connections, not two separate segments at once. With the example I used of an inductor with 7 values in a series, I think it adds up to 28 options. Basically, count from the bottom upwards 7 times, move up to 2 and count upwards 6 times, so on and so forth.
So, we’re good!

No reason at all you can connect between any two points but I am not sure you are going to get the range of values you might expect.

Remember, on a multi-tap inductor, the inductance you get depends on the square of the number of turns between the two points. There will be a fair amount of head scratching to work out the number of turns for each tap to give you intermediate values for none standard tap combinations and as Abbey pointed out you cannot short any.

Cheers

Ian

Yeah that’s the part that is pretty wild haha. If there was one of these multi tap inductors in hand that I knew I had to commit to, I could just measure all of the different series values with a good LCR meter, and design filters that utilize some or all of those values. Surely just having lots to chose from would enable a better EQ system since it just ends up more and more adjustable in between the full range, no matter where it lands. Even if they are weird frequencies like 242.68. Just label it 240 or 245 on the dial and that’ll be great.

This reminds me though.. I’ve been trying to find a calculator that does these filter calculations based around component values too, not just frequency / impedance / slope. Is there one? I’m talking about being able to enter an inductor value, impedance, slope, and see what caps go along with it to create which frequency filter.

 

[systemtruck]

I went ahead and did some quick tests with this tool https://markimicrowave.com/technical-resources/tools/lc-filter-design-tool/
For this Cinemag multitap inductor https://cinemag.biz/eng_docs/CML-2212.pdf
I know it’s not meant for this specific design, but it was a good test..

I figured I’d post them here in case it’s a helfpul lesson for someone else too.

I learned this..
Using two of the same inductors in series in a 3 pole Pi type LPF with one cap, perfectly halves the cutoff frequency vs a LPF with same inductor value that uses two caps and one inductor. See below. I thought that was interesting.
It left me wondering why manufacturers would bother implementing two expensive inductors for the LPF. All of them did it this way, so there must be a clear reason. And I’m assuming the answer is accuracy… I’m taking a guess that the wide tolerance of inductors is balanced tighter when two are used, giving more sense of high cut frequency accuracy. Maybe just 1 inductor in a LPF is laughable in regards to consistency from product unit to unit, and just rarely ends up close to the frequency printed on the front panel.
If that were the case, then i suppose the HPF doesn’t need such accuracy since there are a lot more Henries involved in the key range at the bottom so it’s easier to making windings be accurate. That’s just my guess, I have no info about this.

1 inductor Pi LPF (2 caps shunt):

24.2mH, 8K
44.3mH, 4.3k
76mH, 2.5k
106mH, 1.8k
148mH, 1.3k
280mH, 680 Hz
409mH, 467 Hz
806mH, 237 Hz

2 inductor Pi LPF (2 in series, 1 cap shunt):

24.2mH, 4K
44.3mH, 2.15K
76mH, 1.25K
106mH, 900 Hz
148mH, 646 Hz
280mH, 340 Hz
409mH, 234 Hz
806mH, 118 Hz


1 inductor Pi HPF (2 caps shunt):

24.2mH, 1970 Hz
44.3mH, 1075 Hz
76mH, 630 Hz
106mH, 450 Hz
148mH, 322 Hz
280mH, 170 Hz
409mH, 117 Hz
806mH, 59.2 Hz

 

[emrr]

do you have an audio cyclopedia?

 

[systemtruck]

do you have an audio cyclopedia?

Hadn’t heard of it, but it’s here for free in full access with a free account.
https://archive.org/details/audiocyclopedia0000trem/page/n3/mode/2up

Edit: holy crap the Equalizer chapter is a goldmine

 

[emrr]

The 1950’s first edition is probably more expansive. No need to reinvent the wheel. Sims are only as accurate as the complexity of the data entered. Inductor Q can wildly modify the results. Spend some time around a fixed L varying the C and watch the shapes and frequency change.

 

[systemtruck]

The Wave Filter chapter is the chapter pertaining to the filters I’m attempting. It answers SO many questions.
Thank you for this reference. I’ll probably read it sooner than i really have time for haha.
Here are two questions i had posted in this thread that they answer in the text…

Interesting that they demonstrate a balanced filter vs unbalanced filter. This one has more sections, i guess it’s 5 poles instead of the more common 3 pole. But regardless, if i do go this route of balancing a 3 pole, i think it just adds a couple caps for each filter, so why not. Maybe that helps in some way to cancel some noise coming from the inductors.

And as for the series vs shunt question i had, they suggest it is the same performance and having one inductor is cheaper so why not go that route.




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